Climate Consensus Sensitivity

A significant post appeared on Bishop Hill last week written by climate scientist Dr. Richard Betts, who heads the Climate Impacts strategic area for the UK Met office, and is a lead author for the IPCC.

Most climate scientists do not subscribe to the 2 degrees “Dangerous Climate Change” meme (I know I don’t). “Dangerous” is a value judgement, and the relationship between any particular level of global mean temperature rise and impacts on society are fraught with uncertainties, including the nature of regional climate responses and the vulnerability/resilience of society. The most solid evidence for something with serious global implications that might happen at 2 degrees is the possible passing of a key threshold for the Greenland ice sheet, but even then that’s the lower limit and also would probably take centuries to take full effect. Other impacts like drought and crop failures are massively uncertain, and while severe negative impacts may occur in some regions, positive impacts may occur in others. While the major negative impacts can’t be ruled out, their certainty is wildly over-stated.

The scientific issue here is that, while most climate scientists do indeed accept a measure of AGW, the question of whether this is likely to be dangerous or beneficial depends on the action of various feedbacks in the climate system, some which may be positive and increase the effects of CO2 while some may be negative and decrease these effects: much of the debate on climate change hinges around these feedbacks, about which there appears to be still a lot of uncertainty amongst scientists.

It is really worth reading the whole thing, in which Betts makes a number of other important points, not least the distinction between climate change scientists– which could mean just about anyone who considers how climate change is effecting their subject (eg. migratory patterns) and climate scientists who actually study atmospheric physics. So when he says “Most climate scientists do not subscribe to the 2 degrees ‘Dangerous Climate Change’ meme” he is talking about the scientists who should really know.

This cuts to the heart about the “scientific consensus”- while most atmospheric physicists do see CO2 as having a warming effect, it would appear that many do not subscribe to the alarmist position championed by Hansen which demands a draconian policy response of drastic CO2 reductions NOW- and this suggests that alarmist statements we heard this week from the IEA may be overplayed, despite the fact that carbon emissions are certainly rising faster than would be required to keep under the IPCC’s “safe” threshold.

Now this seems to be being supported by a new paper from a team of researchers led by Andreas Schmittner of Oregon State University due to be published in Science:

World Climate Report: There is word circulating that a paper soon to appear in Science magazine concludes that the climate sensitivity—how much the earth’s average temperature will rise as a result of a doubling of the atmospheric concentration of carbon dioxide—likely (that is, with a 66% probability) lies in the range 1.7°C to 2.6°C, with a median value of 2.3°C. This is a sizeable contraction and reduction from the estimates of the climate sensitivity given by the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), in which the likely range is given as 2.0°C to 4.5°C, with a best estimate of 3.0°C.

The report on this forthcoming study goes onto suggest “the IPCC is going to be hard-pressed to retain the status quo in its Fifth Assessment Report”.

All this shows that when people talk about the scientific consensus it is important to check what they are actually talking about: for a start, which scientists? and consensus about what?

-that the earth is warming?
-that the earth is warming (partly) because of us?
-that it is dangerous that the earth is warming because of us?
-that we are all doomed unless we implement NOW specific policies of massive carbon emissions reductions and pay for expensive and unreliable renewables and reorgsanise the whole of society in order to fit in with a specific ideology?

You will see that the last statement is a very long way from the first, and even further from the study of actual climate science.

Update: Revkin has posted an extra piece from Dr. Betts here:

The difficulty comes when scientists become expected to either support or denounce that judgment purely on the basis of science. If this happens, there is a risk that the science is retrofitted to support a political standpoint. This is extremely dangerous, because the same science also has to inform other policy areas, particular adaptation to climate change.

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