Experts have predicted that a changing climate will bring more frequent, longer-lasting and more intense heat waves to Europe — a prediction that appears to be coming true rapidly. Less than a month after western Europe suffered a brutal heat wave, France, Britain, Belgium and the Netherlands again face suffocating temperatures, all in countries where few homes are air-conditioned. Highs of almost 109 in Paris and 98 in London on Thursday broke records in those cities. This is also the fifth successive summer of extreme heat in France.

A significant risk of extreme heat becoming a regular part of summer is that it will make high numbers of heat-related deaths seem normal, too, rather than something that requires redoubled attention. Most of these deaths are preventable, even if the climate change contributing to them has no ready remedy. Since 2015, France has recorded between 500 and 3,500 excess deaths from extreme heat each summer; complacency about thousands of deaths ignores the warnings they send about climate. Even the obsolete methods used to calculate death rates pose a risk of minimizing the threat, and consequently the response to it.

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This year’s successive heat waves reveal an ominous pattern, one that looks remarkably like 2003, the deadliest summer in modern European history. That year, short heat waves lasting from a few days to a week struck in June and July. Devastating droughts in some countries exacerbated the effects. At the beginning of August, a system developed over much of western and central Europe and remained in place for two weeks, bringing with it stifling temperatures and ruinous mortality. Across Europe, 70,000 died of excessive heat that summer.

This death rate is shocking because heat deaths are relatively easy to prevent. Just a few hours of air-conditioning per day is enough to allow people to recover, and drinking sufficient fluids is enough to prevent dehydration. In the aftermath of the 2003 disaster, the French government took important steps toward prevention. The state put in place a nationwide alert system to warn of the dangers of high heat and to recommend precautions. Many cities have established cooling centers, and nursing homes are required to provide at least one air-conditioned common room for their residents. Paris developed a telephone response network: those who feel at risk of heat stroke register with the city, and when temperatures soar, social workers call to ensure they are coping. Yet these systems have limits, especially for those most vulnerable. Media alerts don’t reach those without televisions, radios or Internet access, and the telephone systems require self-registration. People with disabilities find it difficult to reach cooling centers. Many homeless shelters are also closed in summer, shutting down an important hub for distributing warnings. Yet the government has done little to mitigate these obstacles, allowing NGOs and charitable organizations that are often insufficiently staffed to meet the needs of those most at risk.

France is on track to repeat the 2003 experience. Peaks of high heat separated by only a few weeks of average temperatures are also striking in the midst of a record drought, with water restrictions in 73 of the country’s 96 departments. Lower water pressures make it more difficult to relieve heat stroke and dehydration, and water restrictions have already had devastating effects for livestock in a country whose agricultural sector is a backbone of the economy.

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So far, the country has attributed only a handful of deaths to the extreme heat. This is in part because of that increased awareness and emergency preparedness. In addition, the extreme heat has not yet hit the traditional August vacation period in France, meaning that health agencies and first-response teams are likelier to be fully staffed now. Moreover, the types of people who die during heat waves are those who are already the likeliest to die of a range of causes: the elderly (and especially the elderly poor), the homeless and those suffering from severe mental and physical disabilities. An increase in deaths among these populations can easily go unnoticed until it becomes impossible to ignore, as it did in the first week of August 2003. One former French health official who played a role in the 2003 heat wave even trivialized the dangers this year, stating this week that the only heat deaths so far this year were among those who “ignored the warnings.” Yet those most vulnerable to the threat of extreme heat are often those who are most difficult to alert, and therefore the likeliest to ignore warnings.

The 2003 heat wave appeared at first to be an outlier. No more. An 18-day heat wave in July 2006 rivaled 2003’s in its intensity, killing some 2,000 people in France. In July 2015, extreme heat in the country killed 3,300. (This same heat wave caused a breakdown in Paris’s suburban commuter train system, preventing me from speaking to an audience in Versailles about my research on the 2003 heat wave.) Since then, extreme heat and elevated mortality has become a regular feature of French summers, with 700 deaths attributed to heat in 2016, 475 in 2017 and 1,500 in 2018. Given the weather so far in 2019, there is every reason to suspect high mortality once again.

Yet there is another danger — the risk that routine summer death tolls will mask the true extent of the threat. Demographers measure heat wave mortality by comparing average deaths for a given period in immediately preceding years with observed deaths in the period under review. For 2003, that meant subtracting the average death toll for Aug. 1-20 in 2000-2002 from the actual death toll in 2003. But comparing observed deaths with averages that are already inflated by excess mortality in prior years — as has been the case since 2015 — will minimize the figure for future years. What had been excess deaths due to exceptional weather in the past will become a normal feature of summer.

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