I have written for quite a while that the most important issue in evaluating catastrophic global warming forecasts is feedback. Specifically, is the climate dominated by positive feedbacks, such that small CO2-induced changes in temperatures are multiplied many times, or even hit a tipping point where temperatures run away? Or is the long-term stable system of climate more likely dominated by flat to negative feedback, as are most natural physical systems? My view has always been that the earth will warm at most a degree for a doubling of CO2 over the next century, and may warm less if feedbacks turn out to be negative.
I am optimistic that this feedback issue may finally be seeing the light of day. Here is Professor William Happer of Princeton in US Senate testimony:
There is little argument in the scientific community that a direct effect of doubling the CO2 concentration will be a small increase of the earth’s temperature — on the order of one degree. Additional increments of CO2 will cause relatively less direct warming because we already have so much CO2 in the atmosphere that it has blocked most of the infrared radiation that it can. It is like putting an additional ski hat on your head when you already have a nice warm one below it, but your are only wearing a windbreaker. To really get warmer, you need to add a warmer jacket. The IPCC thinks that this extra jacket is water vapor and clouds.
Since most of the greenhouse effect for the earth is due to water vapor and clouds, added CO2 must substantially increase water’s contribution to lead to the frightening scenarios that are bandied about. The buzz word here is that there is “positive feedback.” With each passing year, experimental observations further undermine the claim of a large positive feedback from water. In fact, observations suggest that the feedback is close to zero and may even be negative. That is, water vapor and clouds may actually diminish the already small global warming expected from CO2, not amplify it. The evidence here comes from satellite measurements of infrared radiation escaping from the earth into outer space, from measurements of sunlight reflected from clouds and from measurements of the temperature the earth’s surface or of the troposphere, the roughly 10 km thick layer of the atmosphere above the earth’s surface that is filled with churning air and clouds, heated from below at the earth’s surface, and cooled at the top by radiation into space.
When the IPCC gets to a forecast of 3-5C warming over the next century (in which CO2 concentrations are expected to roughly double), it is in two parts. As professor Happer relates, only about 1C of this is directly from the first order effects of more Co2. This assumption of 1C warming for a doubling of Co2 is relatively stable across both scientists and time, except that the IPCC actually reduced this number a bit between their 3rd and 4th reports.
They get from 1C to 3C-5C with feedback. Here is how feedback works.
Lets say the world warms 1 degree. Lets also assume that the only feedback is melting ice and albedo, and that for every degree of warming, the lower albedo from melted ice reflecting less sunlight back into space adds another 0.1 degree of warming. But this 0.1 degree extra warming would in turn melt a bit more ice, which would result in 0.01 degree 3rd order warming. So the warming from an initial 1 degree with such 10% feedback would be 1+0.1+0.01+0.001 …. etc. This infinite series can be calculated as dT * (1/(1-g)) where dT is the initial first order temperature change (in this case 1C) and g is the percentage that is fed back (in this case 10%). So a 10% feedback results in a gain or multiplier of the initial temperature effect of 1.11 (more here).
So how do we get a multiplier of 3-5 in order to back into the IPCC forecasts? Well, using our feedback formula backwards and solving for g, we get feedback percents of 67% for a 3 multiplier and 80% for a 5 multiplier. These are VERY high feedbacks for any natural physical system short of nuclear fission, and this issue is the main (but by no means only) reason many of us are skeptical of catastrophic forecasts.
[By the way, to answer past criticisms, I know that the models do not use this simplistic feedback methodology in their algorithms. But no matter how complex the details are modeled, the bottom line is that somewhere in the assumptions underlying these models, a feedback percent of 67-80% is implicit]
For those paying attention, there is no reason that feedback should apply in the future but not in the past. Since the pre-industrial times, it is thought we have increased atmospheric Co2 by 43%. So, we should have seen, in the past, 43% of the temperature rise from a doubling, or 43% of 3-5C, which is 1.3C-2.2C. In fact, this underestimates what we should have seen historically since we just did a linear interpolation. But Co2 to temperature is a logarithmic diminishing return relationship, meaning we should see faster warming with earlier increases than with later increases. Never-the-less, despite heroic attempts to posit some offsetting cooling effect which is masking this warming, few people believe we have seen any such historic warming, and the measured warming is more like 0.6C. And some of this is likely due to the fact that the solar activity was at a peak in the late 20th century, rather than just Co2.
I have a video discussing these topics in more depth:
This is the bait and switch of climate alarmism. When pushed into the corner, they quickly yell “this is all settled science,” when in fact the only part that is fairly well agreed upon is the 1C of first order warming from a doubling. The majority of the warming, the amount that converts the forecast from nuisance to catastrophe, comes from feedback which is very poorly understood and not at all subject to any sort of consensus.