Climate Letter #2035

James Hansen’s prediction for future global warming, tucked away in the deep interior of his July Temperature Update, differs sharply from much of what we hear in the messaging being delivered by mainstream science. The explanation he offers in support of his prediction goes even farther. It flatly contradicts one of the most sacred of all the teachings of climate science, about the burning of fossil fuels. Science teaches us that the burning of fossil fuels is the main reason why the planet is now warming and why we must stop burning them by mid-century in order to minimize further warming. That’s because their burning produces vast amounts of carbon dioxide as a by-product, resulting in increases in the concentration of carbon dioxide in the atmosphere, which is proven to have a pronounced positive effect on global temperatures.

Hansen is quietly telling us that this is not the whole story, and that science is missing a big part of the actual reality. He reminds us that burning two of the most important fossil fuels, coal and oil, produces vast quantities of another gas as a by-product, sulfur dioxide. This gas goes on to create sulfate aerosols, which interact with clouds, causing them to brighten and thus block the passage of sunlight, allowing the Earth to become cooler. He is not sure about exactly how much cooler, but seems quite confident that in a primordial sense, with no modifying interference, the cooling impact of the SO2 significantly exceeds the warming impact of the CO2. On balance, this means that burning these fuels must have a considerable negative effect on global temperatures for as long as it continues. Lacking modification, the historical effect probably remained at that high level for a full two centuries after the burning began. Any actual warming that did occur during those two centuries would thus need to have been produced by factors other than emissions from the burning of coal and oil.

By the middle of the last century humans remained unsure about this net cooling effect, but did notice that the sulfur emissions had all kinds of other unwanted effects, and began the search for solutions, which was successful. SO2 emissions could be independently brought under control at reasonable expense, and major programs were undertaken at once. By 1970 the programs had begun accelerating, and they have continued in that mode ever since. The result has been a steady diminishing of the cooling effect, on a scale large enough to translate into strong net additions to planetary warming, which have been realized in a dominating way over all other active considerations. CO2 emissions have meanwhile continued to grow, but so have the dirty sulfur emissions that had to be cleaned up.

Now I want to give you a personal opinion. I think Hansen has made a real contribution to climate science by revealing improved estimations of the strength of sulfur emissions as a temperature coolant. There is no better way to explain the actual trend of high-powered warming that has occurred since the early ’70s. I am much less able to follow his reasoning for why the rate of increase in the warming caused by further declines in sulfate aerosols should soon be doubling. There is good reason to believe that the current pace of decline will at least continue, and quite possibly increase, but the potential for doubling needs further elaboration. Meanwhile, the energy created by greenhouse gases that exist in the atmosphere should soon be starting some kind of a decline in intensity even if it remains high.

Hansen puts much of his emphasis on expected changes in Earth’s energy imbalance.  When taking a further look at the July update, there were a couple of things that bothered be, and need to be explained more fully.  One of them is embedded in this sentence: “Von Schuckmann et al. (2020)5 report that the average imbalance over the period 1971-2018 was 0.47 ±0.1 W/m2, but in period 2010-2018 the imbalance was 0.87 ±0.1.”  A little farther on Hansen tells us the imbalance “has approximately doubled to about 1 W/m2 since 2015.”  In the following sentence we read, “This increased energy imbalance is the cause of global warming acceleration.” This comment is puzzling.  I somehow get the feeling that it might really be the other way around, which would have put the imbalance much lower in 1971, perhaps not far from zero–but need some real data as evidence. If that were the truth, then the further warming acceleration that Hansen predicts would be expected to push the imbalance well past 1C, creating an even greater mess to be corrected in later years, or whenever the oceans are ready to cool down.  A little more elaboration on the part of Dr. Hansen would be helpful.

Carl