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Question 3

The IPCC has stated that there is 90% chance that the current warming is caused by human activity. The sceptics say it is down to natural cycles e.g. solar intensity. Could it be possible that the warming is a mixture of the two and, if so, what percentage of the temp rise can we attach to each of the two possible causes?
Neil Tuffin

Professor Mitchell: The simplest approach to answer this is to estimate the relative heating due to increases in greenhouse gases, changes in solar output and other factors. The increase in heating due to greenhouse gases since the pre-industrial period is about 2.4 Wm-2, which is about four or five times the estimated increase in solar output. Changes in the Sun's output in the early part of the 20th century are uncertain because of being largely based on proxy data, such as number of sunspots. However, direct measurement of solar output made by satellites over the last 30 years shows no discernible increase in recent times. In summary, both increases in greenhouse gases and solar intensity may have contributed to the warming observed over the last 100 years, but we would expect that the dominant contribution to have come from greenhouse gases, particularly in the last few decades.

A refinement of this approach is to include all the factors we believe may have affected climate over the last century in model simulations including, for example, the effect of the oceans in slowing down the response to increased heating. The model patterns are then scaled and combined to give the best fit to the observed evolution of the geographical patterns of temperature change. One can also allow for the uncertainty due to natural (unforced or chaotic) variations in climate which can again be estimated from climate models. Using this approach the recent IPCC report deduced that there was a 90% probability that that most (not all) of the recent warming was due to increase in greenhouse gases. Increases in solar intensity may have contributed to the early 20th century warming.

The main sources of uncertainty arise from our lack of knowledge of the level of natural variability and the size of some of the forcing factors including the cooling due to anthropogenic aerosols and changes in solar intensity.