I wrote about climate sensitivity a while back, and for those who are new to this term, it refers to the assumption that while human activity in clearing forests, making cement and burning fossil fuels adds to the carbon dioxide in the air, that addition is multiplied by something else, which is the ‘sensitivity’ of the climate system to that increase. Climate sensitivity (I’ll call it CS from now on) is assumed also to be positive — that is, the effect of CO2 is magnified, not minimised. If you want to know more — and there is more to know — click on the link above.
CS is most important in the policy debate about what to do about whatever anthropogenic global warming (AGW) there is, because the Inter-governmental Panel on Climate Change (IPCC) has steadfastly kept to the view that the power of CS ranges between 1.5 and 4.5 degrees C for a doubling of atmospheric carbon dioxide. It is widely accepted that by itself a doubling of CO2 would lead, all other things being equal, to a rise in average global temperature of about 1.1 degrees C. If CS were, say, 3, then a doubling would lead to an increase of 3.3 degrees or thereabouts. I’ve written about all this before, and want quickly to pass to the new material.
Many readers will know that one of the many ‘explanations’ offered for the failure of the climate models to have predicted the ‘hiatus’ in global warming is the presence of ‘aerosols’ in the atmosphere that are thought to mask the warming. Areosols are colloids, that is, fine particles evenly distributed in suspension in a liquid. Fog is an example, as are clouds themselves, dust in the atmosphere, and of course chemicals of various kinds that are emitted through chimneys and in other ways. The orthodox argument is that while CO2 has continued to increase, its effect has been masked by a growth in aerosols, especially from industrial activity in China and India.
Areosols are exceedingly hard to measure (just think of clouds, which even now the IPCC accepts we know much less about than we ought), and the orthodox argument has gone more or less untested. Until now. Bjorn Stevens, an American who now works at the Max-Planck Institute for Meteorology at Hamburg, is highly respected in his field, and contributed to the IPCC’s AR5. He is not a sceptic about what he calls ‘robust evidence of an anthropogenic influence on climate’. But his very recent paper, only the abstract of which I have read, has caused a great deal of debate.
Stevens called his paper ‘Rethinking the lower bound on aerosol radiative forcing’, and to do it he used as much observational evidence as possible, though models also played their part. The outcome is a strong suggestion that aerosols aren’t all that important. If CS is large, then the effect of aerosols has to be large too, otherwise there would be no masking of the effect of CO2; if the effect of aerosols is small, then CS cannot be large — at least, if it is, then there has to be another powerful masking effect, unknown and unmeasured as yet.
And Stevens found that indeed the likely effect of aerosols was small. He left it at that, but Nic Lewis, who has a real interest in this topic (see again the first link above) took Stevens’s results and interpolated them into his own work, to produce the following graph. The shaded portion represents the IPCC’s estimate of the range of ‘equilibrium climate sensitivity’ (ECS), which you can think of as the long-term change in average global temperature caused by a doubling of CO2. There is a comparable graph for ‘transient climate sensitivity’ (TCS), which is the short-term effect of a doubling, but one is enough here.
The base and final periods are time intervals thought to be similar in terms of volcanic activity and ocean heat content (again, see the first link above for more information). What is plain from the graph is that Stevens’s work, employed by Lewis, shows a likely distribution of CS outcomes than are closer to 1 than to even the lower bound of the IPCC’s wide range.
A couple of sceptic sites have said that this work puts paid to the whole ‘climate change’ endeavour. I don’t think so. It is simply another paper, well-worked so far as I read the critiques, that attacks this most central proposition in climate science. Lewis himself says that there is a lot of work still to be done, and that, as always, the findings are tentative. What is important, at least to me, is that the author of the aerosol work comes from within the climate club. It seems that he first tried to get the paper published in Nature, where it was rejected. Fortunately, he persevered.
As I write, there is a conference going on in Germany about the whole question of climate sensitivity, and I expect to see some outcomes in the blogosphere pretty soon. Interestingly, Nic Lewis is one of the participants, and Bjorn Stevens is one of the convenors. So far, as far as I can tell, Stevens has not commented on Lewis’s use of his aerosol paper.
Why make such a fuss about a single paper? Well, to repeat, CS is at the heart of the AGW scare, The slow and irregular warming that the planet has experienced over the past century has been, so far as anyone can tell, a boon, not a worry. Worry would only come if warming were to race away, and we were to experience say 4 degrees C of warming over the present century. There seems no present danger of that occurring, but the prospect is frequently trotted out by the orthodox.
And the basis for it is the high bound of the IPCC’s range for CS. Without that, there is no support for a climate apocalypse at all — at least one that points to a hot future rather than a cold one. Hence the importance of the Stevens + Lewis papers, and the need to understand what they are saying. I’ll report on the German conference when there is something to say.