Climate countdown | Greenpeace

Countering The Sceptics

Introduction
Counter Arguments
General Statements
Scientific Observations
Modelling Inaccuracies
Detailed Explanations
Additional Reference Index

Introduction

The Climate Sceptics are a handful of scientists, many directly subsidised by the fossil fuel lobby and promoting what numerous mainstream scientists regard as blatant misinformation on climate science, thereby contesting the urgent need to tackle the problem of global warming.

Most of the sceptics have neither the credibility nor the science to mount a plausible challenge to the consensus of 2,500 scientists - including eight Nobel Laureates - who comprise the UN Intergovernmental Panel on Climate Change (IPCC). Whilst some of the sceptics are credible scientists they have chosen to align themselves with one side of the debate over global warming and have promoted viewpoints which have been found to lack credibility. However, many of the sceptics have been able to access industry funding in order to promote their on-going work aimed at undermining the UN Climate Summit in Kyoto.

During 1997, most of the world's major oil, coal and automobile multinationals have gathered forces behind a host of industry front groups and mounted a multi million dollar campaign world-wide to derail agreement on global climate protection. One single US advertising and internet initiative alone reportedly cost US $13 million. A few weeks before the Kyoto summit, several members of this small group of sceptics were known to be on a world tour. This was part of an increasingly intensive campaign, re-inforced by the fossil-fuel and automobile industries backed campaign in order to prevent any mandatory CO₂ or other greenhouse gas emissions being agreed at that meeting.

The so-called science on which most of the sceptics base their arguments is in the main a combination of deliberate misrepresentation of IPCC reports, contextual inaccuracy and unsubstantiated conclusions.

This Background Brief examines the major arguments and statements made by the sceptic scientists over recent years and counters each of those arguments, citing specific source material and, where appropriate, the relevant IPCC Reports. Additional source references for individual sceptic statements appear on page 39.

In it's Second Assessment Report, the IPCC concluded: "the balance of evidence suggests .... that there is discernible human influence on global climate". This statement is a negotiated agreement amongst governments, including Saudi Arabia & Kuwait, based on the IPCC scientific reports; Many of the lead scientists wanted stronger findings reported but were over-ruled by governments as a result of heavy pressure from the OPEC countries and the Global Climate Coalition.

For ease of reference, each sceptic statement is countered by a short answer; most are further countered by a longer more detailed scientific explanation and sourced quotation or reference where appropriate.

In July 1996, as part of its ringing endorsement of the IPCC's report, the US delegation's statement to the Second Conference of Parties to the Convention pulled no punches when it came to attacking the climate sceptics.

are not swayed by and strongly object to the recent allegations about the integrity of the IPCC's conclusions. These allegations were raised not by scientists involved in the IPCC, not by participating governments, but rather by naysayers and special interests bent on belittling, attacking and obfuscating climate change science.

Let me make clear the US view: The science calls upon us to take urgent action; the IPCC report is the best science we have, and we should use it".

The tide would appear to be turning against the sceptics; their statements and arguments are being seen for what they are. This brief is intended as further rebuttal to their misinformation.

Contents

Counter Arguments

General Statements

1. The IPCC Report is imprecise in its measurements and unscientific in its terminology. It is the Policymaker Summary, itself overstated and leaving out crucial caveats which underpins on-going efforts to impose energy taxes and introduce mandatory limits on fossil fuel emissions, not the scientific report (Fred Singer)

Short Answer:
IPCC Reports and Summaries do not measure anything; they represent a carefully balanced assessment of work published to date on all aspects of climate change. They also contain numerous caveats insisted on by the scientists themselves. The Summaries for Policy Makers (SPMs) are negotiated and approved line by line by governments and therefore represent the lowest common denominator view of the IPCC reports. SPMs are heavily influenced by the pressure that the OPEC countries bring to bear by exploiting the consensus rule required for their adoption. Therefore the opposite is true: Often the SPMs contain statements or caveats the scientists themselves objected to but which governments agreed in order to finish the negotiations on the SPM.

2. " Calculations, based on the 1992 FCCC, were known to greatly overestimate warming at the time when the Convention was ratified" & "new calculations support the view that global warming would be relatively modest" (Michael's Congressional Testimony)

Short Answer:
Some of the IPCC 1996 estimates for climate change are indeed lower than the IPCC 1990 estimates. This is mainly because different emission scenarios were used for the "business-as-usual case" and because the effect of atmospheric aerosol pollutants in global climate were included for the first time. If the same "business-as-usual" scenario as in 1990 had been used then the central temperature and sea-level rise projected for 2100 would have been higher than that estimated in 1990. More fundamentally the IPCC has not changed it's best -estimates of warming for a doubling of CO₂ i.e. 2.5 oC. In any event, even with the new emission scenarios, the projected warming is greater than any seen in the last 10,000 years.

3. Technologies already exist for the separation and subsequent collection and disposal of CO₂ from large-scale combustion plant. There are no insurmountable environmental concerns and costs are not prohibitive (McMullan, ESEF)

Short Answer:
Technologies do exist for the separation of carbon dioxide from the flue gas of coal and natural gas plants, but these processes greatly reduce efficiency and consequently incur large extra costs. There are also major environmental problems with CO₂ disposal.

In it's Second Assessment Report the IPCC found that " CO₂ capture and disposal may be ultimately limited for technical and environmental reasons, because not all forms of disposal ensure prevention of carbon re-entering the atmosphere" (IPCC Working Group III, Summary for Policy makers)PM.

4. Noting that 550 ppm level of CO₂ in the atmosphere is a political target, there has been no scientific determination of "dangerous" levels of greenhouse gas concentrations". Furthermore, "there is no useful guide on how to respond to human influence on global climate". (Fred Singer) (Gray; Heartland Institute)

Short Answer:
Solving the climate problem is indeed a political process, based however on advice provided by scientific experts.

The IPCC points out that defining "dangerous" requires value judgements as well as scientific ones and thus deliberately leaves this for policy makers to decide on the basis of evidence presented in their reports.

A level of 550 ppm CO₂ in the atmosphere is close to a doubling above pre-industrial levels. The IPCC has found that impacts for CO₂ doubling include the loss of entire forests, significant loss of human life due to the direct and indirect health effects of climate changes and increased famine and hunger in the poorest parts of the earth. It is a value judgment as to whether or not these are dangerous changes. Greenpeace believes they are.

More explicit guidance on what might constitute a dangerous climate change was given by WMO/ICSU/UNEP's Advisory Group on Greenhouse Gases in 1990 which found that that "temperature increases beyond 1 degree Celcius may elicit rapid, unpredictable and non-linear responses that could lead to extensive ecosystem damage".

It is disingenuous to suggest that there is no useful guide on how to respond to climate change. The IPCC clearly lay out the options for steming the rise in greeenhouse gases, as do many other reports. Which are applied is again a matter of political judgement based on local circumstances.

5. An inner circle of IPCC science policy makers dominate discussions so as to achieve the necessary consensus for politicians to take action, so legitimising the need for a Treaty (Flannery)

Short Answer:
A very large number of scientists have participated in the IPCC process, both for the drafting of the reports and also for their peer reviewing. The IPCC assesses peer-reviewed and scientific sources and its reports are also peer-reviewed. The IPCC involves scientists with a range of views, including those who have some doubt as to the range and extent of human-induced climate change as well as representatives from the industry lobby and other NGOs. In the end it is governments who negotiate every word in the Summary for Policy Makers (SPM) of each IPCC report. The SPMs are generally less compelling than the scientific material which they summarize because they are the product of a UN consensus negotiation and hence areoften only as strong as the OPECcountries will allow. It is common knowledge that the OPEC delegations are directly advised, and in some cases apparently directed, by US fossil fuel lobbyists, including from Exxon, Flannery's own Company.

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Scientific Observations

6. The science of global warming is not settled. "Scientific ...understanding remains limited by enormous uncertainty"

Short Answer:
Uncertainty - which of course has a very specific meaning in science - is inherent in any scientific experiment or model. As is standard practice, the question of accuracy is fully discussed by the IPCC and presented within a clearly defined range.

The IPCC discuss these uncertainties in the last section of WG1 SPM, entitled "There are still many uncertainties". It is notable that the draft title for that particular section ran along the lines of "There are still many uncertainties and surprises may come". Thanks to the fossil fuel lobby, only the uncertainties were emphasised.

The IPCC Chair, Emiritus Professor Bert Bolin, has taken great care to emphasize that the uncertainties work both ways, not just reducing the risk. In its Second Assessment Report, the IPCC found that whilst uncertainties remain, the risk of damage and the precautionary principle provide a basis for action.

7. Ice-core data would suggest that CO₂ responds to global temperature changes, rather than causes them. (Corbyn, ESEF)

Short Answer:
This is contrary to the IPCC findings. The IPCC has found, in its review of the ice-core-record, that the relationship between CO₂ and climate during past warm periods indicates that future warming is likely to lead to higher CO₂ concentrations.

