Does physics give a climate sensitivity to carbon dioxide of about 3°C?

Question

A letter to the editor of the Economist from Professor Anders Levermann of the Potsdam Institute for Climate Impact Research stated the following:

The IPCC’s range on sensitivity is supported by, but not merely based on, models. It is deeply rooted in physics. Quantum physics and thermodynamics, the same physical laws that underlie the functioning of our computers and power plants, yield a baseline climate sensitivity of about 3°C. This is based on the facts that carbon dioxide, water vapour and methane absorb infra-red; a warmer atmosphere holds more water; and ice and snow melt under warming.

Do quantum physics and thermodynamics yield a baseline climate sensitivity of about 3°C?

Answer 1

Very few people disagree with the basic fact that the greenhouse gas CO2 warms the climate, but without some kind of positive feedback mechanism, it doesn’t add very much: around 1°C-1.2°C per doubling of CO2.

The statement is correct, but very misleading. In the absence of the climate system's feedbacks, the climate sensitivity is 1 to 1.2 deg C for each doubling of CO2 (per the IPCC). However, the impact of feedbacks is greater than the direct impact of increased CO2. This can be seen without relying on climate models by looking at the powerful temperature cycles of historical ice ages (or glacial-to-interglacial transitions). These cycles are caused by variations in the Earth's orbit, which only produce a small change in incident radiation. See here for example. Without climate feedbacks, it would take a much greater change in solar output, or an enormous change in CO2 concentration, to cause an ice age. Chris Colose of Univ. of Wisconsin, Madison provides a good explanation of feedback effects:

To put this in perspective, it would take about five doublings of CO2 or a 7% increase in the total solar radiation hitting the Earth to produce the magnitude of climate change typical of glacial-to-interglacial transitions.

Thus it is clear - without relying on models - that the feedback effect must be greater than the direct effect, or else there could never have been ice ages.

The phrase "without some kind of positive feedback mechanism" gives the impression that scientists think there's an alternate mechanism that drives their projected temperatures higher. Climate sensitivity without feedbacks can be calculated fairly accurately on a single sheet of paper, (see here for example) and the result will be approximately 1 deg C. But the result is not relevant to anything, because the real world has many feedbacks; that's why the focus in climate science over the past several decades has been understanding the feedbacks and quantifying their effects. Most of them are well understood, their net effect is overwhelmingly positive (i.e. they amplify temperature fluctuations), and of course they are all based on physics, as demonstrated by Chris Colose.

According to James Hansen, the best source of information about feedbacks is not models, but paleoclimate data:s

Models are imperfect and we will never be sure that they include all important processes. Fortunately, Earth's history provides a remarkably rich record of how our planet responded to climate forcings in the past. Paleoclimate records yield, by far, our most accurate assessment of climate sensitivity and climate feedbacks.

Hansen calculates the climate sensitivity in various units, and expresses it as 2 to 4 degrees C for a doubling of CO2:

The empirical fast-feedback climate sensitivity that we infer from the LGM-Holocene comparison is thus 5°C/6.5 W/m2 ~ 3⁄4 ± 1⁄4 °C per W/m2 or 3 ± 1°C for doubled CO2. The fact that ice sheet and GHG boundary conditions are actually slow climate feedbacks is irrelevant for the purpose of evaluating the fast-feedback climate sensitivity.

This empirical climate sensitivity incorporates all fast response feedbacks in the real- world climate system, including changes of water vapor, clouds, aerosols, aerosol effects on clouds, and sea ice. In contrast to climate models, which can only approximate the physical processes and may exclude important processes, the empirical result includes all processes that exist in the real world – and the physics is exact.

Water vapor provides a great example of climate feedbacks. Because it absorbs infrared radiation very well, water vapor is a very powerful greenhouse gas. Also, warmer air holds more water vapor. So, suppose temperature increases because of higher CO2; the atmosphere will hold more water vapor, which will cause it to absorb more solar radiation, causing it to heat up still further. Calculating the impact of the CO2 while disregarding the water vapor feedback would be a significant underestimation for no good reason. Other positive climate feedbacks are also well known to climatologists, such as changes in reflectivity due to melting of snow and ice, release of CO2 from ocean water because of increasing ocean temperatures, and release of methane from melting permafrost.

Answer 2

François Gervais of the Physics Department at François Rabelais University in Tours, France wrote in 2014 that the anthropogenic contribution to global warming upon the present trend is 0.1°C in the absence of climate feedbacks.

He arrived at this number by finding the present rate of anthropogenic carbon dioxide increase in the atmosphere and applying a simplified physics calculation to the temperature based on radiation.

Gervais' estimate of the rate of anthropogenic carbon dioxide increase is based on two methods, which he claims to support each other: the C13/C12 ratio and also the change in annual average atmospheric CO2 concentration, after subtracting annual fluctuations of up to 600% increase.

Gervais' radiation model is an analysis of the anthropogenic carbon dioxide's contribution to earth's total infrared radiation using data from satellite-based infrared spectrometers.

Answer 3

I've been reading more and more climate denialists pointing to the lack of rise in the past ten years, and claiming it as evidence that the theory of climate change is flawed and previous 100+ years of trend is a fluke.

This real meaning of this temperature plateau is explained in a 2011 articel from the International Business Times

However, these periods would likely last only about a decade or so, and warming would then resume. This study illustrates one reason why global temperatures do not simply rise in a straight line.

The University Corporation for Atmospheric Research gives a good summary of various issues. In particular, their FAQ directly addresses the ten year plateau:

It’s important to note that models are not the only reason why scientists are concerned about climate change. For more than a century—long before many recent advances in science, and long before computer models—we’ve known that increased greenhouse gases could produce a global temperature increase. Observations of the last century of climate, including those from instruments and from the behavior of ice and plants, concur that the planet is warming.

Harvard News also explains:

The primary driver of the warming hole is the aerosol pollution — these small particles,” says Leibensperger. “What they do is reflect incoming sunlight, so we see a cooling effect at the surface.”-climate-change/:

The point of the articles is that the ten year period of non-warming is a direct result of various atmospheric emissions associated with global warming. The result of this is that if in the future emissions are reduced, global warming trend lines will continue to increase for a lengthy period of time, and in fact increase much faster once those "blocking" factors created by emissions are curtailed. Additionally, it means that this 'hidden' warming value will result in a larger temperature overall increase than current temperatures are measuring would indicate.

Answer 4

From the IPCC's AR5 summary report "No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies." I'm not a physicist, nor I suspect is anyone else here, but this change in reporting would seem to indicate a lack of confidence in the actual impact of additional atmospheric CO2 on the climate.