2. The Potential Effects Of A Global Warming
On Rainfall Mechanisms
The most widely used indicator of the expected climate change due to enhanced
greenhouse gas concentrations is the global mean temperature value. Many
authors have concentrated on this variable. In both the 1990 and 1992 reports
published by the IPCC, it is suggested that over the past 140 years global
temperature has risen by about 0.45oC. A recent report of the Max-Planck-
Institut fŸr Meteorologie in Hamburg (Hegerl et al., 1994) shows that the
probability that the latest 20-year warming is due to the (estimated) natural
variability is less than 5%.
It is certain that enhanced greenhouse gas concentrations increase the radiative
forcing (IPCC 1990, 1992). The total forcing is the sum of those from the
individual greenhouse gases (CO2, CH4, N2O, CFCs). For simplicity, total forcing is
expressed in terms of the amount of CO2 which would give that forcing, the
equivalent carbon dioxide concentration. Greenhouse gases have increased since
pre-industrial (mid-18th century) by an amount that is equivalent to about a
50% increase of CO2, whereas CO2 itself has risen by only 30%*.
Precipitation is a second major indicator of climate change. The implications of
a global warming for the amount of precipitable water in the atmosphere, and
for the geographical distribution are introduced in the following subsections.
Footnotes:
* Recently, there has been increasing interest in the role of tropospheric aerosols
(IPCC, 1994; 1995). During the past few decades, human activity has led to an
increase of sulphate aerosols in the atmosphere, mainly due to fossil fuel
combustion and biomass burning. These aerosols affect the radiation balance
directly, by scattering and absorption of solar radiation, or indirectly, by being a
major contributor to cloud condensation nuclei, and hence influencing the size of
cloud droplets and the cloud reflectivity. It is suggested that the enhanced presence
of aerosols reduces the estimated rate of global warming (most pronounced over the
northern mid-latitude continents), and precipitation. Current model estimates
(Jones et al., 1994) indeed indicate a negative radiative forcing when the aerosol
effect is included in the GCM. However, the estimates are highly uncertain. With
regards to rainfall, modelling indicates a lower increase in the global mean
precipitation. (IPCC, 1995). This implies that reducing aerosols, as is presently done
in Europe, would cause an increase in rainfall. As all these suggestions are highly
speculative, they are not considered in the present report's conclusion. Return