In its 1994 Special Report the IPCC found that ice-core records over the past 220,000 years imply the existence of a significant positive feedback:

"Additional insights into climatic feedbacks come from ice core records going back over many thousands of years (known as palaeo-records). A clear correlation between atmospheric CO₂ concentration and global temperature (especially during warming periods) is evident in much of the palaeo-record over long time-scales, with increases of about 80 ppmv occurring during deglaciations. This relationship between CO₂ concentration and temperature may carry forward into the future, possibly causing a significant positive climate feedback on CO₂ fluxes."

Similar relationships for methane are observed from the palaeorecords, with the IPCC finding that that the positive relationship between CH4 concentration and temperature, as for CO₂, may also carry forward into the future.

8. Water vapour in the troposphere (lower atmosphere) is one cause of the greenhouse effect. Warming could make clouds produce more rain, thereby leaving less water vapour behind in the troposphere . Changes in tropospheric water might therefore act to reduce rather than re-inforce warming. (SEPP)

Short Answer:
This argument, while theoretically plausible, is not supported by any actual evidence for such a negative feedback. In contrast, modelling results suggesting that surface warming will increase evaporation from the oceans and thus the amount of water in the troposphere (lower atmosphere), are supported by observational evidence for the last couple of decades which shows that the water content of the troposphere has indeed increased.

9. Solar output and sunspot activity could well have played a major role in climate change as observed over the last century (Friis-Christensen & Lassen, ESEF; Butler; ESEF; and also as detailed in the Nigel Calder's "The Manic Sun" , Pilkington Press, 1977)

Short Answer:
Solar variability may well play a role in determining temperature variations. However, it is of limited importance in comparison with the effect of increased greenhouse concentration.

10. The IPCC Climate Change 95 conclusions on discernible human influence use selective data between 1963 and 1986 and ignore longer term data which would have given a different result; thus the recorded trends only reflect the specific time frame selected. (Michaels' critique of IPCC Results; Kerr: Article in May 16 issue of Science "Greenhouse Forecasting still cloudy")

Short Answer:
The IPCC findings were not based on a single piece of evidence or a single data set as is implied in Michael's allegation, but rather on a combination of observations and modelling results.

In describing this conclusion the 1995 IPCC Science report IPCC SAR WGI Summary for Policy Makers, op.cit. p. 4.) cites three key pieces of evidence:

The 20th Century mean temperature is "at least as high" as the mean temperature in any other century since 1400 A.D.

Statistical assessments of the significance of the 0.3-0.6oC global warming since 1860 have found that "the observed warming trend is unlikely to be entirely natural in origin".

Comparison of the observed pattern of climate changes with the expected pattern of effects from the combined influence of greenhouse gases and sulphate aerosols with the most advanced computer models of the climate.

Pattern-based studies are those in which the modelled climate response to combined forcing by greenhouse gases and anthropogenic sulphate aerosols is compared with observed geological, seasonal and vertical patterns of atmospheric temperature change. These studies show that observed changes in global temperature increasing match the expected response of the combined affects of greenhouse gases and anthropogenic sulphate aerosols. As a consequence the IPCC found that the "probability is very low" that the correlation between the expected and observed patterns of temperature change "could occur by chance due to natural variability"

Michael's comment refers to one of the climate change pattern studies published by Santer et al. (1996). This study used the best quality dataset that contained information about the vertical structure of temperature through the atmosphere, a dataset that spanned the period 1963-87. The dataset Michael's suggests using contains known data quality problems and has no information about the vertical structure of temperature.

Since the IPCC report was concluded further published work has confirmed its findings.

11. There is still no evidence that warming can be attributed to an enhanced greenhouse effect. The IPCC statement on discernible human influence was produced by government negotiators, not scientists. "Contrary to IPCC Reports, warming - which amounts to 0.5 Degrees Celsius over the past 140 years - falls well within natural climatic variability". (Flannery)

Short Answer:
It is simply not true that there is no evidence to attribute recent warming to an enhanced greenhouse effect ( see above).

It is true that the much-hyped statement "the balance of evidence suggests that there is a discernible human influence on global climate" in the Policy Makers Summary of the IPCC's Working Group I report is a product of political negotiation - It is however the minimum the scientists would accept and many of the lead authors preffered a stronger conclusion.

The IPCC found that whilst there is "already limited evidence for the existence of an anthropogence climate signal", it also pointed out that few would be willing to argue that completely unambiguous attribution of ( all or part) this change has already occurred." ( IPCC WGI Chapter 8, pp 438-443)

The observed patterns of change are not consistent with either solar variations or volcanic effects. The fact there has not been a completely unambiguous detection of a human induced climate signal does not justify Flannery's assertion.

12. The human contribution to atmospheric CO₂ is small and its effect on global warming is equally small.

Short Answer:
Not true. The IPCC has found that there is incontrovertible scientific evidence exists showing that the increase in CO₂ levels since pre-industrial times is due to human activities. The 25% increase in CO₂ concentration since pre-industrial times is due to human activity. The effect of this and the increase in other greenhouse gases is significant.

Whilst there are large natural fluxes into and out of the atmosphere each year from the oceans and plants these are known with certainty not to be the cause of the increase. It is intellectually dishonest to categorically make such blatantly inaccurate claims, given the weight of evidence to the contrary.

The oil company Mobil is also making these claims and flouting the solidly understood science of this aspect of the climate issue.

13. Singer has argued that large and natural variations in temperature and CO₂ in the last 3,000 years have produced no apparent ill effects. [Sources: Tree Ring Data by G C Jacoby, (Science, Vol 272, 1996), A Scientific Review of Measurements by R.A. Berner (Science, Vol 276, 1997) & a Woods Hole Institute Study of Sargasso Sea Sediment (Science, Vol 274, 1996)]

Short Answer:
The 'large variations' Singer refers to are estimated to be of the order �0.5�C in global temperature and �20 ppmv in CO₂ concentration. Over the next 100 years we are looking at between +1 and +3�C of warming and an increase of between 200 and 400 ppmv of CO₂. By comparison, from the last glacial maximum to the current interglacial period CO₂ levels rose by about 90 ppm over 10,000 years. Over the same periods the global mean surface temperature increased by 4-5oC. In other words the changes projected for the next century are some 20-30 times faster than those which occurred at the end of the last ice age. Nevertheless, the small changes refered to by Singer did have significant impacts at the time i.e the little Ice age of 150 -450 years ago.

14. There is no firm geological evidence to support global warming. The geological record from near and remote past does not provide the necessary evidence (Harry Priem: ESEF) .

Short Answer:
Priem argues that the geological past does not provide evidence in support of the notion that greenhouse gas increases will warm the planet. This argument is deeply flawed as the records he bases this assertion on are for many millions of years ago and are not legitimate analogies between current and future global warming, owing to an entirely different set of environmental conditions which existed then. The ice-core records of the last 220,000 years shows a clear danger that warming will itself lead to higher CO₂ levels - a positive feedback.

15. Inaccuracy of IPCC CO₂ concentration increase projection of 1% per year in the next century, against an actual increase of 0.4% in the 1990's (Gray; Heartland Institute)

Short Answer:
The claim that the IPCC are over-estimating future changes in the composition of the atmosphere (and thus the risk of climate change), implicit in the above, is fundamentally flawed.

The rate of increase of CO₂ alone in the 1990's is approximately 0.4-0.5%/yr, which with the effects of other greenhouse gas emissions brings the total rate of increase to around 0.6-0.7%/yr. in CO₂ equivalent terms. The IPCC emission scenario often used as the "business as usual" scenario for the next century, IS92a, projects an increase in equivalent CO₂ concentration of about 0.8%/yr. compound. Other IPCC scenarions project increasess in the range of 1.0-1.1% on the high side and on the low side 0.4-.5%. However, the latter are not widely regarded as realistic.

The IPCC present a wide range of projections of future concentrations of carbon dioxide over the next century ranging from very slow growth to very rapid growth over the next century. All but the very lowest of these show that growth rates are likely to increase throughout the next century.

16. With annual growth of 3 GtC per year, yet fossil fuel emission totalling between 5 - 6 GtC, including an additional 1 - 2 GtC from deforestation which are quite uncertain, there is no solid understanding as to why the accumulation is 3 GtC when emissions are 8 GtC" (Flannery, 1997)

Short Answer:
So what? What is important for climate is specifically the proportion of emissions which do accumulate in the atmosphere. It is known with certainty that the accumulation is due to human activities.

That said, the IPCC has estimated that difference in emissions and concentrations is accounted for by increased uptake of carbon dioxide by the oceans and vegetation - although there some uncertainty over the precise role of the various processes involved. Looking to the future, one concern is that the proportion of emissions taken up by these "sinks" could go down due to limits on the capacity of individual sinks, deforestation and/or die-back of plants in response to climate change, thus actually increasing the risk of climate change.

17. There has been a marked growth in downward trend in methane (CH4) growth during recent years and there has been a distortion of data on both atmospheric methane and nitrous oxide concentration (Gray, Heartland Institute)

Short Answer:
The simple fact is that methane concentration continues to grow. As is the case for carbon dioxide, concentrations of methane and nitrous oxide in the atmosphere are regularly recorded from many air-sampling sites, and concentrations are known to a high degree of precision. Concentration levels have increased steadily throughout this century. These measurements can be compared with many ice-core records which date to several hundred thousand years before the present.

