Reuters has published a global temperature analysis that proposes a new category for the largest heat waves, reporting that these planet-scale events have become dramatically more common since the 1960s.

The news organization calls them heat storms: heat waves that, at their peak, cover at least one per cent of Earth’s surface—about 5.1 million square kilometres, or roughly half the size of Canada.

That is a Reuters-created analytical term, not an official meteorological classification. The project’s methodology says the one-per-cent threshold is partly a communication choice intended to capture unusually large events.

From about five a year to 102

Reuters says heat storms averaged about five active events a year in the 1960s. This decade, its analysis finds an average of about 102 annually.

The project also estimates that heat storms accounted for about three per cent of days in the hottest tenth of a location’s historical range during the 1960s. This decade, the share was almost 43 per cent.

Those figures come from Reuters’ own method and should be read as results of that model—not as an established count produced by a government weather agency. The analysis uses temperature data from the European Centre for Medium-Range Weather Forecasts.

How the analysis works

Reuters says it processed more than 600 billion hourly temperatures for 1.04 million grid squares worldwide, using 1961–1990 as the baseline period.

A location enters a heat wave after five consecutive days at or above the 90th percentile of its historical temperature range for that date, with smoothing and a limited grace-day rule. Adjacent qualifying grid cells are combined to measure a heat wave’s area.

The project was developed after consultations with two dozen researchers and scientists. Its authors also published a detailed methodology and open-source tracking code so others can test or reproduce the approach.

A record event reached most of the globe

The analysis names the largest event it identified Heat Storm Vera. Reuters says it lasted 367 days from July 2023 to July 2024 and, over that lifespan, reached almost 85 per cent of the planet.

That does not mean every affected location experienced extreme heat continuously for the full year. It describes the changing geographic footprint of one tracked system as qualifying areas joined, separated and moved over time.

Reuters connects the event with extreme heat and related impacts in several regions. Individual disasters still have multiple causes, and the project’s shared storm label is an analytical framework rather than proof that heat alone caused every reported consequence.

Sources: Reuters, Part 1: the largest heat storm on record, filed October 20; Reuters’ full methodology, filed October 20; The Age of Heat Storms series hub.

Editorial illustration of connected global heat zones; not a weather map, forecast or documentary image of a specific event.