The world is heating up, and a new study warns that almost one-third of people could face the double-whammy of a heatwave and drought five times more often by the 2090s.
Researchers in Germany and China looked at compound hot-dry extremes, events where severe drought and intense heatwaves happen at the same time, in the same place.
Based on the team's modeling and our current emissions trajectory, 28 percent of the global population – nearly 2.6 billion people – will experience at least five times as many of these extremes by the century's end compared to partway through.
When heat and drought combine, the impacts are far worse than either event alone: not just in the number of heat-related deaths, but also for increased risks of wildfires, greater agricultural losses, and higher levels of socio-economic instability.
"Heat and drought amplify each other," says climate scientist Di Cai, from the Ocean University of China. "In compound hot-dry extremes, they lead to water restrictions and unstable food prices. For outdoor workers, it is dangerous."
The researchers combined data from 152 climate simulations based on eight different climate models, used in the Intergovernmental Panel on Climate Change's (IPCC) Sixth Assessment Report to project population growth and climate change.
Based on the government policies in place today, the data indicates a temperature rise of 2.7 °C by 2100, driving that heightened level of compound hot-dry extremes. These events were defined as having temperatures in the top 10 percent range for an area, based on historical records, plus drought classified as moderate or worse.
The researchers' modeling suggests that across the planet as a whole, we'll face 2.4 times as many compound hot-dry events by the end of the century as we do now, and those events will last, at their maximum, nearly 3 times longer than today.
There will be a lot of disparity in terms of how those increases are felt from region to region, though. Tropical nations and low-income countries – those who have contributed the least to global warming – will be hit the hardest.
"For lower-income countries, there is a huge unfairness here," says Cai. "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."
The model simulations were very clear that human-driven greenhouse gas emissions would be largely responsible for the continued rise in global temperature, which is already 'supercharging' extreme weather events.
"The choices we make today will directly affect the daily lives of billions of people in the future," says Cai.
The research shows there is another way, reaffirming how much of a difference emissions reduction policies can make.
The analysis found that renewed commitments to the 2015 Paris Agreement and additional binding long-term pledges could reduce the number of people affected by these compound events by one-third.
If those pledges are fully implemented, the proportion of the global population exposed to five times as many hot-dry extremes would shrink from 28 percent to 18 percent – or nearly 900 million fewer people impacted.
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While further analyses could provide more granularity on the risks, the researchers say their work "shows the urgent need for fair and immediate climate action that protects those most at risk."
"When you get to almost 30 percent of the global population affected by this, it's very critical," says climatologist Monica Ionita, from the Alfred Wegener Institute in Germany. "It should make us consider much, much more deeply our actions in the future."
The research has been published in Geophysical Research Letters.