18. Singer found that sea levels dropped with increasing temperature over the last century rather than rise. Comparing data on temperature and sea-level, the on-going sea-level trend shows a drop between 1925 and 1940, a fall at a time when temperatures were rising. If confirmed, this would indicate that during temperature increases, any glacial melting would be more than off-set by increased ocean evaporation and precipitation at the poles. (SEPP)

Short Answer:
But why then has sea-level not fallen during the 1980s and 1990s? Global temperature and global sea-level will not necessarily correlate well if the correlation is performed on annual data. Both records are subject to various influences over the short-term, mostly natural, which will confound any simple such relationship. It is whether there is correlation over the long-term - decades or more - which matters. The long term trend for an increase in both global temperature and sea surface temperature is unequivocal.

19. Warming all occurred before the 1950s and thus before the major greenhouse gas increases.

Short Answer:
Not true. Of the 0.3 to 0.6degC surface air temperature warming which has occurred since the late-19th century, between 0.2 to 0.4degC (i.e., 67 per cent) has occurred over the last 40 years. Warming over the last two decades has been at the rate of about 0.15degC per decade.

20. Non-climatic influences such as urbanisation can change the local temperature (heat island effect) The studies of temperature used have not been uniformly distributed across the globe or up through the atmopshere. The fact that data have been taken at different times means that temporal changes in the climate are not considered. All of these effects need to be considered and resolved. (Corbyn & Golipour, ESEP)

Short Answer:
This comment refers to the global surface air temperature record for land areas. All of these sources of uncertainty have been extensively examined and in most cases quantified. Based on this record (when blended with the marine data), IPCC state that: "Global surface temperatures have increased by about 0.3 to 0.6degC since the late-19th century, and by about 0.2 to 0.3degC over the last 40 years." [IPCC, 1996, p.137] The marine record shows a similar trend to the land areas.

21. "Signs of temperature levelling off have been evident since 1990."

Short Answer:
Not only is it impossible to deduce the start of a trend from just a few years of data as suggested, but the statement is untrue. The six years since 1990 (1991-96) have averaged +0.25degC warmer than the 1961-90 average, the warmest six year period in the entire global instrumental record starting in 1856. 1995 was the warmest year on record and 1997 may yet be the seond warmest, which clearly shows that there is no trend towards temperature levelling off.

22. Satellite measurements cast doubt on the rates of warming produced by computer models.

(a) In stark contrast to ground-based meteorological stations, satellites and radiosondes picked up little evidence of warming. (Flannery)

Short Answer:
This is an invalid comparison as the various techniques are measuring different things. At face value this is true, but satellites are measuring the temperature through a depth of the atmosphere - typically between 1 and 8 km above the Earth's surface - and not the surface air temperature which the meteorological stations are measuring. When the satellite records are corrected for known variations, they produce a similar trend to the surface records.

In any case, when the effects of ENSO events and volcanic eruptions are removed from the satellite and surface records, the trend agreement from 1979 to 1995 between the two improves. This implies that mid-atmosphere and surface temperatures are affected differently by ENSO and volcanic eruptions - a fully understandable notion. Furthermore, the satellite and radiosonde records are much shorter than surface observations (17, 40 and 140 years respectively). Trend detection in a short 17-year record is not easy; the longer-term 140-year surface record is much more appropriate for this purpose. Even the 40-year record from radiosondes picks up a warming trend in the mid-troposphere of between 0.08�C and 0.11�C/decade since the 1950s.

Source:: IPCC (1996) pp.147-148 in, Climate change 1995: the science of climate change (eds.) Houghton,J.T., Meiro Filho,L.G., Callendar,B.A., Harris,N., Kattenburg,A. and Maskell,K. Cambridge University Press, Cambridge, UK, 572pp.

(b) Satellite records are far more reliable than ground records. (Flannery)

Short Answer:
This claim cannot be substantiated. It is difficult to create a continuous, consistent climate record from satellite observations alone. The main reason is that satellites and instruments have a finite lifetime and have to be replaced. Again it should be noted that satellites and ground records are measuring different things.

For the current Microwave Sounding Unit (MSU) satellite record, eight different satellites have been used. A significant problem is that each new satellite has a different orbit so that a different part of the diurnal cycle is sampled. Since MSU-derived temperatures depend on surface emissions, and since surface emissions vary substantially with the diurnal cycle, so the observed orbital drift mean that the diurnal cycle can get aliased with trends, especially over land. Surface instrumental records also have measurement biases and instrument changes. The point here is that both satellite and instrumental surface records need careful scrutiny and adjusting for biases. One record cannot be claimed to over-ride the other.

Source:: Trenberth, K.E. and Hurrell, J.W. (1997) How accurate are satellite 'thermometers'? Nature, 389 (25 September), 342-343.

(c) The heat-island effect, which shows a wide disparity of temperature readings between large, medium and small population countries, has not been addressed by the IPCC. (Goodridge, SEPP)

Short Answer:
This claim is manifestly false. The effects of urbanisation - the heat-island effect - on the surface temperature record over land have been extensively analysed and results are fully discussed by the IPCC. The question of disparity in trends between different countries is irrelevant in the context of global climate changes.

Additional Explanation:
A large amount of work has been published to address and quantify these problems. For example, land stations with strong urban warming influences have been removed from the record and the remaining bias due to urbanisation has been estimated at no more than 0.05degC over 100 years (Note: urbanisation cannot effect sea surface temperatures, which comprise 70 per cent of the planet). On the basis of this and other work (see Q.24 above), the IPCC cite a range of warming since the 19th century (between 0.3 and 0.6degC) rather than a single value with spurious precision.

Source:: IPCC (1996) pp.141-143 in, Climate change 1995: the science of climate change (eds.) Houghton, J.T., Meiro Filho, L.G., Callendar, B.A., Harris, N., Kattenburg, A. and Maskell, K. Cambridge University Press, Cambridge, UK, 572pp,

(d) New evidence of regional climate warming at northern mid-latitudes shows a startling correlation to patterns of commercial airline traffic. Singer theorises that thin contrails of ice particles are creating a regional warming at the surface. This effect has also never been addressed by the IPCC. (SEPP)

While it is true that the emissions from aircraft could be having a climate impact and more work is needed in this area, this argument probably overblows the scale of the problem. IPCC made no comment beyond stating that "The climate impact of aircraft ....remains a serious concern" (IPCC WG 1 1996), since there were virtually no peer-reviewed papers in print to review. But it's notable that the handful that were suggested a relatively weak effect.

The phrasing of the arguments is particularly disingenuous in the implicit suggestion that the warming trend may be all down to aircraft. While airline traffic and regional warming may correlate well, this is not necessarily "startling" given that alongside the growth in airline traffic, emissions of greenhouse gases from other sources have also escalated.

Additional Explanation:

The most recent paper examining this theory concludes: "Several remaining questions must be answered before a reliable assessment of the impact of soot particles on cirrus can be made." (Jensen et al., 1997, p.252). This work is ongoing.

Notwithstanding the above comment, were such an effect on surface temperatures to be demonstrated, it would hardly be a cause for complacency. It would indicate another mechanism whereby human emissions into the atmosphere contributes to global warming, a mechanism which, with the forecast growth in air traffic, can only worsen.

Source:: Jensen, E.J. and Toon, O.B. (1997) The potential impact of soot particles from aircraft exhaust on cirrus clouds Geophys. Res. Letts., 24, 249-252.

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Modelling Inaccuracies

23. "The climate model results are known to be wrong: there is inconsistency with current climate trends and with parallel historic trends" (Flannery)

Short Answer:
Climate models are NOT known to be wrong. On the contrary, they are proven to be increasingly successful at reproducing trends in the behaviour of the climate back to when records began - and before.

This in turn gives confidence in the models to predict future climatic trend. Different models give varying results because they use different physical mechanisms to represent the respective interactions of the key climatic influences which they are studying.

24. "Studies on the impacts of climate change on ecosystems, health and the economy are based on old models which are in error...." (Michaels: Congressional Testimony, November 16th, 1995).

Short Answer:
Michaels is referring to the use of climate scenarios for impact studies deriving from global climate model (GCM) experiments completed prior to the Earth Summit in Rio in June 1992. GCM experiments in the last two to three years yield slightly lower estimates of global warming. The reason for these differences are two-fold, neither of which is related to 'errors' in the models. Far from being "model errors", these factors depend on what a priori assumptions are made about the future world economy. First, different emissions scenarios are being used to force the models and, second, the effects of sulphate aerosols are now being simulated in addition to the effects of greenhouse gases. New impact studies are being performed which use these later scenarios.

25. "Newer models have yet to be properly reviewed", "or to be properly verified against actual, current or past temperature records"; they also have well-known limitations: limited resolution and incomplete data & methods with which to validate the models" (Michaels' Congressional Testimony ; Gray; Flannery )

Short Answer:
Unlike sceptics' statements, climate models are extensively reviewed and validated before being allowed to be published

Limitations are openly discussed by the IPCC.

26. Different climate change models produce significantly different results, especially for critical factors such as precipitation, soil moisture, drought and storms" (Flannery)

Short Answer:
Different models converge on key climatic features, although it is true that there is considerable divergence vis-a- vis regional scenarios.

Different models converge on many key climatic features, although it is fair to say they diverge on diverge on details such as specific regional context.

