Deadly heat in Mexico and US made 35 times more likely by global heating

The deadly heatwave that scorched large swaths of Mexico, Central America and the southern US in recent weeks was made 35 times more likely due to human-induced global heating, according to research by leading climate scientists from World Weather Attribution (WWA).

Tens of millions of people have endured dangerous daytime and nighttime temperatures as a heat dome engulfed Mexico, and the large, lingering zone of high pressure stretched north to Texas, Arizona and Nevada and south over Belize, Honduras, Guatemala and El Salvador.

A heatwave can be caused by several factors including a heat dome, which traps hot air close to the ground, blocking cool air from entering and causing temperatures to rise and stay high for days or weeks. In May and early June, the heat dome hovered over the region, breaking multiple daily and national records, and causing widespread misery and disruption, especially among the poorest and most marginalized communities.

Such extreme heat spells are four times more likely today than they were at the turn of the millennium, when the planet was 0.5C cooler, the WWA analysis found.

“Unsurprisingly, heatwaves are getting deadlier … we’ve known about the dangers of climate change at least since the 1970s. But thanks to spineless politicians, who give in to fossil-fuel lobbying again and again, the world continues to burn huge amounts of oil, gas and coal,” said Friederike Otto, co-author of the study and senior lecturer in climate science at Grantham Institute, at Imperial College London.

According to the study, without meaningful political action to stop fossil fuels, deadly heatwaves will be “very common in a 2C world”, Otto said.

Extreme heat increases the rates of cardiovascular, respiratory and renal diseases, as well as threatening to overwhelm power supplies, healthcare facilities and other infrastructure.

At least 125 people have died and thousands more have suffered heatstroke in Mexico since March, when the temperature hit almost 52C on 13 June – the hottest June day ever recorded in the country. The extreme heat exacerbated severe drought and air pollution, stoking power outages, water shortages, thousands of wildfires and a mass die-off of endangered monkeys and birds. The actual mortality and morbidity toll is still unknown.

In Phoenix, Arizona, the hottest major city in the US, 72 suspected heat deaths were under investigation by the Maricopa county medical examiner’s office by 8 June – a 18% rise compared with the same period last year. Across the south-west US, more than 34 million people were under heat warnings, and dozens have suffered heat exhaustion at political rallies.

In Guatemala’s dry corridor, the hottest and driest part of the country where most people eke out a living from back-breaking farm work, schools were shuttered as temperatures hit 45C, and some of the region’s poorest communities faced crop failures and severe water shortages.

In Honduras, electricity has been rationed, and smoke from uncontrolled forest fires contributed to the worst air quality ever recorded in the capital, Tegucigalpa.

The death toll from across Central America – one of the most vulnerable regions in the world when it comes to the effects of the climate crisis due to its geography, high levels of poverty and inequality, poor infrastructure and governance, and a lack of heat-warning systems – is unknown.

Previous studies have shown that the frequency and intensity of heatwaves, the deadliest form of extreme weather, have been increasing in recent years due to the climate crisis, which is caused by burning fossil fuels and by other human activities such as deforestation and industrialized agriculture.

This year’s May was the hottest on record globally – and the 12th consecutive month that records for average monthly temperatures were broken.

To quantify the effect of human-caused warming on the recent extreme temperatures across North and Central America, a team of international scientists analysed weather data and climate models using peer-reviewed methods to compare how these types of events have changed between today, with a climate that has warmed by approximately 1.2C, and pre-industrial times, which had a cooler climate.

The WWA researchers looked at the five-day maximum temperatures across North and Central America in May and June. The analysis found that the climate crisis made the excessive heat spell about 1.4C hotter during the day – and 35 times more likely than in pre-industrial times.

The effect on night temperatures is even stronger, with the analysis finding temperatures about 1.6C hotter – a 200-fold increase due to global heating. Hot nights are particularly dangerous for human health, as the impact of heat is cumulative and the body only begins to rest and recover when temperatures drop below 80F (27C).

If fossil fuels are not phased out, the frequency and intensity of heatwaves will continue to rise, leading to more deaths, illness, economic losses, hunger, water shortages and forced migration among the world’s worst-affected communities – who have contributed least to the climate crisis.

“As long as humans fill the atmosphere with fossil-fuel emissions, the heat will only get worse. Vulnerable people will continue to die and the cost of living will continue to increase,” said Izidine Pinto, co-author and researcher at the Royal Netherlands Meteorological Institute.

Yet so much damage to the planet has already been done that heat-related deaths and disruption will continue to rise unless local and national governments rethink every aspect of life including urban planning, water conservation, shade, school sports and outdoor worker protections.

Karina Izquierdo, urban adviser for the Latin American and Caribbean region at Red Cross Climate Centre, and a co-author of the study, said: “Every fraction of a degree of warming exposes more people to dangerous heat … As well as reducing emissions, governments and cities need to take steps to become more resilient to heat.”

? This article was amended on 20 June 2024 to correct that May was the 12th consecutive month a hottest-month record was broken, not the 13th as stated in an earlier version; and to clarify that these records are based on average monthly temperatures.