A new study that caught my attention was published today in the journal Nature Communications, having the title, “A less cloudy picture of the inter-model spread in future global warming projections.”(https://www.nature.com/articles/s41467-020-18227-9, open access) One of the authors works at Floricda State University, which prompted the school to issue a press release with the headline, “Polar ice, atmospheric water vapor biggest drivers of variation among climate models.”
(https://phys.org/news/2020-10-polar-ice-atmospheric-vapor-biggest.html) The mention of water vapor is what really got my attention. The researchers have analyzed the content of 25 recent top-rated climate models from all over the world, with the aim of evaluating the uncertainties related to various feedbacks that are expected to contribute to future warming projections that are primarily based on greenhouse gas emissions. Different models all have their own way of treating uncertainties inherent to each of the feedbacks, which can be considerable. Feedback issues are indeed basically responsible for the wide spread in IPCC climate forecasts, a spread that no one is happy with. Cloud cover is commonly thought to be the one such category causing the widest amount of uncertainty, but that is not what the researchers found to be the case in their analysis.
From the release, “They found that climate models that predicted higher average temperatures for the Earth’s surface overall also yielded results that showed more polar ice loss and more water vapor in the atmosphere…..The research also found that cloud cover is less important than scientists previously thought for explaining variation among models…..Knowing that polar ice and water vapor in the atmosphere are the most important drivers of variability in different climate models will help climate scientists further refine those models.” This excited me because it suggests that climate scientists may be starting to take water vapor more seriously. Could they even be considering the fraction that travels at high altitude toward each of the polar regions? That idea was quickly abandoned after reading the study’s Abstract, which says: “We show that the ice-albedo feedback spread explains uncertainties in polar regions while the water vapor feedback spread explains uncertainties elsewhere.”
The main purpose of the study is to show that when all of the known cloud cover uncertainties are combined, some positive and some negative, they tend to cancel each other out. Different studies do not combine them in the same way, but if all the studies were unified—and equally treated—complete cancellation would apparently be the likely outcome. If this finding is widely accepted it would be good news for forecasters because the remaining uncertainties like those associated with ice-related albedo and water vapor are not thought to be as large. We might be seeing an effect of this study on the next range of forecasts to be delivered by the IPCC, along with an acknowledgment that cloud effect research is still ongoing and not yet final.
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Carl