Additional Information:
Differences between model results should not be exaggerated; they show many common features:

" All model simulations ... show the following features: greater surface warming of the land than of the sea in winter; a maximum surface warming in high northern latitudes in winter; little surface warming over the Arctic in summer; an enhanced global mean hydrological; increased precipitation and soil moisture in high latitudes in winter" (IPCC, WG1, 1966, p 6)

" At the hemispheric-to-continental scale, confidence in climate models are higher at the regional scale. In addition, there is more confidence in temperature projections than in hydrological changes." (IPCC, WG1, 1966, p 6)

27. Likewise ".... in the models used, the surface and free troposphere are very closely coupled; hence their temperatures would be expected to move together. However, the surface and the mid-troposphere appear to be much less coupled than the models assume." (Michaels, New Scientist 19 July, 1997)

Short Answer:
At face value this could appear true, but it is important to understand what temperatures satellites and radiosondes are measuring.

They are, in fact, measuring the temperature through a depth of the atmosphere - typically between 1 and 8 km above the Earth's surface - and not the surface air temperature which the meteorological stations are measuring.

28. With regard to agriculture, plant and CO₂ fertilisation effects are such that the increase of CO₂ in the atmosphere both promotes growth and reduces their demand for water. These effects could be expected to lead to a greening of the Earth and there are signs that it is already happening. (Idso, ESEP)

Short Answer:
This attractive picture of a greener, more productive world is unproven. While elevated CO₂ concentrations have been shown to increase plant growth and water use efficiency under controlled conditions (such as in a laboratory or a greenhouse), doubts exist over how much of this benefit will be realised in the real world due, amongst other things, to limits on nutrient availability and other environmental conditions.

Moreover, in some cases the overall effects of CO₂ fertilisation could actually be negative and result in, for example, in a reduction in the quality of forage crops in rangeland areas. Still more fundamentally, no account is taken of the potential adverse effects of climate changes on, say forest growth, which may result from the increases in carbon dioxide - a dead plant is hardly in a position to benefit from carbon dioxide fertilisation.

29. "Climate models cannot predict regional climate change, and are especially poor on hydrology, so impacts are poorly known." (Flannery)

Short Answer:
It is true that climate models validate better at larger-scales than at smaller-scales and that parts of the hydrological cycle are amongst the hardest to model. But most studies treat results from climate models as scenarios rather than as predictions and then proceed to examine the sensitivity of an impact sector to these scenarios. No serious impact scientist is claiming that we have a definitive prediction of the regional impact of future climate change.

Nevertheless the overwhelming conclusion is that there will be significant negative impacts on many natural and human systems even if the tuning and exact character of these impacts cannot be predicted with certainty.

30. "Concerns have been expressed about catastrophes like increases in hurricanes and sudden climate shifts. These are largely speculative without scientific confirmation." (Flannery)

Short Answer:
This argument underplays the what is a real risk of more extreme events and abrupt changes in climate. While it is fair to say that the jury is still out on hurricanes, it is certainly conceivable that global warming will be associated with increases in hurricane frequency or intensity.

More serious, is the apparent dismissal of the risk of sudden climate changes. Not only is there evidence of abrupt climate changes in the recent and distant past, but model results suggest that if current emissions of greenhouse gases continue unabated then the North Atlantic ocean circulation could shut down some time in the next 100 years, causing rapid cooling across Europe.

While the likelihood of such events is uncertain, even a low risk of a catastrophic outcome must be taken seriously.

31. "Recent estimates of health impacts have fallen considerably, many show gains. It appears that adaptation could counter many impacts on managed systems in wealthy nations." (Flannery)

Short Answer:
In northern temperate countries such as the UK and Japan, there is empirical evidence that a decrease in deaths in warmer winters has, in the past, more than offset projected increases in heat-wave deaths. However, the gains and losses in a warmer world would vary by region and studies in the USA point towards a death rate that increases more steeply with rising temperatures than the fall in winter mortality.

Health impacts have always been projected to be much lower in wealthy countries. The real issue is in relation to poor populations living in countries without a strong health system. This is the majority of the world's population.

In many regions of the world, however, negative health impacts are expected to dominate, directly through heat waves and extreme weather events, and indirectly through changes in the incidence and an extension in the range of many vector- and water-borne diseases, such as malaria, dengue fever and meningoencephalitis. The extent to which this becomes a major health hazard depends on both the availability and effectiveness of appropriate pesticides and drugs - a major concern is that resistance to both is growing.

Contents

Detailed Explanations

In order to effectively reply to many of the scpetics' statements in the previous section, additional information and explanation helps further clarify specific rebuttals.

Statement 1. The IPCC Report is imprecise in its measurements, unscientific in its terminology .........

The IPCC does not make measurements. It was created by the World Meteorological Organisation and the United Nations Environment Programme in 1988, in order to :

assess available scientific information on climate change

assess the environmental and socio-economic impacts of climate change, and

formulate response strategies (IPCC WGI, 1996, Foreword).

Following IPCC rules, the SPM (Summary of Policy Makers) was approved in detail at the fifth session of IPCC Working Group I, (Madrid, 27 -29 November 1995). An "approved " report has been subject to detailed, line-by-line discussion and agreement in a plenary meeting of the relevant IPCC Working Group. Participants in the Madrid meeting included 177 delegates from 96 countries, representatives from 14 NGOs (including Dr Singer) and 28 lead authors (IPCC WGI, 1996, Preface). The initial text of the SPM, written by scientists, was subjected to hundreds of amendments, many of them coming from oil exporting countries and industrial lobbies. As a result, the approved text of the SPM contains even more caveats than in the original version.

Dismayed by the " dilution " of their text, which they resented to a certain extent as a misrepresentation of their conclusions, some lead authors were on the verge of resigning and disapproving the final text of the SPM. For example, the original proposal for the famous sentence "the balance of evidence suggests that there is a discernible human influence on global climate " was much stronger :"The weight of evidence indicates a significant human influence in global climate change" (GPI, 1966, The Scourge of the Sceptics).

Heavy lobbying from numerous quarters including among others Saudi Arabia, Kuwait, the Climate Council, GCC & Dow Chemical resulted in the final version being less precise and a long way from what the scientists wanted and had included in the original.

Statement 2. Calculations were known to greatly overestimate warming at the time the Convention was ratified .... new calculations ..... relatively modest global warming

The statements which comprise the sentence are vague; probably intentionally so. The assumption must be that Michaels is referring to the IPCC 90 calculations, which served as a basis for negotiating the UNFCCC. These calculations were the best available at the time, and were not known to overestimate the warming due to greenhouse gases. The IPCC WGI (1996) Report explains why the newer projections for the mid-range IPCC emission scenario, IS92a, including the effects of aerosols, project an increase in global mean surface air temperature relative to 1990 which is approximately one third lower than the 1990 " best estimate ":

It is due primarily to lower emission scenarios (particularly for CO₂ and the CFCs), to the inclusion of the cooling effect of sulphate aerosols and to improvements in the treatment of the carbon cycle " (p 6). But the 1990 IPCC assessment that for a doubling of carbon dioxide concentrations in the atmosphere - or an equivalent increase of a mixture of greenhouse gases - climate models give an equilibrium warming of 1.5-4.5 degrees C, which remains unmodified by the SAR. Furthermore, the IPCC WGI (1996) report observes that : " In all cases the average rate of warming [over the next century, as computed in the SAR] would probably be greater than any seen in the last 10,000 years ".

N.B.

These explanations also put into context the often-quoted Flannery contention that the 1990 IPCC best guess of a warming of 3 deg C and of a sea level rise of 65 cm were subsequently revised downwards by some 30 per cent.

Statement 3 Dangerous climate change

The ultimate objective of the UNFCCC (Article 2) requires "...stabilisation of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system."

The IPCC explicitly leave the definition of what constitutes "dangerous ... interference" for policy makers to decide on the basis of evidence presented in their reports. It specifically highlights the fact that " the definition of "dangerous" will depend on value judgments as well as upon observable physical changes in the climate system and that such policies will not rest on purely scientific grounds"(IPCC WW1, 1996 Preface).

As yet there is no political consensus over what level of climate change may be considered tolerable. But, some indication of what could be viewed as dangerous interference was given by WMO/ICSU/UNEP's Advisory Group on Greenhouse Gases (AGGG) in 1990.

Based on assessments of historic rates of change, adaptation rates and climate stability, the AGGG found that global temperature change of above 0.1oC per decade and 1oC in total (above pre-industrial levels) are linked with extensive damage and bring with it a risk of climate instability. Rates of temperature change above 0.2oC per decade and 2oC in total are associated with major social and economic disruptions and a large risk of climate instability.

According to stabilisation scenarios presented by the IPCC based on best estimates of climate sensitivity, keeping rate of change below 0.1 degC per decade over the next century requires atmospheric concentrations of greenhouse gases to be stabilised at less than 440ppmv - assuming that emissions of other greenhouse gases are kept constant at 1990 levels. Allowing for increases in emissions of other greenhouse gases, this means stabilising carbon dioxide at less than 440ppmv..

