Quantifying the physical processes leading to atmospheric hot extremes at a global scale.

Heat waves are among the deadliest climate hazards. Yet the relative importance of the physical processes causing their near-surface temperature anomalies (𝑇')-advection of air from climatologically warmer regions, adiabatic warming in subsiding air and diabatic heating-is still a matter of debate. Here we quantify the importance of these processes by evaluating the 𝑇' budget along air-parcel backward trajectories. We first show that the extreme near-surface 𝑇' during the June 2021 heat wave in western North America was produced primarily by diabatic heating and, to a smaller extent, by adiabatic warming. Systematically decomposing 𝑇' during the hottest days of each year (TX1day events) in 1979-2020 globally, we find strong geographical variations with a dominance of advection over mid-latitude oceans, adiabatic warming near mountain ranges and diabatic heating over tropical and subtropical land masses. In many regions, however, TX1day events arise from a combination of these processes. In the global mean, TX1day anomalies form along trajectories over roughly 60 h and 1,000 km, although with large regional variability. This study thus reveals inherently non-local and regionally distinct formation pathways of hot extremes, quantifies the crucial factors determining their magnitude and enables new quantitative ways of climate model evaluation regarding hot extremes.