27 March 2018

Changes in Extremely Hot Days Under Stabilized 1.5oC and 2.0oC Global Warming Scenarios as Simulated by the HAPPI Multi-Model Ensemble


We present a projection of future extremely hot days for both 1.5oC and 2.0oC stabilized warming scenarios using six different global climate models by examining extremes of high temperatures averaged over three consecutive days. Changes in this measure of extreme temperature are also compared to projected changes in hot season temperatures. We find that over land this measure of extreme high temperature increases from about 0.5 to 1.5oC over present-day values in the 1.5oC stabilization scenario depending on location and model. We further find an additional 0.25 to 1.0oC increase in extreme high temperatures over land in the 2.0oC stabilization scenario. While model structural uncertainty is larger than uncertainties due to natural variability and statistical fitting details, it is significantly smaller than the projected changes themselves.


The United Nations Framework Convention on Climate Change (UNFCCC) invited the scientific community to explore the impacts of a world where anthropogenic global warming is stabilized at only 1.5oC above pre-industrial average temperatures. This work is to inform the special report of the Intergovernmental Panel on Climate Change..


The Half A degree additional warming, Prognosis and Projected Impacts (HAPPI) experimental protocol was designed to rapidly inform the Intergovernmental Panel on Climate Change about the differences between stabilized climate at 1.5oC and 2.0oC above pre-industrial global temperatures. However, it does not isolate all of the effects of forcing changes required to stabilize the climate from the present day conditions. Results from the HAPPI models are consistent with similar results from the one available fully coupled climate model (CESM). However, a complicating factor in interpreting extreme temperature changes across the HAPPI models is their diversity of aerosol forcing changes.

William D. Collins
Lawrence Berkeley National Laboratory (LBNL)
Wehner, M, D Stone, D Mitchell, H Shiogama, E Fischer, L Graff, V Kharin, L Lierhammer, B Sanderson, and H Krishnan.  2018.  "Changes in Extremely Hot Days Under Stabilized 1.5C and 2.0C Global Warming Scenarios as Simulated by the HAPPI Multi-Model Ensemble."  Earth System Dynamics 1-22, doi:10.5194/esd-2017-89.