Welcome to the world of triple-digit spring weather

Vox

24-04-2025

is a correspondent at Vox writing about climate change, energy policy, and science. He is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.
A man walks along a road in Karachi, Pakistan, protecting himself amid the region's ongoing heat wave. In April alone, hundreds of millions of people across Pakistan and India have been experiencing scorching temperates. Asif Hassan/AFP via Getty Images
We're only midway through spring, yet searing summer temperatures have already started baking some parts of the world.
Heat waves are a distinct weather phenomenon where high temperatures linger for days at a time. As global average temperatures climb higher, the frequency and duration of periods of extreme heat are also growing, which is already hurting people around the world.
But the human impacts of heat waves also vary depending on their timing. Climate change is leading to shorter winters, earlier springs, and earlier arrivals of extreme temperatures. Heat waves that occur early in the warm season, well before summer sets in, tend to cause greater harm to health.
Related The Texas heat wave is even worse because of its timing
'These early events can cause more heat-related illnesses and fatalities than later heat waves in June or July, even if temperatures are similar,' Davide Faranda, a climate scientist at the French National Center for Scientific Research studying extreme weather, wrote in an email.
There are several factors behind this. One is acclimatization. When winter ends, people are less used to high temperatures at a physiological level. When ambient temperatures are higher than body temperatures, individuals absorb more heat, which can lead to heart and lung problems, first in vulnerable people — the very young, the very old, and those with underlying health concerns — then in everyone. In regions like South Asia, spring is when millions of farmers head outdoors to plant crops, where they can face dangerous temperatures while doing intense physical labor. Gradual exposure to heat over time can help people better withstand it, but without this familiarity, an early season heat wave can pack an unexpectedly strong punch.
Heat has a cumulative and compounding effect on the body too when it doesn't let up.
Humans acclimatize through infrastructure and behavior as well. Drinking water helps limit the dangers of high temperatures, but someone might not be in the habit of staying adequately hydrated in the spring. A person may not recognize that heavy sweating, light headedness, and severe fatigue are symptoms of heat illness.
Many buildings may still be set to heat rather than cooling when the first heat wave of the year sets in. The low availability of air conditioning in the Pacific Northwest contributed to the death toll of a severe heat wave in 2021 that killed at least 868 people.
That's not to say that mid- or late summer heat waves aren't dangerous, too. Heat has a cumulative and compounding effect on the body too when it doesn't let up. Spells of high temperatures that last weeks and persist long after the sun has set have proven deadly.
To reduce the dangers of springtime heat, it's important to pay attention to weather forecasts and prepare accordingly. That means avoiding direct sunlight, proactively staying hydrated, and taking breaks during high temperatures. Ease into the warm weather. It's also essential to recognize the warning signs of heat-related complications and not to try to push past your limits.
South Asia is a window into the future of extreme heat
The region spanning Afghanistan, Bangladesh, India, Iran, and Pakistan is home to more than a quarter of the world's population. It's also where scientists can see some of the strongest effects of human activity on temperature. 'South Asia is one of the regions where the climate change signal in heat waves is particularly strong,' Faranda said.
Along with an international team of researchers, Faranda analyzed the factors behind the recent heat wave across India and Pakistan. The group found that events like the severe heat wave in April 2025 are 4 degrees Celsius warmer over the past three decades than they were in the period between 1950 and 1986. The team controlled for other factors that influence temperatures like urban air pollution and changes in land use. The recent scorching temperatures also took place at a time when the El Niño-Southern Oscillation, a major driver of global weather variability, was in its neutral phase. In its warm phase, it tends to drive up global temperatures, so seeing this heat wave without this additional boost adds to the evidence that climate change is a major contributor.
Humidity is another important driver of heat risk. It can get pretty muggy in South Asia. One of the ways scientists track this is with the wet-bulb globe temperature, a metric that accounts for heat, humidity, and sunlight exposure. For a healthy, young person, the upper survival limit for wet-bulb globe temperature is 95 degrees Fahrenheit. Cities in Iran, India, and Pakistan now regularly cross that threshold.
Precipitation also appears to be shifting in the region, with more spells of severe rainfall followed by drought. And as the planet continues to heat up, these trends will continue. 'Future projections indicate that heat waves in South Asia are likely to start earlier in the year, last longer, and reach even higher peak temperatures,' Faranda said.
While many factors are unique about South Asia, other regions of the world are on the same course. The same pattern of more frequent heat waves earlier in the season is also playing out in countries like Indonesia, Malaysia, Thailand, and Vietnam, leading to similar problems.
'These impacts include increased mortality rate, and heat-related illnesses, disruption to local food supply and agriculture, and potential overload of power grids due to increased electricity demands,' Gianmarco Mengaldo, a professor at the National University of Singapore who co-authored the India-Pakistan heat wave analysis.
The US is also facing an increase in the number of heat waves, with warming starting earlier in the year.
The US is seeing an increase in the frequency, duration, and timing of extreme heat. Environmental Protection Agency
It's leading to more complications from extreme heat as well as leading to longer, more intense pollen allergy seasons.
Communities can take steps to mitigate the impacts of heat with design elements like green spaces and cool roofs that reflect sunlight rather than absorbing it. It's also critical to limit greenhouse gas emissions to slow the warming of the planet as a whole.