(b) Temperature increase above pre-industrial levels
For the purposes of evaluating a budget for the period to 2100 a long term warming commitment is used as the fundamental constraint. This can be translated into an atmospheric CO2 concentration making assumptions in relation to several factors including the climate sensitivity, the role of other greenhouse gases and the time horizon for the calculation. The cumulative CO2 emissions that would lead to the level of CO2 corresponding to these assumptions is then derived essentially using IPCC best estimates of the carbon cycle. Table 15 summarises the results of the `carbon budget' analysis for specific concentrations levels and temperature targets. The Appendix to this report contains the basis for calculating the `carbon budget' described here for different long term warming limits.
Uncertainties surround these calculations. The effects of those mentioned above and that of the trajectory of emissions and of carbon cycle model uncertainties, have been estimated in assessing the overall `carbon budget'. If all sources of uncertainty are independent then the overall uncertainty in the `carbon budget' for a given temperature target is around 50%. Uncertainties associated with the `carbon budget' for a given CO2 stabilization level are smaller, of the order of 15%. Much of the difference is due to uncertainty in the climate sensitivity.
Numbers in parenthesis in this section refer to estimates of climate change or carbon budgets for a climate sensitivity of 2.5oC.
The total allowed `carbon budget' up to 2100 to limit long term global average temperature increase to below 1oC above pre-industrial levels is in the range of 110-340 GtC for a climate sensitivity of 3.5oC (150-455 GtC for a climate sensitivity of 2.5oC). Central estimates respectively are 225 GtC and 295 GtC.
The central estimate of the total allowed `carbon budget' up to 2100 that would limit the long term global average temperature increase below 2oC above pre-industrial levels is 410 GtC for a climate sensitivity of 3.5oC[118].
Discussion has occurred in the international climate negotiations on long term CO2 concentration limits of, for example, 450 ppmv or 550 ppmv. Stabilizing actual or equivalent CO2 at either of these levels would require that total carbon emissions be limited in the range of 500-870 GtC respectively over the next 100 years (See Table 15) The long term temperature rise corresponding to these levels would span 2.4 oC (1.7 oC) to 4.2 oC (3.0 oC), far in excess of the ecological limits outlined above.
5.1.3.1.1.1.1.1.1 Table 15 CO2 Concentrations, cumulative emissions, global temperature increase
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This table shows cumulative carbon emissions corresponding to CO2 stabilization levels along the corresponding long term temperature increases taking into account the effects of other greenhouse gases. The carbon budgets 1oC and 2oC temperature limits, for both the 2.5 and 3.5oC climate sensitivity levels, are shown. Equivalent CO2 levels are calculated assuming that the additional forcing of the other greenhouse gases s is 23% of the CO2 forcing. The uncertainty range for temperature targets is +/- 50%
An allowed fossil fuel budget corresponding to temperature limits is the total `carbon budget' less net deforestation for the budget period. The IPCC CO2 stabilization calculations assumed approximately 80 GtC of deforestation emissions over the period to 2100. The range in the IPCC IS92 emission scenarios was 30-95 GtC. In these calculations 80 GtC has been used as the standard deforestation estimate.
With no action to stop deforestation the fossil `carbon budget' is 145 GtC (215 GtC) for the ecological target of a 1oC maximum long term temperature increase.
For the EU limit of a 2oC increase, the comparable fossil fuel budget is 330 GtC (505 GtC) or much less than half the currently known economically recoverable fossil fuel reserves.
Action to substantially reduce deforestation and to expand afforestation programmes would help stabilize the climate system. Mathematically, for a given total `carbon budget' the more action than can be undertaken to limit deforestation the more of the total budget that would be available for fossil fuel use. If, for example, a combination of programmes to halt deforestation and to re-afforest were undertaken over the next century that resulted in no net deforestation over that period then the fossil fuel budget would equal the total budget for that period i.e. 225 GtC (295 GtC).
A 225 GtC budget with emissions constant at around 6.5 GtC from 2000 would be extinguished by 2025. With emissions growing at about 2%/year this budget would be used up by about 2020. On the other hand a reduction in Annex I emissions approximately in line with the "safe emissions" corridor analysis by 2010, followed by a steady, but rapid, decline in global emissions thereafter would limit global emissions to around 225 GtC.
