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Scientists say (6)

03 Feb 2021 | Science Notes

Herewith our latest installment of stuff that actual scientists actually say, as opposed to stuff journalists make up then adorn with the phrase “scientists say” to make it sound legit. This item comes from a recent paper by Mitchell et al. in which they compared observed warming rates in the lower atmosphere over the tropics after 1979 (when satellites used for temperature monitoring were launched) to the trends simulated in climate models. The older generation of climate models are called “CMIP5” and the bigger, better, more expensive new ones are called CMIP6. The authors of the paper had shown back in 2013 that the CMIP5 models warmed too much. And the new ones? “Focusing on the CMIP6 models, we have confirmed the original findings of Mitchell et al (2013): first, the modeled tropospheric trends are biased warm throughout the troposphere (and notably in the upper troposphere, around 200 hPa) and, second, that these biases can be linked to biases in surface warming. As such, we see no improvement between the CMIP5 and the CMIP6 models.” Scientists say.

The tropical region matters because that’s where key atmospheric processes operate that determine how much greenhouse gases will warm the whole planet. And for a long time climate scientists have been wrestling with the fact that the models keep showing too much warming in the tropics at the surface and in the troposphere (1 km to 10 km altitude). Unfortunately the newest models are still as biased as the old ones.

The warm trends bias in the models is seen throughout the entire troposphere, but is greatest in the upper troposphere (peaking around 200 hPa), where the modeled trends are—on average – 4 to 5 times greater than the observations. We draw attention to the CanESM5 model: it simulates the greatest warming in the troposphere, roughly 7 times larger than the observed trends. We note this model is known to have a high climate sensitivity compared to others (Swart et al 2019, Forster et al 2019). Throughout the depth of the troposphere, not a single model realization overlaps all the observational estimates.

And who built the CanESM5 model, with its 7 times-too-large warming trend? None other than the experts at Environment and Climate Change Canada. But they are in good company, since not a single model run overlapped with all the observed data. Scientists say.

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