Santer has been searching for proof of the human fingerprint for close to three decades. He has a lot time invested in finding it, from 1996:

The observed spatial patterns of temperature change in the free atmosphere from 1963 to 1987 are similar to those predicted by state-of-the-art climate models incorporating various combinations of changes in carbon dioxide, anthropogenic sulphate aerosol and stratospheric ozone concentrations. The degree of pattern similarity between models and observations increases through this period. It is likely that this trend is partially due to human activities, although many uncertainties remain, particularly relating to estimates of natural variability.https://www.nature.com/articles/382039a0 July 4th, 1996

It is worth reading a rebuttal of Santer’s 1996 paper by the now deceased John Daly.

And still to this day.

Dr. Roy Spencer posted on Santer’s new paper.

So, why mention stratospheric cooling in the context of climate change?

Climate researchers have been searching for “human fingerprints” of climate change for decades, something measurable that cannot be reasonably explained by natural variations in the climate system.

I will agree with the authors that stratospheric cooling (especially in the mid- to upper-stratosphere) is probably the best evidence we have of a human fingerprint, at least up where there is very little air, where no one lives, and where there are no observable resulting impacts on weather down here where life exists. Water vapor remains an uncertainty here, because more water vapor would also cause cooling, and our understanding of natural variations in stratospheric water vapor is quite poor. But for the sake of argument, I will give the authors the benefit of the doubt and agree that most of the observed cooling is probably due to increasing CO2, which in turn is likely mostly due to burning of fossil fuels.

Infrared radiative cooling by water vapor and carbon dioxide has long been known to be the primary way the stratosphere (and even higher altitudes) lose heat energy (gained from sunlight absorption by ozone) to outer space. This cooling mechanism is part of the so-called greenhouse effect: greenhouse gases warm the lower altitudes of planetary atmospheres, and cool the higher altitudes. In fact, without the greenhouse effect, weather as we know it would not exist. The greenhouse effect is energetically analogous to adding insulation to a heated house in winter: for a given rate of energy input, the inside of the house becomes warmer, and the outside of the house becomes colder.

The stratospheric cooling effects of CO2 and water vapor was first described theoretically by Manabe and Strickler (1964). Adding more CO2 to the atmosphere enhances upper atmospheric cooling, lowering temperatures. The temperature effect up there is large, several degrees C, meaning it is easier to measure with current satellite methods, as the authors of the new study correctly point out.

But what then happens in the troposphere (where we live) in response to more CO2 is vastly more complex. Theoretically, adding more CO2 should warm the lower troposphere radiatively. This warming then gets mixed throughout the depth of the troposphere from convective overturning (basically, “weather”).

But just how much tropospheric warming will be caused by increasing CO2?

After 30 years and billions of dollars expended on the effort in research centers around the world, the latest crop of climate models (CMIP6) now disagree on the expected amount of tropospheric warming more than ever before. This is mostly because of the insufficiently understood effects of water, especially the response of clouds (the climate system’s sunshade) and precipitation processes (which limit the most abundant greenhouse gas, water vapor) to warming.

I consider it irresponsible to conflate stratospheric cooling with the global warming issue. Yes, strong cooling in the upper stratosphere is likely a fingerprint of increasing atmospheric CO2 (putatively due to fossil fuel burning), but for a variety of reasons, that is not reason to believe climate models in their predictions of tropospheric (and thus surface) warming trends. That is a very different matter, and the models themselves demonstrate they are not yet up to the task, now disagreeing with each other by a factor of three or more.

So now you hopefully understand why entitling such a paper “Exceptional stratospheric contribution to human fingerprints on atmospheric temperature” is essentially a non sequitur on the issue of global warming.

https://www.drroyspencer.com/2023/06/climate-fearmongering-reaches-stratospheric-heights/