Tropical nights: A growing risk as global temperatures rise

As Lorna Powell, a National Health Service (NHS) urgent care doctor in London, watched a weather forecast for extreme heat across Britain in June, one prediction in particular caught her attention.

Meteorologists warned that the British capital could experience a slew of so-called tropical nights, in which temperatures remain above 20 deg C. The prediction came true: In June, London endured a record-equalling five consecutive such nights. For Powell, that meant more patients.

Britain is far from alone as climate change drives more extreme weather.

Across the world, the hottest nights are warming at a faster rate than the hottest days – with increasingly severe consequences for health and economic repercussions also set to get worse.

Much of Europe has just experienced the most sweltering June on record, with hundreds of weather stations in at least 15 countries reporting their hottest nights ever, a Financial Times (FT) analysis of provisional temperature data shows.

“What stands out about June’s heatwave is not only the daytime intensity but (also) the extraordinary warmth overnight,” says Robert Rohde, lead scientist at Berkeley Earth, a US non-profit climate research organisation.

More than half of the longstanding monitoring sites across Germany, France and Britain registered their warmest June night on record, many smashing previous highs by several degrees. In Kubschutz, just east of Dresden in Germany, the mercury did not dip below 29.4 deg C on June 27, setting a new national record for the country’s warmest night.

How night-time heat stress affects health

Night-time heat is harder to mitigate than its daytime equivalent, while its health effects can be devastating.

“If the night does not cool down, the body loses its main opportunity to recover,” says Dominic Roye, a climate scientist who has examined the impact of hot nights across 44 countries and is affiliated with the Biological Mission of Galicia, a Spanish research institution. “Unusually warm nights have their own independent impact on mortality.”

Since the body relies on a temperature drop of about 1 deg C to initiate deep sleep, tropical nights can deny people rest.

While heartbeats and blood pressure normally fall during sleep, on a hot night the heart is forced to pump twice the normal volume of blood to cool down the body. The heat and additional strain lead to excess sweating and potential dehydration.

“During the June heatwave, we struggled to keep pace with the sharp rise in life-threatening illnesses triggered by dangerously high temperatures, particularly overnight,” says Powell, the NHS doctor, who also helps lead Mothers Rise Up, a climate action group.

In Europe and other parts of the world where domestic air-conditioning remains a rarity, there is also an economic cost.

“That underlying night-time heat stress on the body is costing money,” says Kathy Baughman McLeod, chief executive of Hera, a non-profit focusing on climate adaptation. “People go to work, their hand-eye coordination is off, they make mistakes, they hurt themselves, they hurt other people.”

Parts of Asia, including China, Iran and Saudi Arabia, have also experienced significant increases in overnight temperatures.

In Jakarta, the world’s largest city according to the UN, the number of nights with minimum temperatures above 24 deg C increased from about three per year in the 1970s to 60 per year during the past decade. 

But across the world’s 500 most populous cities, FT analysis of data from the European Union’s Copernicus Climate Change Service shows, the sharpest rises in night-time temperatures have been concentrated in southern and eastern Europe. Night-time temperatures represent the minimum across a 24-hour period, typically reached in the hours before sunrise.

In some European cities, the hottest nights have warmed by more than 0.5 deg C per decade, more than twice the average increase across all locations analysed. 

Naples, Barcelona and Athens have gained about one additional tropical night each year since the 1970s. In Milan, there were about three nights above 20 deg C each year in the 1970s; now, there are about 33.

Mark McCarthy, a science manager at Britain’s Met Office, says that historically, tropical nights were very rare for much of the continent. “What we’re seeing is that (hot nights are) increasingly frequent,” he says. “And our climate projections suggest that that’s a trend that we would expect to continue.” 

Research published in June in Nature Climate Change, a scientific journal, found that the 10 hottest nights of each year have warmed faster than the 10 hottest days – by a global average of 0.32 deg C per decade compared with 0.27 deg C per decade, respectively. 

The study also found that during one in 10 tropical nights, people are now hit by “heat stress”, when humidity and other factors add to discomfort, up from about one in 30 in the 1970s.

