Hot-dry extremes could hit 28% of humanity five times more often by end of century

by Sean Cummings, American Geophysical Union

edited by Sadie Harley, reviewed by Robert Egan

In their current state, climate policies around the world could leave a significant chunk of the global population exposed to simultaneous extreme heat and drought over five times more often by the end of this century than during the mid-to-late 20th century.

In a new study, researchers project the increase will affect 28% of the global population overall, concentrated in low-income, tropical nations that have contributed only a small fraction of humanity's greenhouse gas emissions to date.

"Heat and drought amplify each other," said Di Cai, a climate scientist at the Ocean University of China and lead author of the study. "In compound hot-dry extremes, they lead to water restrictions and unstable food prices. For outdoor workers, it is dangerous."

The study appears in Geophysical Research Letters.

Amplified extremes

When heat and drought strike together, the damage often exceeds the sum of what they can inflict separately. Wildfire risk, agricultural losses, and heat-related mortality can all spike.

These extreme combos are already on the rise. When the researchers divided Earth's land into cells on a grid and compared heat and drought occurrence in each cell, they found that, on geographical average, Earth's land areas weathered roughly four hot-dry events per year from 2001 to 2020. By their estimates, that's about twice as often as in the preindustrial period from 1850 to 1900.

To see how conditions might evolve through the end of this century, the team analyzed 152 existing simulations based on eight climate models, considering various scenarios of population growth and global warming outlined in the Sixth Assessment Report by the Intergovernmental Panel on Climate Change.

For this study, they defined hot-dry events as days with a high temperature in the top 10% and at least moderate drought, both relative to records from the 1961 to 1990 baseline.

The effort required processing terabytes of data, a significant challenge. "The more chaotic the climate becomes, the more difficult it becomes to make forecasts," said Monica Ionita, a climatologist at the Alfred Wegener Institute and senior author of the study. "It's very difficult to keep up with what's going on now."

In the climate and population growth scenario most aligned with our current trajectory, the team found, hot-dry extremes become "heightened" (over five times more probable on any given day than during 1961 to 1990) for 28% of the global population—nearly 2.6 billion people—by the 2090s. For comparison, they expect only about 6.6% to suffer that level of exposure in the 2030s.

"When you get to almost 30% of the global population affected by this, it's very critical. It should make us consider much, much more deeply our actions in the future," Ionita said. She had anticipated a slightly slower pace of change, ending at a figure of maybe 10% or 15%. "By the end or middle of the century, maybe my children will not be able to experience the life that I have now."

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Some reap what others sow

Globally, compound hot-dry extremes may strike nearly 10 times per year on average by the end of the century, with the longest lasting around 15 days—increases of 2.4 and 2.7 times from the conditions of the past 25 years, respectively.

Human emissions of greenhouse gases drive those changes. When the researchers analyzed simulations with only natural forces at play, no significant trends in the frequency or duration of hot-dry extremes emerged.

However, those who emit the most likely won't suffer the greatest impacts. According to the geographical distribution of risk in the simulations, low-income nations around the equator and tropics, including islands such as Mauritius and Vanuatu, will feel the most exacerbated hot-dry extremes despite contributing far fewer emissions than wealthier nations.

For context, the team estimated the climate impact of the carbon 1.2 average U.S. citizens emit over their lifetimes could expose one additional person to heightened hot-dry extremes by the end of the century.

"For lower-income countries, there is a huge unfairness here," Cai said. "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."

Limiting emissions could avert a lot of risk, the researchers found. If all nations fully implement the climate action plans they contributed under the Paris Agreement, as well as more binding long-term pledges, about 18% of the global population would face heightened exposure to hot-dry extremes by the century's end. That equates to roughly 1.7 billion people, nearly a third fewer than the number under the current trajectory.

"The choices we make today will directly affect the daily lives of billions of people in the future," Cai said.

photo: Outdoor laborers are especially vulnerable to extreme heat and drought. The impacts amplify when the two occur together, an increasingly frequent phenomenon due to climate change, especially in tropical regions. Credit: TruongDinhAnh, Pixabay