Study projects nearly 2.6 billion people could face frequent ‘hot‑dry’ heat-and-drought extremes by 2090s

On a warming planet, days that are both searingly hot and bone dry are becoming more common — and could become routine for billions by century’s end.

A peer‑reviewed study published April 7 in Geophysical Research Letters projects that, on current greenhouse‑gas and population trajectories, roughly 28% of the world’s population — about 2.6 billion people — could be living by the 2090s in places where “hot‑dry” extremes occur more than five times as often as they did in the late 20th century.

What the study measured

The analysis, led by Di Cai of Ocean University of China and the Alfred Wegener Institute in Germany, examines "compound hot‑dry events": days when high temperatures and drought coincide. The authors defined a hot‑dry day as one when the daily high temperature falls in the hottest 10% of days and at least a “moderate drought” is present, each relative to the 1961–1990 climate at a given location.

To estimate future exposure, the team combined 152 simulations from eight climate models with socioeconomic and emissions scenarios used by the U.N. Intergovernmental Panel on Climate Change. They compared preindustrial (1850–1900), recent past (2001–2020) and future decades through the 2090s.

Key findings

• The world has already seen a sharp rise in compound heat‑and‑drought days: between 2001 and 2020 locations averaged about four hot‑dry events per year, roughly double the preindustrial frequency.

• Under a current‑trajectory scenario for emissions and population growth, those extremes may occur nearly 10 times per year on average by the 2090s, with the longest spells lasting around 15 days. Frequency increases by about 2.4 times and duration by about 2.7 times compared with the recent past.

• By overlaying climate projections with population maps, the study estimates that about 6.6% of the global population could face a greater‑than‑fivefold increase in exposure as early as the 2030s; that share rises to roughly 28% by the 2090s under the current‑trajectory scenario.

• If countries fully implement Paris Agreement pledges and pursue more ambitious long‑term commitments, the researchers estimate about 18% of the global population (around 1.7 billion people) would experience that level of increased exposure by the end of the century — roughly one‑third fewer people than in the current‑trajectory case.

Who will be hit hardest

The study finds impacts are uneven. Low‑income, tropical countries — many of which have contributed little to historical greenhouse‑gas emissions — are projected to see the most dramatic increases in exposure. Small island states such as Mauritius and Vanuatu are highlighted among projected hotspots.

"For lower‑income countries, there is a huge unfairness here," Cai said in an American Geophysical Union statement accompanying the paper. "It’s hard to fund air conditioning. It’s hard to fund health care. There is no backup if water runs out. It’s not just a climate science issue; it is about basic, daily life."

Co‑author Monica Ionita of the Alfred Wegener Institute said in the release that conditions are changing quickly: "It’s very difficult to keep up with what’s going on now."

Impacts and implications

Previous research shows that extreme heat and drought each strain crops, water systems and human health; together they amplify harms. Combined extremes can reduce crop yields more sharply, raise wildfire risk and increase heat‑related illness and death — especially among outdoor workers and people without access to cooling.

The new study adds a global estimate of how many people could live under those combined stresses and explicitly ties exposure to inequality and policy choices. The authors also note a personal‑scale translation included in their materials: based on their calculations, the lifetime greenhouse‑gas emissions of an average U.S. resident could be linked to exposing "one additional person" to heightened hot‑dry extremes by 2100.

Attribution and caveats

The authors tested model runs that include only natural forcings (for example, volcanic eruptions and solar changes) and found no clear historical trend in hot‑dry extremes; runs that incorporate human‑caused greenhouse‑gas emissions produced strong increases. From that, the team concludes the rise in compound heat and drought is largely attributable to anthropogenic warming.

Scientists not involved in the study are likely to scrutinize some methodological choices: using a 90th‑percentile temperature threshold and a particular drought index can affect counts of compound days, and different climate models can yield different regional projections. The full paper lists the drought index, model ensemble and socioeconomic pathways used; those specifics are not all included in the AGU press summary.

Conclusion

Even with methodological caveats, the work bolsters a growing body of research pointing to more frequent compound heat and drought in a warming world. It stands out for combining climate projections with population exposure and emphasizing inequality: without faster emissions cuts and significant investments to help vulnerable communities adapt, hundreds of millions more people — many in countries that have contributed least to climate change — could face recurring stretches of dangerous heat and scarce water before the century ends.