Climate response to aerosols
Changes in aerosols and other climate forcing mechanisms fundamentally cause a perturbation to the Earth's energy budget, either by scattering or absorbing solar radiation, or by absorbing infra-red radiation that would otherwise be emitted to space. We call this perturbation "radiative forcing".
Climate sensitivity parameter
Climate models have suggested that to first order the global mean surface temperature change can easily be found by multiplying the global mean radiative forcing by a quantity called the "climate sensitivity parameter". This means that we do not need necessarily need a complex and expensive climate model if all we want to do is compare the likely effect of a change to the atmosphere on global mean surface temperature. We can calculate the forcing (much easier since we don't have to worry about complex processes such as cloud formation), and as long as the climate sensitivity parameter is independent of the change causing the forcing, we don't need to go any further!
Several studies have shown that the climate sensitivity parameter is indeed broadly constant, independent of the forcing mechanism (e.g. the same size forcing from a carbon dioxide increase or a change in the solar constant would produce the same global mean surface temperature change, (see Joshi et al., 2003 Click here for a postscript (.ps) version (1300 kb)). However, there are indications that this is not true for either absorbing aerosol or some ozone changes.
We (Jolene Cook) have recently been able to show that in fact the climate sensitivity parameter varies substantially for aerosol that absorbs even a small amount of solar radiation. Radiative forcing cannot be used reliably to indicate even the sign of the global mean temperature change. The reason is due to a mechanism involving cloud whereby the presence of aerosol in the atmosphere reduces the low level cloud cover and this provides an additional warming. This work has been published in the Quarterly Journal of the Royal Meteorological Society. Click here for the Acrobat (.pdf) version of Cook and Highwood (500 kb)).
The semi-direct effect in other GCMS
Following this work, we (Nicola Stuber) are investigating the magnitude and mechanism of the semi-direct effect in the Unifed Model HadSM3 as part of the EU Framework 6 QUANTIFY Integrated Project. Initially we have repeated similar experiments as in Cook and Highwood and examined the different mechanisms involved. An important part of this is a comparison with ECHAM simulations from our DLR partners. Subsequently we will quantify the semi-direct magnitude, as well as that of more conventional radiative forcing and climate response to emissions of pollutants from transport sectors over Europe.
Is this the end for radiative forcing?
Although this work spells bad news for the concept of radiative forcing, these runs and other work in the Radiation Group has led to the development of a possible new metric for climate change. This work has been accepted for publication in Geophysical Research letters. Click here for an acrobat (.pdf) version (90 kb)