The season is stretching at both ends
A global analysis published in Nature Climate Change on 23 September finds that heat waves over land are beginning earlier in the year, ending later and covering a longer season. On average, the first heat wave of the year begins 3.29 days earlier per decade, while the last one ends 5.41 days later per decade, extending the season by 8.71 days per decade. Over the 45 years of records studied, the first heat wave arrived about two weeks earlier, the last one came about 24 days later in the calendar, and the season lengthened by roughly 39 days.
The trends are widespread. Earlier onset appeared over 72.0% of global land, later ending over 79.6%, and a longer season over 92.1%, with statistically significant changes across 13.4%, 20.3% and 34.4% of land respectively. Drylands showed more pronounced shifts in timing than humid regions. The study was led by researchers at the Chinese Academy of Sciences with international co-authors.
Faster onset is the new part
The findings also describe a recent change in how heat waves behave. The first heat wave of the season has shifted toward faster onset, meaning temperatures climb into dangerous territory in less time. That matters for people and systems that adapt on a scale of days: emergency plans, hospital staffing, power reserves and crop irrigation schedules are built around a warning window, and a compressed onset shrinks it.
The year has already provided examples. UN News described 2026 as a succession of stark reminders, from deadly floods in the Himalayas to wildfires and extreme heat across Europe. Cities across the continent ran cooling centres and issued night-time heat alerts as overnight temperatures stayed high.
Why timing belongs in planning
Most heat planning is built on how hot it gets and how long the heat lasts, not on when it arrives. The researchers argue that heat wave timing should be folded into early-warning systems and risk assessments for public health, agriculture, ecosystem management and energy supply, so that hospitals, grid operators and farmers know which part of the calendar is now dangerous. A season that starts two weeks early and ends three weeks late changes the answer.