Behind or ahead of committed warming: what it means for future hot extremes

Virtually all land regions have warmed over recent decades, yet heatwave trends show striking regional differences. The thermodynamic rise of hot extremes can be strongly modulated by atmospheric circulation, a phenomenon that has received increasing attention for regions such as Europe and parts of North America, where observed trends in hot extremes have been amplified and dampened, respectively. But what about other regions? How persistent are these circulation anomalies? And what are the implications for future heatwaves?

Using dedicated climate model experiments, we quantify how atmospheric internal variability has modulated historical heatwave trends globally. Building on a large ensemble framework, we interpret observed circulation contributions as placing regions on unusual warming trajectories — either well below or above the ensemble mean expectation. Regions currently displaying less warming compared to climate model simulations are effectively "lagging behind" the warming already committed to by anthropogenic forcing; those running warm are "ahead".

This warming trajectory position has profound implications for the pace of future change. Regions currently lagging behind, including much of North America, face substantially faster increases in hot extreme probability between now and the mid-century than ensemble mean projections suggest. Conversely, other regions have already experienced much of the expected probability increase. We illustrate these divergent futures through the evolving return period of what was once a 1-in-100-year hot extreme, showing how the present trajectory position determines the pace of change over the coming decades.