A hot day could be expected to have an extra four hours of extreme heat by the end of the century if the planet continues to warm under a high-emissions scenario, a study has found.

Researchers, including those from China's Sun Yat-sen University and Southern Marine Science and Engineering Guangdong Laboratory, also projected that days not categorised as 'hot days' -- having a heat exceeding 90 per cent of those on previously measured days -- could potentially gain a further three hot hours.

Low and middle-income countries are likely to be the worst hit, findings published in the journal Nature Climate Change show.

Extreme heat events are typically assessed using daily mean average, maximum, or minimum temperatures. However, the measures can neglect shorter but potentially important periods of extreme heat, the researchers said.

The modelling study shows that hourly assessments can capture heat exposure overlooked by daily metrics and could support a more effective heat risk management and public health planning, they said.

Global hourly data, combining weather observations and model estimates during 1981-2023 and coupled with future climate projections to the end of the 21st century, was analysed.

Land areas around the world experience an average of approximately 269 hours of extreme heat each summer, the analysis found.

An hourly heat event was defined as temperature exceeding 90 per cent of previous measurements of that hour during the baseline of 1981-2010.

More than 80% of the world's land area showed an increase in heat, with an average of 62 additional hours per decade of extreme heat between 1981 and 2023 -- 28 per cent of the events were also found to occur on days not identified as heat extremes by daily metrics.

"HHEs (hourly heat extremes) are emerging worldwide and exhibit marked unequal exposure across regions and generations," the authors wrote.

By late century, hourly heat extreme events could be expected to increase fourfold with a sixfold increase in population exposed to such instances under a high-emissions scenario, they found.

"Each hot day will gain about four hot hours, and a further three hot hours will occur on non-hot days, which daily metrics overlook," they wrote.

Low and middle-income countries were found to account for more than three-quarters of both current and projected heat exposure and face the largest increases in lifetime exposure across generations.

The researchers said future research should focus on mechanisms causing hourly heat extremes, such as heat bursts or flash heat events.

An improved understanding of short-term heat events on an hourly scale could improve early warning systems, potential health risk assessments, and adaptation to a warming climate, they said.