Abstract/Summary

Transient climate response (TCR), transient response at 140 years (T140) and equilibrium climate sensitivity (ECS) indices are intended as benchmarks for comparing the magnitude of climate response projected by climate models. It is generally assumed that TCR or T140 would explain more variability between models than ECS for temperature change over the 21st Century, since this timescale is the realm of transient climate change. Here we find that TCR explains more variability across CMIP5 than ECS for global temperature change since pre-industrial, for 50- or 100-year global trends up to the present, and for projected change under representative concentration pathways in regions of delayed warming such as the Southern Ocean. However, unexpectedly we find that ECS correlates higher than TCR for projected change from the present in the global mean and in most regions. This higher correlation doesn’t relate to aerosol forcing, and the physical cause requires further investigation.