On the southern outskirts of Paris, a cemetery holds the bodies of the city’s unclaimed dead. Until recently, there lay a hundred whom some consider to be the first victims of global climate change. They were mostly elderly and poor, the forgotten people of the worst weather disaster in contemporary European history: the heatwave of August 2003, which killed nearly 15,000 in France alone and thousands more across the Continent.

Experts have predicted that a changing climate will bring more frequent, longer-lasting and more intense heatwaves to Europe — a prediction that appears to be coming true rapidly. Less than a month after Western Europe suffered a brutal heatwave, France, Britain, Belgium and the Netherlands again face suffocating temperatures, all in countries where few homes are air-conditioned. Highs of almost 109F in Paris and 98F 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 minimising the threat, and consequently the response to it.

This year’s successive heatwaves reveal an ominous pattern, one that looks remarkably like 2003, the deadliest summer in modern European history. That year, short heatwaves 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 towards 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 centres, 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 heatstroke 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 centres. 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 organisations 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 amid a record drought, with water restrictions in 73 of the country’s 96 departments. Lower water pressures make it more difficult to relieve heatstroke and dehydration, and water restrictions have already had devastating effects for livestock in a country whose agricultural sector is a backbone of the economy.

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 heatwaves 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 heatwave even trivialised 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 heatwave appeared at first to be an outlier. No more. An 18-day heatwave in July 2006 rivalled 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 heatwave 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 heatwave.) Since then, extreme heat and elevated mortality have become regular features 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 heatwave 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 August 1 to 20 between 2000 and 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 minimise the figure for future years. What had been excess deaths because of exceptional weather in the past will become a normal feature of summer.

The bodies of the unclaimed victims of 2003 are now gone from the cemetery, cremated to make room for new occupants. Their deaths are now truly invisible. But a changing climate is intersecting with an ageing population and increasing economic inequality, putting ever more people at risk.

Better health surveillance and wide-reaching alert systems have reduced deaths from extreme heat, but the very frequency of such intense heatwaves has made the tools we use to measure mortality increasingly outdated. Most, if not all, of these deaths are preventable, provided we recognise them for what they are and ensure greater access to cooling, fluids and early medical care. Given the horrific conditions taking shape this week, it is critical to see heat deaths not as normal but instead a measure of the costs of a changing climate, and avoid a repeat of the devastation of 2003.

• Richard Keller is a professor in the Department of Medical History and Bioethics at the University of Wisconsin-Madison. He is the author of Fatal Isolation: The Devastating Paris Heat Wave of 2003