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Heat waves are often assessed by how frequent or intense they are, but vulnerability involves more than that. A key factor in the amount of harm heat waves cause is people’s capacity to adapt with measures like cooling technology and the power to run it. It is found that while wealthy countries can buffer their risk by rapidly investing in measures to adapt to climate change, the poorest quarter of the world – areas likely to be slower to adapt – will face escalating heat risk.

Heat waves are among the deadliest climate and weather-related disasters, and they can be destructive to crops, livestock and infrastructure. Currently, about 30% of the global population lives in areas where heat and humidity levels can be deadly on at least 20 days a year, studies show, and the risk is rising.

Adaptation measures, such as cooling centers, home-cooling technology, urban planning and designs focused on reducing heat, can lower a population’s heat exposure impact. However, a country’s ability to implement adaptation measures generally depends on its financial resources, governance, culture and knowledge. Many developing countries struggle to provide basic services let alone protections from escalating disasters in a warmer future.

It is estimated that the poorest quarter of the world lags the wealthiest in adapting to rising temperatures by about 15 years on average. This estimate is based on the pace of preparation and support for adaptation plans described in the U.N. Environment Program’s Adaptation Gap Report. The actual lag will vary because of wealth _________________, but that estimate provides a broad picture of the rising risks. Heat risk is up globally, but more in poor regions. Looking back over recent decades, there is found a 60% increase in heat wave days in the 2010s compared with the 1980s. We defined a heat wave as extreme daily temperatures above the 97th percentile for the area, for at least three consecutive days.

It is also found that heat wave seasons were getting longer, with more frequent early- and late-season heat waves that can increase heat-related deaths. People’s average heat wave exposure in the poorest quarter of the world during the 2010s was more than 40% greater than in the wealthiest quarter – roughly 2.4 billion person-days of heat wave exposure per year compared with 1.7 billion. This heat wave risk in poor countries has often been overlooked by the developed world, in part because heat deaths aren’t consistently tracked in many countries.