"Non-optimal" temperatures kill more than 52,000 people in the UK every year, research suggests.
On a global scale, June 2021 brought "extraordinary" extremes in climate, with Canada and parts of the US experiencing an unprecedented heatwave. Despite the UK's recent rain, last June was Europe's second warmest on record.
Sweltering summers and bitter winters can be life-threatening in extreme cases, with elderly people often most at risk.
To better understand how weather affects a person's health, scientists from Monash University in Melbourne compared death and temperature data across the world from 2000 to 2019 – when global temperatures rose by 0.26°C (32.4°F) per decade.
Results, published in The Lancet Planetary Health, suggest nearly one in 10 (9.43%) deaths could be attributed to non-optimal temperatures – with 8.52% of fatalities being cold-related and 0.91% down to severe heat.
This equates to more than 52,000 fatalities in the UK and over 5 million across the world, a situation that is set to worsen with "the inevitability of climate change".
The research comes as scientists from the London School of Hygiene & Tropical Medicine warn malaria and dengue will affects billions more people if global warming continues.
Since 1880, Earth's average surface temperature has risen at a rate of 0.07°C (32.1°F) per decade, with this nearly tripling since the 1990s. As a result, 19 of the 20 hottest years on record occurred after 2000.
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In 2019, the Global Burden of Diseases, Injuries, and Risk Factors Study revealed non-optimal temperatures were among the 10 leading causes of death worldwide. Nevertheless, the "burden" of this was "not well quantified".
Before the Monash research, the second-largest study was published in 2015. Based on 74 million deaths across 13 regions, the results suggested 7.7% of deaths were due to abnormal temperatures.
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Speaking of the Monash paper, co-lead author Professor Yuming Guo said: "This is the first study to get a global overview of mortality due to non-optimal temperature conditions between 2000 and 2019, the hottest period since the pre-Industrial era.
"Importantly, we used 43 countries' baseline data across five continents with different climates, socioeconomic and demographic conditions – and differing levels of infrastructure and public health services – so the study had a large and varied sample size, unlike previous studies."
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Overall, non-optimal temperatures caused 74 "excess deaths" per 100,000 people, largely due to cold exposure, the results suggest.
Worldwide, just over half (51.4%) of these deaths occurred in Asia, particularly its eastern and southern countries.
Nevertheless, Europe had the highest excess deaths per 100,000 people when it came to heat exposure, while Sub-Saharan Africa fared worst for cold-related fatalities.
While cold-related deaths were high, these fatalities decreased by 0.51% from 2000 to 2019, while heat-related casualties rose by 0.21%.
"In the long-term, climate change is expected to increase the mortality burden because hot-related mortality would be continuing to increase," said Professor Guo.
The results illustrate "the importance of taking data from all points of the globe, in order to get a more accurate understanding of the real impact of non-optimal temperatures under climate change."
The scientists have concluded their findings "call for decisive and coordinated action to raise public awareness of temperature as a health risk".