There is only one important scientific question regarding climate change: Is atmospheric sensitivity high or low? If it is high the planet has big problems. If it is low we can deal with global warming with the technologies and societal mechanisms that currently exist.
So where do we stand on this issue? Enthusiasts for both extremes have very wrongly claimed the issue has been settled in their favor. This has not helped anyone. We don’t know what atmospheric sensitivity is. We don’t even know if there is one and one only value for atmospheric sensitivity. We are not likely to know for sure for 30 or more years, according to climate scientist Judith Curry, among others.
But remember that the very definition of Lukewarmer is one who believes that atmospheric sensitivity is lower than claimed by the activists–I for one think it is around 2C or a bit lower. Why on earth, if I say it cannot be determined with accuracy at this point, am I willing to ‘bet the planet’ on my belief/intuition/opinion/prayer/ limited understanding of incomplete science that it is low?
For starters, remember that the range of potential sensitivity values given by the Intergovernmental Panel on Climate Change (IPCC) includes low values as well as high. They estimate that the value of sensitivity could fall anywhere between 1.5C and 4.5C. So my ‘preference’ of 2C is well within the accepted range of possibility.
Furthermore, there is recent data that helps buttress my opinion. One finding is my own, explained in detail over at the other weblog I maintain, 3,000 Quads. I show that the Carbon Dioxide Information Analysis Center (CDIAC) estimates that one-third of all human emissions of carbon dioxide have occurred since 1998, a date I chose because it marks a plateau in temperature rises–after a disconcerting rise in temperatures between 1976 and 1998, temperatures have stalled. (This happens and is not really unexpected, although the plateau has lasted a bit longer than some would have thought.)
The point I made there and repeat here is that if temperatures aren’t moved by the emission of one-third of our global total of CO2 emissions, it is hardly an argument for high sensitivity. It doesn’t make it impossible–but it doesn’t reinforce the opinion of the activists.
The other, more widely publicized piece of news is reporting of new work done by Nic Lewis that finds some evidence of sensitivity ranges even lower than mine. As published by science writer Matt Ridley, “We can now estimate, based on observations, how sensitive the temperature is to carbon dioxide. We do not need to rely heavily on unproven models. Comparing the trend in global temperature over the past 100-150 years with the change in “radiative forcing” (heating or cooling power) from carbon dioxide, aerosols and other sources, minus ocean heat uptake, can now give a good estimate of climate sensitivity.
The conclusion-taking the best observational estimates of the change in decadal-average global temperature between 1871-80 and 2002-11, and of the corresponding changes in forcing and ocean heat uptake-is this: A doubling of CO2 will lead to a warming of 1.6°-1.7°C (2.9°-3.1°F).
This is much lower than the IPCC’s current best estimate, 3°C (5.4°F).”
Not surprisingly, Lewis’ finding and Ridley’s piece got quite a bit of press. Ridley’s piece was published in the Wall Street Journal, disputed by Climate Progress (Joe Romm has a habit of automatically criticizing any evidence that threatens the consensus and is not above ignoring facts when he does so) and discussed more rationally at places like Climate Etc. and The Way Things Break.
And again, neither my mashing up of data nor Nic Lewis’ more methodical examination of observations are going to settle the issue. But all of the argument is now looking at the possibility of low sensitivity. Just a year or two ago, the argument was about high sensitivity.
It may well be a pendulum-like discussion that swings back and forth. But for now, data seems to be on the Lukewarm side of the discussion.