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Models overstate Arctic warming

26 Feb 2020 | Science Notes

A new study in the peer-reviewed journal Geophysical Research Letters says... well, let’s let the authors do the talking. They titled their article “The Amplified Arctic Warming in the Recent Decades may Have Been Overestimated by CMIP5 Models” and it is about how observed warming since 1968 in the Arctic has been faster than the rest of the world, but not nearly as fast as climate models said it would be. The problem, they surmise, is that models over-predict the loss of Arctic sea ice: “Further analysis indicates that the overestimation mainly comes from the exaggerated heating contribution from the Arctic sea ice melting.” Which means, they add, “future secular Arctic warming may have been over‐projected.” Add one more item to the “Why do all the mistakes seem to go in one direction?” file.

The authors calculate that the Arctic warmed at a rate of about 0.14 degrees C per decade starting in the 1890s, then cooled mid-century, then the warming trend rose again to about 0.21 degrees C per decade after 2010, making a net acceleration of 0.07 degrees C per decade over the century. But the climate models showed a different pattern. They have essentially no warming going on as of the 1890s, they underestimate the warming rate until about 1940, after which they predict too much acceleration of warming, so the trend rises to 0.3 degrees per decade by 2010. Looking at simple trends over the entire century it appears models and observations approximately agree, but that’s because the models underestimate the warming rate early in the century and overestimate it after 1970. Since we’re now in a period when the Arctic is not warming as fast as the models claim, the authors, say, projections of Arctic heating over the coming few decades are likely exaggerated.

Getting this science right matters, and not only because some people live in the Arctic and might want to know what the future holds. It's important for models to get basic climate mechanisms right if we are to rely on them for policy purposes. Arctic ice melt is an important feedback that determines the total global warming response to greenhouse gases. If the models are overstating it, they will make unreliable global projections. It's one more piece of the puzzle needing to be put in the right place in the right way.

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