Separately, when a very hot day is followed by a tropical night, the human body has little opportunity to recover. In Europe, occurrences of a so-called heat-stress day followed by a tropical night have increased by 73 per cent since the 1970s.

“It’s not just a future problem,” says Rebecca Emerton, a scientist at the European Centre for Medium-Range Weather Forecasts and the study’s lead author. “This has already happened.”

McCarthy at the Met Office says hot nights are among “the most acute ways in which we are impacted by our changing climate”. He warns: “Our infrastructure, our buildings and our homes are not well adapted to some of the extremes that we’re already experiencing.”

Cost of sleep deprivation

In Spain, which has baked in night-time temperatures above 25 deg C in summer, Maria Isabel de Castro describes trying to sleep through Madrid’s sweltering nights as “insufferable”.

The 50-year-old tax adviser takes cold showers in the middle of the night and goes to bed without drying off in an attempt to stay cool. The lack of sleep often leaves her exhausted. “It’s hard to focus at work the next day,” she says.

In Britain, a YouGov survey commissioned by Greenpeace UK found that two-thirds of adults struggled to sleep during June’s heatwave, with about half saying they lost three or more hours of sleep each night. 

Another study estimated the costs of temperature-caused sleep loss in the country at about 0.04 per cent of gross domestic product per year – or just more than £1 billion (S$1.7 billion).

Such figures are only likely to increase. Patrick Bigler of the University of Lausanne has suggested hot nights could increase fivefold by the end of the century, based on a climate change scenario in which emissions peak by around 2040 and then decline.

Elevated night-time temperatures have also been linked to a rise in domestic violence and crime, says Baughman McLeod of Hera. People are “irritable, frustrated, can’t get comfortable, and want to take it out on somebody”, she says.

Then there are the health challenges. Those with underlying problems, the elderly and the very young are especially vulnerable. 

The risk of strokes increases by 7 per cent during tropical nights, a 2024 study found, while there are also higher instances of mental disturbances compared with hot days.

Powell, the NHS doctor, says that as well as disrupting sleep and causing dehydration, hot nights can “trigger cardiovascular stress”, especially in older adults with pre-existing conditions such as high blood pressure or diabetes.

After the June heatwave, Germany recorded almost 5,500 excess deaths, according to provisional data. France registered 2,000 and Belgium 1,200. Madrid’s Carlos III Health Institute reported at least 1,028 heat-related deaths in Spain.

The latest research from Roye at the Biological Mission of Galicia found the risk of mortality rose by 2.6 per cent globally for extreme night heat; in some instances, such as the Cypriot capital of Nicosia, the figure increased by as much as 17 per cent.

Air-cons and the limits of night-time options

One of the biggest issues is that night-time “adaptation options are more limited than during the day”, Roye says.

“During the day, people can seek shade, reduce activity or enter an air-conditioned building,” he adds. “At night, opening windows, the most basic and accessible strategy, stops working when it is just as hot outside as it is inside.”

Air-conditioning is more effective in reducing heat than such elementary responses, and the International Energy Agency expects the number of air-conditioners installed across the EU to more than double from about 130 million in 2023 to 275 million by 2050.

But the devices can increase the temperature in neighbouring houses, are often expensive to run and, even when fuelled by clean energy, increase the greenhouse gas emissions that contribute to climate change. 

Installing cooling systems is often particularly challenging because of the age of Europe’s housing stock and some countries’ propensity for renting. Many European homes, especially in northern parts of the continent, have been designed to retain heat rather than repel it.

Other solutions to excessive night-time warmth include planting trees, preventing rooms from getting hot during the day by using shade, shutters or curtains, or painting roofs white to reflect heat. 

Powell, the NHS doctor at Mothers Rise Up, says ultimately countries need to cut emissions. In the meantime, “vulnerable people need to understand the health risks, try to stay well hydrated and cool one room in their home if possible”, she advises.

With London sweltering through its third heatwave in less than two months – and recording another tropical night on July 9 – she is feeling on edge.

“As the hot nights just keep on coming, I brace myself before every shift, knowing we are going to be inundated with heat-exhausted babies and a spike in urgent cases.” Financial Times