As summer kicks into high gear, heatwaves have already gripped parts of the United States, Europe and South Asia with recording-breaking temperatures and deadly conditions.
In the past 100 years, heatwaves have become hotter and more frequent across the world, according to the latest report from the United Nations’ Intergovernmental Panel on Climate Change (IPCC), the global authority on climate science.
The average global temperature has risen roughly 1.2 degrees Celsius (2.2 degrees Fahrenheit) since the Industrial Revolution began in the 19th century.
The IPCC is unequivocal about what’s causing this additional heat in the atmosphere: emissions created largely from humanity’s burning of fossil fuels.
Some 90 per cent of that heat is being absorbed by the ocean, the consequences of which are already dire. Warmer waters can lead to coral reef bleaching, where the corals lose the algae that populate them and the reefs turn white and devoid of much life.
And higher temperatures can decrease oxygen levels in the water – oxygen that marine life needs to live, putting stress on some ocean ecosystems.
Warmer water also expands, contributing to global sea level rise, in addition to water that comes from glacial melt in rapidly-heating polar regions.
Marine heat can be devastating for wildlife. During the 2021 Pacific Northwest (PNW) heatwave, the deaths of millions of marine animals were linked to the extreme temperatures
The damage extends far beyond sea-level rise and marine life. In 2021, that heatwave in PNW and western Canada killed hundreds of people, saw roads buckle and wildfires erupt. This spring, a heatwave in India and Pakistan killed 90 people, devastated crops and led to power shortages.
Even polar regions, more commonly associated with snow and ice, have been hit with heatwaves that send temperatures far above normal, leading to further melting ice and permafrost.
The Pacific Northwest and South Asian heatwaves were found to have been hotter, and more likely, because of human-caused climate change, according to the World Weather Attribution network, an international group of climate scientists who are now able to swiftly diagnose such events.
The US heatwave in 2021 was found to be “virtually impossible” without the climate crisis — and in South Asia this year, the deadly heat was made 30 times more likely by humanity’s actions.
The IPCC also found that some extreme heat experienced worldwide over the past ten years would have been “extremely unlikely to occur without human influence”.
Some impacts of heat are plainly visible, like ice melting and wildfires erupting. But heatwaves also stress water, energy and transportation infrastructure, as well as food systems and livelihoods from crop and cattle losses, says the World Health Organization.
And heat is a silent killer. Between 1992 and 2021, heat was the deadliest type of disaster in the US with an average 158 people dying, in comparison to 88 in floods, 71 in tornadoes and 45 from hurricanes.
Individual heatwaves can be extremely deadly too. The 2003 European heatwave killed over 70,000 people, and the 2010 Russian heatwave resulted in over 50,000 dead.
A recent study looking at mortality in hundreds of locations in 43 different countries found that about 37 per cent of heat-related deaths between 1991 and 2018 were attributable to the climate crisis.
Extreme heat is especially dangerous for vulnerable groups like the elderly, young children and people with medical conditions. Worryingly, the climate crisis is increasing night-time temperatures, eliminating the ability for people and buildings to cool down, according to the US Center for Disease Control.
Global heating is also causing potentially deadly spikes in humidity. When combined with extreme heat, high humidity can make it much more difficult for people to cool themselves by sweating.
The so-called “wet-bulb” temperature combines temperature and humidity to measure the potential for heat stress. Any wet bulb temperature above 95F (35C) is considered unsurvivable for someone in the sun for more than six hours.
Most of the time, wet-bulb temperature is below the actual temperature — an 80F (27C) day with 50 per cent relative humidity has a wet-bulb temperature around 67F (19C), for example.
But a 100F (38C) day with 85 per cent humidity would have a wet-bulb temperature over 95F (35C). A recent study found that these instances, while still somewhat rare, have doubled in frequency since 1979.
Cities are hit especially hard by heatwaves — a rising problem, considering that two-thirds of all people will live in urban areas by 2050.
In the US, cities are around 1-7F (0.5-4C) hotter than surrounding areas during the day, according to the Environmental Protection Agency. This “urban heat island effect” stems from manmade materials like asphalt and concrete that absorb and retain more heat than natural landscapes like forests and fields.
Heatwaves can cause positive feedback loops that fuel even more heat. As temperatures rise, more water evaporates into the atmosphere. And more water vapor in the atmosphere means more warming—in turn, leading to more evaporation and more warming.
When excessive heat melts permafrost in polar regions, it can lead to the release of the potent greenhouse gas methane. In everyday life, more heat means more people cranking up air conditioning units, leading to more power usage derived from burning fossil fuels – and even more potential feedback loops.
Under current emissions cuts pledged by nations, the world is tracking for an average temperature rise of 2.7C by the end of the century, according to the Climate Action Tracker, an independent scientific analysis tool.
At 2C, heatwaves that once occurred every decade would happen about every two years and be 2.6C hotter, the IPCC says. Far more rare, intense heatwaves that happened every 50 years would happen about 14 times as often and be 2.7C hotter.
If the world were to reach 4C of warming — which is possible if emissions continue to rise through the rest of the century — both the 10-year and 50-year heatwaves would occur a little less than once per year, the IPCC said.
And the climate crisis isn’t just causing extreme heat events — it may also be making the average day just a little bit hotter.
The Climate Shift Index, a tool created by the non-profit Climate Central, shows how more or less frequent each day’s high and low temperature is in the lower 48 US states has become due to global heating. The map shows how even mildly hotter days may have become more likely as the planet has heated.