The proposal for a target carbon dioxide concentration of 550ppmv (parts per million by volume) comes from France. Even before allowance is made for increases in other greenhouse gases, IPCC scenarios show that this could cause global temperatures to rise by about 2 oC above pre-industrial levels by 2100, suggesting a rate of increase in excess of what might be considered tolerable.

Response options for reducing the risk of climate change and reducing potential adverse impacts are laid out in the IPCC reports on Impacts, Adaptations and Mitigation of Climate Change, and on Economic and Social Dimensions of Climate Change.

References: Vellinga, P. and Swart, R.G, 1991. The Greenhouse Marathon. Proposal for a Global Strategy. In: Climate Change: Science, Impacts and Policy: Proceedings of the Second World Climate Conference, eds J. Jaeger and Ferguson, p.129-134.; Rijbersman, F.R. and Swart, R.G. (eds), 1990. Targets and Indicators of Climate Change. Stockholm: Stockholm Environment Institute.; Krause, F., Bach, W. and Koomey, J., 1990. Energy Policy in the Greenhouse. Volume 1: From Warming Fate to Warming Limit. Benchmarks for a Global Climate Convention. London: Earthscan.; Karas, J., 1991. Back from the Brink. Greenhouse Gas Targets for a Sustainable World. London: Friends of the Earth..; IPCC, 1996: Watson, R.T. and others (eds). Climate Change 1995. Impacts, Adaptations and Mitigation of Climate Change: Scientific-Technical Analyses. Cambridge: Cambridge University Press.; IPCC, 1997. Houghton, J. and others (eds). Stabilisation of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications. WMO/UNEP.; IPCC, 1996. Bruce, J.P. and others, (eds). Climate Change 1995. Economic and Social Dimensions of Climate Change. Cambridge: Cambridge University Press.

Statement 4 The Inner IPCC Circle

As succinctly documented in the Preface to IPCC WGI (1996):

" The SAR was compiled between October 1994 and November 1995 by 78 lead authors from 20 countries. Formal review of the chapters and the summaries by governments, non-governmental organisations (NGOs) and individual experts took place during May to July. Over 400 contributing authors from 26 countries submitted draft text and information to the lead authors and over 500 reviewers from 40 countries submitted valuable suggestions for improvement during the review process. "

Somewhat large for an inner circle, one would have thought.

Statement 5 Water Vapour

All scientists agree that water vapour is an important greenhouse gas and that warmer conditions will increase evaporation and thus the transfer of water from the oceans into the atmosphere. What is at issue is how much will stay there to increase global warming (a positive feedback) and how much will be rained out and thus reduced (a negative feedback). This, in turn, depends on both how much extra water the troposphere can hold and how much reaches the free troposphere (the atmosphere above the weather systems) which depends partly on the complex physics of clouds.

Climate models treat these processes in some detail and in nearly all cases models agree that the water vapour feedback effect is positive and will amplify the warming induced by greenhouse gases alone. But as the IPCC freely acknowledges, processes for maintaining water vapour in the atmosphere remain an area of major uncertainty.

Lindzen has been most vocal in challenging the consensus view, arguing that warmer clouds will become more efficient at producing rain, leaving less water vapour behind to moisten the free troposphere. This is a fine theory, but there is simply no evidence to support it. Lindzen's views were taken into account by the IPCC, but still they concluded, "There is no compelling evidence that the water vapour feedback is anything but the positive feedback indicated by the models." [IPCC 1996, p.210].

This viewpoint is further supported by observations over the last couple of decades. Nearly all available studies show that the water vapour content of the free troposphere has been increasing, regardless of whether the studies were over North America, over the western Pacific or over the Equatorial Pacific. These tests agree, in essence, with the climate model results, thus increasing confidence in their predictions of an amplifying water vapour feedback on the climate sensitivity

[see p.162 of IPCC 1996 for discussion of the observational evidence of increasing water vapour, and pp.200-210 for a discussion of the theory].

References: IPCC (1996) Climate change 1995: the science of climate change (eds.) Houghton,J.T., Meiro Filho,L.G., Callendar,B.A., Harris,N., Kattenburg,A. and Maskell,K. Cambridge University Press, Cambridge, UK, 572pp,; Lindzen,R. (1990) Some coolness concerning global warming Bull. American Meteor. Soc., 71, 288-299

Statement 6 Solar Sunspot Activity

Again, this area is succinctly covered in IPCC documentation:

IPCC WGI (1996, p 115) : " Recent observations and theoretical calculations imply that the radiative forcing due to changes in the Sun's output over the past century has been considerably smaller than anthropogenic forcing. "

Page: 118 : " The available evidence indicates that natural variations in the radiative forcing, due to volcanic eruptions and changes in solar output, may have been important in determining some of the decadal scale variations in global climate over the past 150 years. Nevertheless, the cumulative radiative forcing due to human activity remains large compared to these and on this evidence such radiative forcing would be expected to have played a more significant role in determining the long-term trends in climate over the past 150 years. "

Statement 7 Selectivity of data

Santer's study, published in July 1996, demonstrates an increasing pattern of correlation between two different climate models and global observations of weather balloon (radiosonde) data from 1963 to 1987, data that reveal the vertical structure of temperature changes (called the Oort dataset). This study was one of several that were used by the IPCC to establish a 'discernible human influence' on climate. Although there is another dataset (constructed by Jim Angell) that spans a longer period from 1957 to 1995, this dataset has four limitations that prevented it being used in the Santer study:

It contains no information about vertical temperature structure

The years prior to 1963 have known instrumentation problems and contain very little Southern Hemisphere data

The Angell dataset contains far fewer weather balloon observations and so is less comprehensive that the Oort dataset

The Angell dataset has not been scrutinised carefully for internal consistency, whereas the Oort dataset has (at least until 1987).

If Santer had used the unscrutinised Angell dataset, the sceptics would (should) have claimed the study was invalid being based on inferior data. This is the claim they frequently make about the global surface temperature record, so they cannot here criticise Santer for using the best quality dataset.

References:: Santer,B.D., Taylor,K.E., Wigley,T.M.L., Johns,T.C., Jones,P.D., Karoly,D.J., Mitchell,J.F.B., Oort,A.H., Penner,J.E., Ramaswamy,V., Schwarzkopf,M.D., Stouffer,R.J. and Tett,S. (1996) A search for human influences on the thermal structure of the atmosphere Nature, 382, 39-46.

Statement 8 No evidence of an enhanced greenhouse effect

Again, the IPCC refutes this statement with absolute clarity:

IPCC WGI (1996, p 5) : " The balance of evidence suggests a discernible human influence on global climate. Since the 1990 IPCC Report, considerable progress has been made in attempts to distinguish between natural and anthropogenic influences on climate. ....Assessments of the statistical significance of the observed global mean surface air temperature trend over the last century have used a variety of new estimates of natural internal and externally forced variability. ......Most of these studies have detected a significant change and show that the observed warming trend is unlikely to be entirely natural in origin. ....More convincing recent evidence for the attribution of a human effect on climate is emerging from pattern-based studies, in which the modelled climate response to combined forcing by greenhouse gases and anthropogenic sulphate aerosols is compared with observed geographical, seasonal and vertical patterns of atmospheric temperature change. These studies show that such pattern correspondences increase with time, as one would expect as an anthropogenic signal increases in strength. "

The much cited "balance of evidence" quote in the Policy Makers and technical Summaries of the IPCC WGI report does not reflect the subtleties or the strength of the conclusion of chapter 8. The latter states, "The body of statistical evidence in Chapter 8, when examined in the context of out physical understanding of the climate system, now points towards a discernible human influence on global climate." (IPCC WGI, pp.439)

Such is the level of mis-information, taken in combination with the evidence already sourced in this document in reply to General Statement (2) on Page X, it is almost as though the sceptics naively assume that nobody can remember what was in the IPCC Reports.

Reference: IPCC WG I, 1996. Houghton, J. and others.

Statement 9 Natural Climatic Variability

Again, the most effective rebuttal is through a selection of IPCC quotations:

IPCC WGI (1996, p 14) " CO₂ concentrations have increased from about 280 ppmv in pre-industrial times to 358 ppmv in 1994. There is no doubt that this increase is largely due to human activities, in particular fossil fuel combustion, but also land-use conversion and to a lesser extent cement production. "

IPCC WGI (1996, p5) : Assessments of the statistical significance of the observed global mean surface air temperature trend over the last century have used a variety of new estimates of natural internal and externally forced variability. These are derived from instrumental data, palaeodata, simple & complex climate models and statistical models fitted to observations. Most of these studies have detected a significant change and show that the observed warming trend is unlikely to be entirely natural in origin.

'' ...Our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural variability, and because there are uncertainties in key factors. Nevertheless, the balance of evidence suggests that there is a discernible human influence on global climate "

Statement 10 Temperature variations produce few ill effects

The palaeoclimatic records for the last three millennia are too limited to afford a complete reconstruction of global temperatures and even more limited to enable reconstruction of regional climate variability and extremes. The natural variations Singer refers to are estimated to be of the order �0.5�C in global temperature and �20 ppmv in CO₂ concentration. Far from being large, in the context of the anticipated climate change for next century these changes over 3,000 years are rather small.