Using the central assumptions adopted in this work for the climate sensitivity (3.5oC) and with this budget if CH4 emissions are cut by 20% by 2015, N2O 30% by 2020, (with both remaining constant thereafter), and halocarbons are phased out by 2025 then the increase in warming from the 1990 to 2100 is 0.5oC or approximately 1.2oC above the pre-industrial global mean. Temperature would be falling from a peak of about 1.4oC above pre-industrial levels (Figure 12). The best-estimate of the increase in mean sea-level above 1990 levels by 2100 would be around 19 cm and beginning to stabilize[119] (Figure 12). The rate of global temperature change for this scenario is shown in Figure 9 (above), where it is apparent that this scenario leads to quite a rapid drop in the rate of warming.
The effects of other greenhouse gases is quite significant. If the emissions of CH4, N2O and fluorocarbons from IS92a scenario are used (i.e. no action on other gases) the warming above pre-industrial levels would be about 2oC and the sea-level rise about 31 cm above 1990 levels..
An example of this kind of scenario is the Greenpeace Fossil Free Energy Scenario (FFES)[120] which combines action to reduce fossil fuel emissions with action to halt deforestation and an extensive reafforestation programme. This scenario was designed using the IPCC best-estimate of climate sensitivity of 2.5oC and had a fossil fuel budget of approximately 305-345 GtC. Under this scenario deforestation was reduced by half to 40 GtC and afforestation projected to sequester 80 GtC over the period to 2100. The net emissions over this period were in the range 265-300 GtC.
A higher climate sensitivity of 3.5oC implies a significantly lower total `carbon budget' than was used in the Greenpeace FFES. If the same deforestation and afforestation programme is assumed then the fossil `carbon budget' would be around 265-270 GtC.
Assuming that the FFES afforestation scenario is close to an upper limit on
what might be possible in this area the range of allowable fossil fuel
emissions that would enable the ecological targets to be met is 145-265 GtC.
Figure 12 Sea-level rise and temperature increase for 225 GtC carbon budget
(a) Sea-level rise above 1990 levels
(b) Temperature increase above pre-industrial levels
See text for explanation of calculation. A climate sensitivity of 3.5oC was used in this calculation along with IPCC " best-estimates" for sea-level rise parameters.
From a policy perspective it would be necessary to actually achieve gains in emission reductions from deforestation over reasonable time periods (10-20 years) before it would be safe to assume that there could be some offset for fossil fuels within a total emission budget approach. A consequence of a failure to rigorously ensure that gains were made in this area could be that total emissions exceed the budget.
For the EU temperature limit and assuming no net deforestation, the fossil fuel budget is around 410 GtC. This is somewhat higher than earlier estimates.
Krause[121] estimated a `carbon budget' for fossil fuels for the period 1985 to 2100 based on a similar long term target. His calculations limited warming rates to below 0.1oC and the absolute warming to 2.5oC above 1860 levels, however the budget calculated is lower than that above being around 270 GtC for the period 1991-2100. The differences appear to be due to improvements in carbon cycle modelling. His calculation assumed no net deforestation over this period and a final atmospheric equivalent CO2 level of 430-470 ppmv with an actual CO2 level of 380-400 ppmv. This corresponds to a long term warming of 2.2-2.7oC Using current carbon cycle models a budget of 400-465 GtC would correspond to these concentration limits. With the standard IPCC deforestation assumptions this would correspond to a fossil `carbon budget' of around 320-385 GtC for the period 1991-2100.
The assumptions used in this work yield a higher central estimate for emissions than that of Krause for several reasons. Krause's contribution from other gases is higher (c.a.48% compared to 23% used here). If Krause's assumptions were used here this would decrease the carbon budgets estimated in this work by about 20% (see Appendix). There have been improvements in the understanding of the carbon cycle since 1989 which also change the budget estimates. Taking these and other factors into account Krause's work is consistent with the results found here.
In terms of the overall feasibility of achieving the EU temperature target, the World Energy Council (a body representing the World's Energy Ministers) `ecological scenario'[122] approximates the reductions and `carbon budget' that would correspond to the EU limit. This work confirms the overall qualitative features of the FFES work.
The WEC's so called `ecological scenario' has as its goal to "reduce CO2 emission levels to 2 GtC by 2100 (corresponding to one-third of current emissions or the 60 percent fall from 1990 levels indicated by the IPCC's 1990 Scientific Assessment as required to stabilise the atmospheric concentration)."[123] In this scenario concentrations of CO2 rise to 430 ppmv and decline, stabilising by 2100[124], thereby avoiding a doubling of CO2 levels. In emission terms reductions of some 30% are required in OECD countries by 2020 and nearly 70% by 2050 and over 90% by 2100. In developing countries the rate of emission growth starts to fall below business-as-usual early in the next century and is 25% below this by 2020. Emissions stabilise around 2050 before declining.
