18 April 2018

Early 21st Century Anthropogenic Changes in Extremely Hot Days as Simulated by the C20C+ Detection and Attribution Multi-Model Ensemble

Science

We examine the effect of the 20th and recent 21st century anthropogenic climate change on high-temperature extremes as simulated by four global atmospheric general circulation models submitted to the Climate of the 20th Century Plus Detection and Attribution project. This coordinated experiment is based upon two large ensembles simulations for each participating model. The “world that was” simulations are externally forced as realistically as possible. The “world that might have been” is identical except that the influence of human forcing is removed but natural forcing agents and variations in ocean and sea ice are retained.

We find that most regions experience increases in the frequency and intensity of extremely hot three day periods, but anthropogenic sulfate aerosol forcing changes locally can decrease these measures of heat waves in some models.

Impact

A principal finding is that model treatments of atmospheric aerosols, which is not specified in the C20C+ experimental design, can have large localized influences on the magnitude and even sign of the anthropogenic change in extreme high-temperature events. Statements based on models that prescribe atmospheric aerosol concentrations rather than emissions may erroneously conclude that the human interference in the climate system increased the risk and magnitude of an actual observed heat wave when in fact the net effect of greenhouse gas (GHG) and aerosol increases were to decrease the risk and magnitude due to rapid increase of localized aerosol concentrations in stagnant conditions. A more complete understanding of the role of aerosols on extreme temperature will require a modification to the coordinated experimental protocol.

Summary

Most land areas of the planet are simulated to have exhibited an increase of between 1 and 3oC for long period return values of the 3 day average of the daily maximum temperature as a result of anthropogenic climate change. While the changes in return value are stable as the rarity of the counterfactual event is increased, relative changes in frequency are not. This consequence of the boundedness of the Generalized Extreme Value (GEV) distributions fitted to the annual maxima causes the so-called Risk Ratio to increase with rarity in those areas where the anthropogenic effect is a warming of extreme temperatures.

Contact
William D. Collins
Lawrence Berkeley National Laboratory (LBNL)
Publications
Wehner, M, D Stone, H Shiogama, P Wolski, A Ciavarella, N Christidis, and H Krishnan.  2018.  "Early 21st Century Anthropogenic Changes in Extremely Hot Days as Simulated by the C20C+ Detection and Attribution Multi-Model Ensemble."  Weather and Climate Extremes.  https://doi.org/10.1016/j.wace.2018.03.001.