Investigative journalists find that such events now cover nearly 43% of unusually hot days worldwide, prompting scientific debate over the terminology and methodology.


On 20 October 2026, a Reuters investigation report asserted that the world’s largest heat waves have become increasingly widespread and persistent. It calls these events “heat storms”: heat waves covering at least 1% of Earth — roughly half Canada’s area — at their peak. Their annual number has risen from about five in the 1960s to about 102 this decade.

The series, “The Age of Heat Storms,” analyzed hourly temperatures since 1960 across more than a million grid squares worldwide. Reuters had consulted two dozen meteorologists and climate researchers, presenting the findings as another way to understand climate change.

Some of the data reported:

  • The largest heat wave of the 1960s covered about 6% of Earth over 46 days. This decade’s largest, named Heat Storm Vera by Reuters, reached almost 85% during its 367-day span from July 2023 to July 2024.
  • Reuters connects Vera with more than 1,300 Hajj pilgrim deaths, Louisiana wildfires and heat that helped power the 2023 cyclone preceding the deadly dam collapses in Derna, Libya.
  • Heat storms account for almost 43% of unusually hot days worldwide, compared with 3% in the 1960s.
  • At least one has been active continuously since July 6, 2008 — a stretch Reuters describes as exceeding 18 years, or 6,681 days. That period included 56 storms reaching more than a tenth of Earth. Meanwhile, cold snaps have contracted almost 80% in area. Victoria University of Wellington researcher James Renwick described heat as climate change’s biggest killer.

In an accompanying article, Reuters data journalist Maurice Tamman said the project had sought to connect discussions of 2-degree warming targets with everyday experience. With no universal heat wave definition, the team developed one using European Centre for Medium-Range Weather Forecasts ERA5 data and a 1961–1990 baseline. Adapted from marine heat wave research, it requires temperatures to remain within a location’s hottest 10% for that date for five days.

Some scientists have challenged the research because of its established meteorological meaning. Reuters retained the term’s broader meaning of a major natural disturbance. Tamman has acknowledged that the 1% cutoff was somewhat arbitrary, intended to identify exceptionally large, rare events.

Notably, Reuters has released its methodology as open source; assigned names to storms as with hurricanes; and introduced a public Heat Storm Tracker. Anthony Leiserowitz, Director, Yale Program on Climate Change Communication, has said that naming events helps people recognize that change is occurring.