When, in addition, one considers the rapidly growing global population, rising from less than one billion pre-industrial to over 10 billion by 2050, Singer's argument by analogy is seen to be largely irrelevant. A climate warming of 1�C on a planet with 1 billion people could be entirely different when it occurs on a planet with 10 billion people. In any case, we have scant enough documentary evidence over the last 3,000 years to be able to assert that 'no ill effects' have been caused. Indeed, the late Hubert Lamb (1988) in his classic study of what evidence there is from history demonstrates that at certain times and certain places over the last 2,000 years climate change has noticeably influenced human social, economic and political development.

Reference:: Lamb,H.H. (1988) Weather, climate and human affairs Routledge, London

Statement 11 No firm geological evidence

Priem bases his case on examining the relationship between estimates of global temperature in past eras and the reconstructed CO₂ concentrations prevailing at those times. (The relationship between global temperature and CO₂ concentration during the last glacial/interglacial transition has been dealt with in Q.9 above). He cites evidence from the Late Ordovician period (440 million years ago) and from the mid-Cretaceous period (100-120 million years ago), eras with higher CO₂ concentrations than today but with temperatures rather similar to those of today.

These are not legitimate tests of the global temperature/CO₂ relationship, however, since as Priem himself admits, they are many environmental differences between the past and the present which make these past eras illegitimate analogies for current global warming. For example, the geography of the land surface was entirely different during these two past eras to that we observe today, the orbital characteristics of the Earth (an important and well-known factor determining planetary climate) were not the same as today and the cycling of various greenhouse gases between the atmosphere, oceans and biosphere would have been entirely different to that which currently prevails.

Contrary to what Priem implies, Global Climate Models have been successfully used to simulate the climates of the last glacial period 18,000 years ago and also those of the early Holocene period of 9,000 years ago. And when, in fact, and, when these experiments incorporate the appropriate orbital characteristics of the Earth, they are able to reproduce quite well the pattern of global climate reconstructed from various paleoclimate indicators (e.g. COHMAP, 1988).

Reference:: COHMAP (1988) Climatic changes of the last 18,000 years: observations and model simulations Science, 241, 1043-1052

Statement 12 Inaccurate projection increases .....

The overall long-term historic trend in carbon dioxide is towards increasing growth rates in carbon dioxide concentrations over time. Growth rates in the early 1990s were unusually low compared with those in the preceding decade and since then. This anomaly appears to be due to natural variations in release and take-up rates of carbon dioxide and cannot be considered as representative of longer-term trends.

In the words of the IPCC, "As best as can be established, the anomaly of the early 1990s represented a large but transient perturbation of the carbon system" (IPCC 1996, p.82). A better historic comparison is with the average growth rates over the last decade - about 0.7% per year.

The IPCC present six greenhouse gas emissions scenarios, based on a wide range of future assumptions regarding economic, demographic and policy factors (IPCC 1994, pp.21-22). In all but the very lowest scenario - which assumes severe constraints on fossil fuel use and low population and economic growth - growth rates of carbon dioxide concentrations continue to increase throughout the next century.

Future temperature and sea levels are presented for all scenarios, but more detailed climate change experiments generally use either the mid-range scenario (IS92a) or assume a growth rate in equivalent carbon dioxide concentrations of 1% per year (IPCC 1996, pp.289-357). Equivalent CO₂ concentrations take into account the effect of all greenhouse gases.

Reference:: IPCC (1994). Houghton, J. and others, (eds). Climate Change 1994. Radiative Forcing of Climate Change and An Evaluation of the IPCC IS92 Emission Scenarios. Cambridge University Press.; IPCC (1996). pp.69-131 and pp.289-357. In: Houghton, J. and others, (eds). Climate Change 1995: The Science of Climate Change. Cambridge University Press, Cambridge, UK,572pp.

Statement 13 Accumulation figures and sinks

Flannery's statement is probably meant to raise doubts about the rate of carbon accumulation in the atmosphere.

The central reply to this argument is the data about CO₂ emissions from fossil fuel combustion is solid. Data about emissions due to land use changes are more uncertain, but they represent approximately 25 % of total anthropogenic emissions, which the IPCC gives as 7.1 GtC/year (+/- 1.1 GtC) for the period 1980-89.

The fraction of those anthropogenic CO₂ emissions which stays in the atmosphere is called the "airborne fraction". It is close to 50 % (IPCC WGI, 1996, p78) at the moment. The amount is very well known, as all carbon that accumulates in the atmosphere contribute to the increase of atmospheric CO₂ concentration and has been very precisely measured.

It is true that there is some uncertainty about the fate of the fraction of anthropogenic carbon emissions that does not remain airborne.

Some 0.5 GtC (+/ 0.5) is taken up by forest regrowth in the Northern Hemisphere and some 1.3 GtC (+/-1.5) is taken up by vegetation through other processes

Reference: IPCC 1996, pp.78-79, 449 and 487. Houghton, J. and others (eds). Climate Change 1995. The Science of Climate Change. Cambridge: Cambridge University Press.

Statement 14 Methane trends and data distortion

Another example of a highly selective quotation, completely out of context and misinterpreted:

The most likely source would seem to be:

IPCC WGI (1996, p18): "Over the last 20 years, there has been a decline in the methane growth rate: in the late 1970s the concentration was increasing by about 20 ppbv/yr and during the 1980s the growth rate dropped to 9-13 ppbv/yr. Around the middle of 1992, methane concentrations briefly stopped growing, but since 1993 the global growth rate has returned to about 8 ppbv/yr."

(p 88): "There has been considerable speculation and modelling of the possible cause(s) of the 1992/93 anomaly. Many of these factors contributed to the observed methane anomaly in 1992/93, but at present it is not clear what their relative contributions are, whether they can fully explain the observed anomalies in concentration and isotopic composition, or even whether we have identified all of the important processes."

The concentration of atmospheric methane is currently the highest ever observed, including ice-core records that go back 160,000 years. Methane concentration increased sharply during the last two hundred years and has approximately doubled during the last century (Stauffer et al., 1994). During 1992/93, very low growth rates were observed, (see above), but in 1994 global methane growth rates recovered to close to the range of rates observed throughout the period 1984 to 1991. A number of explanations have been forwarded for the 1992 slowdown. These include the possibility of reduced emissions related to large cuts in natural gas production and leakage in the former Soviet Union and decreased biomass burning in the tropics. IPCC Working Group I concluded that not all of the decrease in the growth rate was accounted for by these explanations.

Nitrous oxide is a longer-lived and more potent heat-trapping gas than methane. Concentrations continue to grow in the global atmosphere, increasing from 275 ppbv in the pre-industrial atmosphere to 311 in 1992. The 1993 growth rate was lower than observed in the late 1980s-early 1990s. The lower growth rate has been tentatively related to feedbacks associated with sulphur emissions from the June 1991 Mt. Pinatubo volcanic eruption, rather than with human activities. Nevertheless the changes in growth rates fall within the range of variability seen on decadal time-scales.

Reference:: IPCC (1996) pp.87-88 in, Climate change 1995: the science of climate change (eds.) Houghton,J.T., Meiro Filho,L.G., Callendar,B.A., Harris,N., Kattenburg,A. and Maskell,K. Cambridge University Press, Cambridge, UK, 572pp, Stauffer,B., Neftel,A., Fischer,G. and Oeschger,H. (1994) pp. 251-254. In Trends '93, Oak Ridge National Laboratory, Tennessee, USA.

Statement 15 Sea level rise

Correlation between global temperature and sea-level may or may not show a negative relationship when performed on annual data. Both records are naturally subject to various influences over the short-term and these differing influences will confound any simple relationship. Over the duration of the last 110 years, however, for which we have estimates for both indicators, the long-term trends for an increase in both global temperature and sea-level are unequivocal. The correlation between these long-term trends is very high. [Note: sea-level between 1925 and 1940 did not fall - it merely rose more slowly; source: p.264 of IPCC, 1990].

There are three main contributions which climate warming makes to sea-level change: thermal expansion (i.e., warmer ocean water expands and leads to higher sea-levels), land glacier melt, and changes in the mass balance of the Greenland and Antarctic ice sheets. Of these, thermal expansion and glacier melt are considered to be the most important with respect to observed changes to date. Changes in mass balance of ice sheets due to climate change are hard to estimate and in the case of Antarctica may contribute to fall in sea-level because of increased snow accumulation. This may be what slowed the sea-level rise during the 1920s and 1930s and in this sense Singer may be correct. However, since there is only a limited volume of ice locked up in land glaciers (between 40 and 60 cm only of equivalent global sea-level rise; p.372 IPCC, 1995), the single most important long-term contribution to global sea-level rise is thermal expansion. It is this contribution that will increasingly dominate future sea-level rise as the ocean column slowly expands over the next few centuries in response to the extra heat the atmosphere injects into the world's oceans.

References:: IPCC (1990) Climate change: the IPCC scientific assessment (eds.) Houghton,J.T., Jenkins,G.J. and Ephraums,J.J., Cambridge University Press, Cambridge, UK, 364pp.; IPCC (1996) Climate change 1995: the science of climate change (eds.) Houghton,J.T., Meiro Filho,L.G., Callendar,B.A., Harris,N., Kattenburg,A. and Maskell,K. Cambridge University Press, Cambridge, UK, 572pp,

Statement 16 Current warming timescales

Examination of the global temperature record reveals two periods of noticeable warming, the first between 1910 and 1940 and the second between 1975 and the present. Greenhouse gas concentrations have been rising since the Industrial Revolution, although with an increased rate of increase since the mid-20th century. But one would not expect a precise correlation between greenhouse gas concentrations and global temperature for at least two main reasons.

