Half a million heat deaths, and the two-thirds that is ours

The Lancet Countdown on health and climate change published its tenth annual indicator report this week: 146 experts from 71 institutions, 54 indicators². The headline everyone will quote is heat. Heat was linked to an estimated 540,800 deaths a year between 2016 and 2023, and to a record 710,000 in 2023 alone³. Desk arithmetic: from the Countdown’s own data file, the 2016–23 yearly average of 540,811 is 35.8% above the 2006–15 average of 398,371¹. That is the “nearly 36% since Paris” you will read elsewhere³.

The more interesting number is the one the model subtracts. The Countdown runs the same years twice: once with the temperatures that happened, once with temperatures as they would have been without human-caused warming¹. In that second world, heat deaths were about 179,260 a year in 2006–15 and 170,875 a year in 2016–23¹. Flat, slightly down. Desk arithmetic: the share attributed to climate change rose from about 219,100 a year to about 369,900, an increase of roughly 69%¹. In 2023, 71.5% of the estimated heat deaths were attributed to climate change¹.

Two cautions keep this honest. These are modelled estimates built on mortality data from 129 countries, not a count of death certificates¹. And the model assumes that people’s sensitivity to heat does not change over time; the authors say that if adaptation is working, later years may be overestimated¹. That caveat is the good news hiding in the footnotes: shade, cooling centres, early warnings and checking on an elderly neighbour are exactly the kind of adaptation that would make the next estimate smaller.

The report is not only gloom. Clean-energy investment reached a record $2.1 trillion in 2025, even as fossil fuels still supply more than 80% of the world’s energy³. Health harms “are growing faster than the world’s response to them”, said Marina Romanello, the Countdown’s executive director³. The authors want climate change formally recognised as a global health emergency².

For you, the practical part is unglamorous. The Countdown names older adults, women and people with underlying health conditions as particularly vulnerable to heat¹. A heat plan is a phone list, a thermometer and a cool room. It works in any decade.