The WEC also confirms the findings of the IPCC that early action is needed:
"In each scenario, the discernible shifts after 2020 toward particular energy sources, fuels, and uses, turn out to be driven largely by choices and developments prior to 2020"[125].
Whilst the WEC scenario does not slow the rate of warming fast enough nor does it limit the long term increase of sea-level rise and temperature to within the limits adopted here, it does show that large reductions can be achieved but that strong policy interventions are needed to achieve these.
Economically recoverable conventional fossil fuel reserves total 820-1239 GtC. If burnt over the next 100 years this would lead to a long term temperature commitment well above 3.5oC. Unconventional oil and gas adds a further 233-262 GtC to this total, taking the warming commitment to well over 4.5 oC in the long term.
Table 16 summarizes the relationship between the `carbon budget' for a range atmospheric stabilization scenarios and the total fossil reserve and resource estimates. For the Greenpeace Fossil Free Energy Scenario a total `carbon budget' of around 300 GtC is used to 2100, which is approximately 30% of the total conventional reserves. The WEC so called `ecological scenario' (which produces a temperature increase close to the EU 2oC maximum limit) burns around 550 GtC or close to 50% of the total fossil fuel reserves. Table 16 CO2 emission scenarios vs. estimates of fossil fuel reserves and resources
Scenario |
GtC |
% of IPCC Conventional Reserves Identified /Potentials by 2020-2025 |
% of IPCC Conventional and Unconventional Reserves Identified /Potentials by 2020-2025 |
% IPCC Resource Base Maximum Potentials |
Fossil Free Energy Scenario (FFES) |
300 |
37% |
29% |
7% |
350 ppmv CO2 stabilization scenario |
300 |
37% |
29% |
7% |
World Energy Council (WEC) `Ecological Scenario' |
550 |
67% |
52% |
13% |
450 ppmv CO2 stabilization scenario |
630 |
77% |
60% |
15% |
550 ppmv CO2 stabilization scenario |
870 |
106% |
83% |
21% |
650 ppmv CO2 stabilization scenario |
1,030 |
126% |
98% |
25% |
750 ppmv CO2 stabilization scenario |
1,200 |
146% |
114% |
29% |
IS92a |
1,500 |
183% |
142% |
36% |
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This table compares the carbon budgets for IPCC stabilization scenarios with estimates of fossil fuel reserves and resources.
Economically recoverable coal reserves total over 646-1,034 GtC, and are enough in themselves to breach any of the ecological limits defined above.
Economically recoverable conventional oil reserves total in the range 110-124 GtC, which is quite close to the `carbon budget' for the ecological target with no action on deforestation of 145 GtC. Taking into account unconventional resources boosts this amount to 240-275 GtC, about equal to the ecological targets outlined above with stringent action on deforestation.
Conventional gas reserves total in the range 72-81 GtC with unconventional reserves boosting this to 175-192 GtC.
The total of conventional and unconventional oil and gas reserves is in the range 410-467 GtC. This is in excess of the central estimate of the budget required to exceed the EU's maximum temperature increase target.
For all the cases examined involving the 1oC limit the total oil and gas reserves exceed the carbon budgets even before potential coal use and deforestation are accounted for. In this idealized calculations, if unconventional oil and gas are excluded from the assessment then nearly all scenarios would burn more carbon than occurs in reserves, however, the ratio would narrow considerably once coal use and deforestation are subtracted from the budget.
This table shows that the current economically recoverable reserves of oil and gas are sufficient on their own to breach the carbon budgets applying to both a 1 and 2oC temperature limit.
The resource base is the theoretical maximum potential resources available and has been estimated to be over 4,000 GtC (Table 8). It is clear that only a very small fraction (less than ca. 5%) of this could be burnt over the next century if ecological targets are to be met.
[118] The range is 205- 620 GtC. For a climate sensitivity of 2.5oC the range is 295-875 GtC with a central estimate of 585 GtC
[119] The MAGICC model of Wigley and others has been used here with the IPCC best-estimate gas and sea-level rise parameters. For the emission assumptions described in the text the radiative forcing of the other gases is 21% of that of CO2.
[120] Lazarus op.cit.
[121] Krause, F.; W. Bach and J. Koomey (1989) Energy Policy in the Greenhouse: From Warming Fate to Warming Limit, Earthscan Publications Ltd, London.
[122] WEC/IIASA (1995), Global Energy Perspectives to 2050 and Beyond, World Energy Council/IIASA International Institute for Applied Systems Analysis.
[123] WEC/IIASA (1995) op.cit. p. 52
[124] WEC/IIASA (1995) op.cit. p. 90
[125] WEC/IIASA (1995) op.cit. p. 92-93