It isn’t quite true to say that the science is settled–climate science is in its infancy, and we have only a poor understanding of the Earth’s climate. Just about every proposition is controversial. But we are very close to being able to say that, as to global warming alarmism, the debate is over and the alarmists have lost. (I mean, of course, the scientific debate, not the political one, which never had much to do with science in the first place.)
One fundamental question in the global warming debate is, what is the Earth’s equilibrium climate sensitivity? That is, how much will the Earth’s average surface temperature rise, ceteris paribus, on account of a doubling of the concentration of carbon dioxide in the atmosphere? Global warming hysteria is predicated on the belief that average temperature will rise by up to 6 degrees C as a result of doubling atmospheric CO2. All of the scare headlines you see about polar bears, droughts, flooded cities, etc., rely on that assumption.
The problem for alarmists is that contemporary research doesn’t support any such scenario. The most recent nail in the alarmists’ coffin is a paper by Nic Lewis and Judith Curry titled “The implications for climate sensitivity of AR5 forcing and heat uptake estimates,” which concluded that the best estimate of equilibrium climate sensitivity is 1.64 degrees C. Lewis describes the paper’s methodology here, and you can follow the link to the paper and read it for yourself.
The Lewis/Curry paper is consistent with recent scholarship, which pretty universally finds that the UN’s IPCC projections are far out of line. This Cato post by Pat Michaels and Paul Knappenberger illustrates the findings of recent studies, compared with the politically-motivated IPCC projections. As you can see, science doesn’t support the alarmism of politicians and their Michael Mann-style minions:
If a good estimate of climate sensitivity is 1.64 degrees, what is the significance? That figure is well within the range of natural variability. It will be swamped by other factors that continuously act on the Earth’s climate, and we may never know whether, or to what extent, CO2 had any impact on the Earth’s temperatures. That shouldn’t be too surprising: to the extent that historic temperatures and CO2 concentrations can be reconstructed, there is zero apparent correlation between the two.
Actually, I think the 1.64 degree estimate may be high. Dr. Curry, on her own web site, explains why that may be true:
Is this paper the last word on climate sensitivity estimates? No. The uncertainty analysis in the Lewis and Curry paper relates only to the uncertainty in external forcing, surface temperature and ocean heat uptake. There remains considerable meta uncertainty in the determination of climate sensitivity, including how the problem is even framed.
In particular, the energy balance approach does not account for factors that do not directly relate to the energy balance, e.g. solar indirect effects and natural internal variability that affects forcing (although an attempt has been made in the Lewis and Curry paper to make some allowance for uncertainty associated with these factors) . Further, there was ‘something else’ going on in the latter 19th and early-mid 20th century that was causing warming, that does not seem to relate directly to external forcing. The paper does attempt to factor out the impact of the Atlantic Multidecadal Oscillation through the selection of base and final periods, but this is by no means a complete account for the effects of multi-decadal and century scale internal variability, and how this confounds the energy balance estimate of climate sensitivity.
In other words–as I understand what she and others have said–the Lewis and Curry paper accepts most of the alarmists’ assumptions, and shows that their conclusions are still wrong. I think that when the dust finally settles, perhaps not in our lifetimes, the alarmists’ predictions will be even more discredited. To quote Dr. Curry one more time:
At the heart of the recent scientific debate on climate change is the ‘pause’ or ‘hiatus’ in global warming – the period since 1998 during which global average surface temperatures have not increased. This observed warming hiatus contrasts with the expectation from the 2007 IPCC Fourth Assessment Report that warming would proceed at a rate of 0.2 degrees C/per decade in the early decades of the 21st century. The warming hiatus raises serious questions as to whether the climate model projections of 21st century have much utility for decision making, given uncertainties in climate sensitivity to carbon dioxide, future volcanic eruptions and solar activity, and the multidecadal and century scale oscillations in ocean circulation patterns.
One parting comment: contrast the intelligence and technical skill manifested by Lewis and Curry with the borderline psychopathic ramblings of climate alarmists like Robert Kennedy Jr., and Michael Mann trying to sue his critics into submission, and you get a pretty good idea of who is winning the climate debate.