First, global temperatures will vary quite naturally due either to internal ocean-atmosphere oscillations or to natural external forcing factors like volcanic eruptions and changes in solar energy. For this reason, the global temperature curve will never follow the smooth monotonic increase in greenhouse gas concentrations. Second, is the contribution that other human pollution sources have made to global temperature. The most important of these is now considered to be sulphate aerosols formed from the emissions of sulphur dioxide. Sulphate aerosols have a cooling effect on surface temperature and the concentration of these small particles increased most substantially in the middle decades of the century. During the middle decades of the century these aerosols therefore increasingly counteracted part of the warming effect resulting from increasing greenhouse gas concentrations. Since the 1970s, however, emissions of sulphur dioxide have been decreasing in the major American and European source areas (thus again releasing greenhouse warming), although such sulphur emissions are now on the increase over large parts of Asia.

[For further details on the reconciliation of observed warming trends with model simulations, see IPCC 1996, pp.423-424].

Reference:: IPCC (1996) Climate change 1995: the science of climate change (eds.) Houghton,J.T., Meiro Filho,L.G., Callendar,B.A., Harris,N., Kattenburg,A. and Maskell, K. Cambridge University Press, Cambridge, UK, 572pp,

Statement 17 Additional factors effecting surface temperature records

The global surface air temperature record is a blending of land and sea surface temperature measurements. Each of these sources of data have specific sources of error and, the further back in time one goes, generally the fewer observations are available. For example, there are no observations for Antarctica before 1957. A large amount of work has been published to address and quantify these problems. For example, land stations with strong urban warming influences have been removed from the record and the remaining bias due to urbanisation has been estimated at no more than 0.05degC over 100 years (Note: urbanisation cannot effect sea surface temperatures, which comprise 70 per cent of the planet!). The possible warming effects of desertification on the global record have also been quantified and this bias is again only a few hundredths of a degree Celsius per century. On the basis of all of this work the IPCC cite a range of warming since the 19th century (between 0.3 and 0.6degC) rather than a single value with spurious precision. The conclusion indicates without question a warming, which is on-going.

Reference:: IPCC (1996) pp.141-143 in, Climate change 1995: the science of climate change (eds.) Houghton,J.T., Meiro Filho,L.G., Callendar,B.A., Harris,N., Kattenburg,A. and Maskell,K. Cambridge University Press, Cambridge, UK, 572pp,

Statement 18 Levelling off of temperature

The more general point, however, is that one can't interpret long-term trends from short-term data. A general global warming trend does not imply that each year, or even each 5 or 10 year period, will be warmer than the previous one. There are many reasons why temperatures in individual years and short periods rise and fall, mostly to do with natural variations in the climate system. For example, the years 1992 and 1993 were only +0.14degC and +0.19degC above the 1961-90 average, due mainly to the cooling effects of the Mt.Pinatubo eruption in the Philippines. The year 1996 (+0.22degC) was cooler than 1995 mainly because of a weak La Ni�a event in which the tropical Pacific Ocean cooled substantially. What we anticipate at a global scale is not for 1997 to be necessarily warmer than 1996 (or 1995), but whether, for example, the 1990s decade is warmer than the 1980s decade and whether the first 10 years of the next century will be warmer than the 1990s.

[Data cited here are from the global surface air temperature record compiled by UEA, Norwich, and the UK Hadley Centre, the record most commonly used in all IPCC reports and in international scientific research papers. It should not be confused with the global mid-troposphere air temperature record derived from satellites. This is an entirely different variable and is discussed in Q.26 below.]

Statement 19 Inconsistencies between different models

The most effective, succinct and easily understood evidence to counter these statements comes - but inevitably - the relevant IPPC Report passages:

IPCC WGI (1966, p 30-34): "Several factors give us some confidence in the ability of climate models to simulate important aspects of anthropogenic climate change in response to anticipated changes in atmospheric composition:

The most successful climate models are able to simulate the important large-scale features of the components of the climate system well, including seasonal, geographical and vertical variations which are a consequence of the variation of forcing and dynamics in space and time

Many climate changes are consistently projected by different models in response to greenhouse gases and aerosols and can be explained in terms of physical processes which are known to be operating in the real world

The models reproduce with reasonable fidelity other less obvious variations in climate due to changes in forcing
The model results exhibit "natural" variability on a wide range of time-and-space scales which is broadly comparable to that observed

In short, confidence in climate models has increased since 1990 and further confidence will be gained as models continue to improve"

Statement 20 Impact models in error

Michaels here is totally misunderstanding how climate change scenarios for impact work are arrived at. The single most important unknown in the climate system for determining the likely future climate change is the climate sensitivity (i.e., the equilibrium global temperature warming for a doubling of CO₂ or CO₂-equivalent). The IPCC range for this value lies between 1.5�C and 4.5�C and has done so ever since the IPCC first published in 1990. In fact, this range can be traced back to one of the first ever reports on global warming in 1979 by the National Academy of Sciences. Climate scenarios have been using values of the climate sensitivity within this range ever since impact work started in the late 1980s. What has altered more recent scenarios of climate change is the assumed growth in greenhouse gas concentrations during the next 100 years and whether or not the effects of sulphate aerosols are simulated. Far from being 'model errors', these factors depend on what a priori assumptions are made about the future world economy and whether one can simulate the effect of aerosol forcing. Higher forcing scenarios and the exclusion of aerosol effects will raise global warming rates; lower forcing scenarios and the inclusion of aerosol effects will lower global warming rates.

The latest projections available from IPCC encompass a range of global warming rates of between about 0.07�C and 0.4�C per decade over the next 100 years, with a mid-range value of about 0.2�C (Kattenburg et al., 1996). The ranges are a little lower than those published by IPCC in the early 1990s - in fact by between 10 and 30 per cent lower - but this is due not to 'model error', but to different a priori assumptions. Impacts studies in any case take a wide range of climate change scenarios into account, often spanning all of the range of warming rates cited above.

Reference:: Kattenburg, A., Giorgi, F., Grassl, H., Meehl, G.A., Mitchell, J.F.B., Stouffer, R.J., Tokioka,T., Weaver, A.J. and Wigley,T.M.L. (1996) Climate models - projections of future climate pp.285-358 in, Climate Change 1995: the science of climate change (eds.); Houghton, J.T., Meiro Filho, L.G., Callendar, B.A., Harris, N., Kattenburg, A. and Maskell, K., Cambridge University Press, Cambridge, UK, 572pp.

Statement 21 Accuracy of review

The IPCC assesses available information on the science of climate change on the basis of articles published in peer-reviewed journals and reports of professional organisations such as the International Council of Scientific Unions (ICSU), the World Meterological Organisation (WMO), the United Nations Environment Programme (UNEP), the World Health Organisation (WHO) & the United Nations Food and Agriculture Organisation (FAO) (See: Forward to Synthesis Report, p. vii). According to these rules, the newer models - the results from which form the basis of the SAR - have already been discussed and analysed in major scientific journals and publications.

As already explained in detail, these models succeed in simulating current and past climate with ever increasing effectiveness.

Models, by definition, have limitations as their very function is to isolate specific key sensitivities of a system. Limitations of the IPCC models are not hidden but openly discussed:

"Many factors currently limit our ability to project and detect future climate change. In particular, in order to reduce uncertainties, further work is needed on the following priority topics ... Representation of climate processes in models, especially feedbacks associated with clouds, oceans, sea ice and vegetation ... Systematic collection of long-term instrumental and proxy observations of climate system variables ... for the purposes of model testing ..." (IPCC, 1996, p 7)

"Important uncertainties remain, but these have been taken into account in the full range of projections of global mean temperature and sea level change" (IPCC, 1966, p 5)

Statement 22 Satellite models

When the effects of ENSO events and volcanic eruptions are removed from the satellite and surface records, the trend agreement between 1979 and 1995 is good - +0.09�C/decade for the satellite record and +0.17�C/decade for the surface. This implies that mid-troposphere and surface temperatures are affected differently by ENSO and volcanic eruptions - a fully understandable notion. Comparison with model results must proceed with caution here, since models do not simulate the effect of volcanoes - and only imperfectly the effect of ENSO. The fact that surface and mid-troposphere temperature trends agree much more closely when these two effects are eliminated from the respective records is a result in line with model simulations.

Reference:: IPCC (1996) pp.147-148 in, Climate change 1995: the science of climate change (eds.) Houghton, J.T., Meiro Filho, L.G., Callendar, B.A., Harris, N., Kattenburg, A. and Maskell, K. Cambridge University Press, Cambridge, UK, 572pp.

Statement 23 Agriculture, plant and CO₂ fertilisation

CO₂ fertilisation is most obvious for so-called C3 plants (most crops and also certain annual weed species), but for C4 plants (e.g. maize, sugar cane, sorghum, millet) the effects are much more modest. In general, the most responsive are the annual crop species and the least responsive are long-lived tree species.

Despite the substantial experimental evidence that CO₂-fertilisation occurs, there are several uncertainties about how this effect is realised in the real world. First, the photosynthetical rate is also related to climate, nutrient and water availability. Non-optimal nutrient conditions could limit the CO₂-fertilisation effect in natural ecosystems. Second, in long-term laboratory experiments, adaptation to higher CO₂ levels seems to occur and photosynthetical rates in plants can eventually return to 'normal' levels. Finally, the actual response of a plant depends strongly on its location, function and competitive ability in an ecosystem. Experimental evidence of CO₂-fertilisation on photosynthetic rates cannot therefore necessarily be directly translated into higher ecosystem productivities.

Even where plant growth is enhanced through enhancements in carbon dioxide, there can also be negative repercussions. For example, the IPCC point out that in the case of rangelands, "CO₂ increases are likely to result in reductions of forage quality and palatability because of increasing carbon to nitrogen ratios" (WGII, pp.133).

It is also possible for some species the benefits of CO₂ fertilisation may be outweighed by the adverse effects of climate change. For example, the IPCC point out that, "Forests are particularly vulnerable to extremes of water availability (drought or waterlogging) and will decline rapidly if conditions move towards one of these extremes" (WG II, pp.97). A dead forest is hardly going to "benefit" from enhanced carbon dioxide levels.

Reference:: IPCC (1996) pp.454-455 in, Climate change 1995: the science of climate change (eds.) Houghton, J.T., Meiro Filho, L.G., Callendar, B.A., Harris, N., Kattenburg, A. and Maskell, K. Cambridge University Press, Cambridge, UK, 572pp.; IPCC (1996) pp.431-432 in, Climate change 1995. Impacts, adaptations and mitigation of climate change: scientific-technical analyses (eds.) Watson, R.T., Zinyowera, M.C., Moss, R.H. and Dokken, D.J., Cambridge University Press, Cambridge, UK, 878pp.; IPCC (1996) pp.133 in, Climate change 1995. Impacts, adaptations and mitigation of climate change: scientific-technical analyses (eds.) Watson, R.T., Zinyowera, M.C., Moss, R.H. and Dokken, D.J., Cambridge University Press, Cambridge, UK, 878pp.; IPCC (1996) pp.97 in, Climate change 1995. Impacts, adaptations and mitigation of climate change: scientific-technical analyses (eds.) Watson, R.T., Zinyowera, M.C., Moss, R.H. and Dokken, D.J., Cambridge University Press, Cambridge, UK, 878pp.

Statement 24 Model limitation with regard to regional variation and hydrology

This statement is missing the point. It would be equally true to say that numerical weather prediction models 'cannot predict' regional or local weather tomorrow or for the day after. They can make such predictions, but these predictions have a certain error margin - sometimes they are right, sometimes they are wrong, often they are partly right and partly wrong. This uncertainty does not inhibit a huge range of commercial, leisure and personal decisions being influenced by the numerical forecasts and clear economic benefits resulting from such applications.

What we are doing in climate change impact studies is recognising that there may well be large climate changes ahead, being concerned that these climate changes may have substantial and serious impacts (some positive and some negative), and therefore undertaking to use the best sources of information to guide these studies. Global climate models do undoubtedly provide the best sources of such information, even though we recognise that the 'error margins' increase the smaller the region becomes (see Gates et al., 1996, for IPCCs discussion of the reliability of climate models). The important thing is to be explicit about the uncertainties - as IPCC is scrupulously careful to do - and then to interpret results accordingly. Waiting for 'perfect predictions' of regional climate change before considering the impacts of climate change is as ludicrous an idea as waiting for a 'perfect weather forecast' before making a decision about going on holiday.

Reference:: Gates, W.L., Henderson-Sellers, A., Boer, G.J., Folland, C.K., Kitoh, A., McAvaney, B.J., Semazzi, F., Smith, N., Weaver, A.J. and Zeng, Q.-C. (1996) Climate models - evaluation pp.228-284 in, Climate change 1995: the science of climate change (eds.) Houghton, J.T., Meiro Filho, L.G., Callendar, B.A., Kattenburg, A. and Maskell, K., Cambridge University Press, Cambridge, UK, 572pp.

Statement 25 Catastrophes

Model findings on hurricanes are contradictory, with some researchers finding little or no change in hurricane frequency and intensity, and others suggesting some potential for changes in intensity in some region. A key problem is that the models are not yet good enough to make meaningful predictions of hurricane occurrence.

The potential for sudden climate changes is well established. In the words of the IPCC, "Unexpected external influences, such as volcanic eruptions, can lead to unexpected and relatively sudden shifts in the climate state. Also, as the response of the climate system to various forcings can be non-linear, its response to gradual forcing changes may be irregular" (IPCC, pp.45).

This view is supported by evidence of abrupt changes occurring both in the recent past which shows a sudden shift in the atmospheric circulation of the North Pacific and in the behaviour of the El Nino-Southern Oscillation in about the mid-1970s. Ice core and other records also show that large and rapid climatic changes temperature, precipitation and ocean circulation have occurred in the past.

The risk of abrupt changes resulting from human-induced climate change have become more tangible with the recent finding that the North Atlantic ocean circulation could shut down completely within the next 100 years if current trends in emissions continue - if this occurs, then temperatures in western Europe will plummet.

A further concern is the possible break-up of the West Antarctic ice sheet - this would cause a rise in sea level of about 6m. This threat is generally considered as uncertain and remote in time, but the recent discovery that ice shelves in Antarctica may be melting from beneath due to the presence of warm water from the deep oceans adds to concern over the stability of the continental ice shelves .

While much work needs to be done to firm up the likelihood and form of extremes and abrupt changes that may accompany global warming, the potential serious consequences justifies a precautionary approach to climate change. A measure of urgency is injected into these proceedings as the chances of the unexpected are increasing daily. In the words of the IPCC, "[as] future climate extends beyond the boundaries of empirical knowledge........it becomes more likely that the outcomes will include surprises and unanticipated rapid changes" (IPCC WGII, 1996, pp.5).

[Hurricances: IPCC, pp.354, North Pacific: IPCC, pp.45, El Nino: IPCC, pp.163, North Atlantic: Stocker and Schmittner, 1997. Antarctic: Jenkins and others, 1996]

Reference: Jenkins, A., Jacobs, S.S. and Keys, 1995. Is the little PIG in hot water. Ant. J. of US.; IPCC, 1996. Houghton, J. and others (eds). Climate Change 1995. The Science of Climate Change. Report of IPCC Working Group1. Cambridge: Cambridge University Press.; Stocker, T. and Schmittner, A., 1997. Influence of CO₂ emission rates on the stability of the thermohaline circulation. Nature, 388, 862-865.; IPCC, 1996. Climate Change 1995. Impacts, Adaptations and Mitigation of Climate Change: Scientific-Technical Analysis. Cambridge: Cambridge University Press.

Statement 26 Health Impacts

If extreme weather events were to occur more often, increases in rates of death, injury, infectious diseases and psychological disorders would result (high statistical confidence). Increases in non-vector-borne infectious diseases such as cholera, salmonellosis, and other food-and water-related infections also could occur, particularly in tropical and subtropical regions, because of climatic impacts on water distribution, temperature, and microorganism proliferation (medium statistical confidence). Although the predicted increase in potential malaria transmission encroaches mainly into temperate and upland regions, actual climate-related increases in malaria incidence (estimated by one model to be of the order of 50-80 million additional cases annually), would occur primarily in tropical, subtropical, and less well protected temperate-zone populations.

Some examples of changes in vector-borne viral infections in "wealthy" countries are summarised in IPCC (1996). Some infections spread in response to warming and increased rain in Australia are Murray Valley encephalitis, which can cause severe brain damage, Ross River virus, which can cause long-lasting joint inflammation, and dengue fever, which can cause an average of 15% mortality without proper medical attention. Also water-borne infectious diseases such as meningoencephalitis would tend to spread more rapidly. No suitable vaccines exist for some of the diseases most sensitive to climate change such as dengue fever or schistosomiasis.

Reference:: IPCC (1996) Human population health Chapter 18 in, Climate change 1995. Impacts, adaptations and mitigation of climate change: scientific-technical analyses (eds.) Watson,; R.T., Zinyowera, M.C., Moss, R.H. and Dokken, D.J., Cambridge University Press, Cambridge, UK, 878pp.[Note: the first sentence about UK and Japan is taken from a government sponsored report in the UK in 1996, and Japan in 1997].; McMichael, A.J. and others, 1996. Climate Change and Human Health. Geneva: World Health Organisation.

Contents

Additional Reference Index

A. Dr Brian P Flannery in Global Climate Change (June 10, 1997)

B. The Global Warming Debate, European Science & Environment Forum (ESEF)

C. Testimony of Patrick J Michaels (University of West Virginia) before the US House of Representatives Subcommittee on Energy and Environment, November 16, 1995

D. Heartland Policy Study No 84, "Climate Change 95": "An Appraisal" by Dr Vincent Gray.

E. S. Fred Singer, President of the Science & Environment Policy Project (SEPP), former Director of the US Weather Satellite Service

F. SEPP E-Wire Press Release