Extreme heat events have expanded into spring and fall across roughly half of the world’s land area since the 1980s, according to a study published in AGU Advances.
Global Climate Data Reveals Expanding Extreme Heat Seasons
Extreme heat is no longer just a mid-summer phenomenon. A recent study analyzing 45 years of global climate data shows that hot days have expanded into spring and fall across half of the world since the 1980s. While summer remains intensely hot—with the National Oceanic and Atmospheric Administration announcing that the June-to-August window was the warmest in 132 years of record-keeping across the Lower 48 states—dangerous temperatures now arrive earlier and linger longer.
Previous climate research largely focused on average temperatures rather than the specific timing of extreme weather events. To bridge this gap, a team of researchers set out to examine dangerous heat thresholds throughout the calendar year. They compared baseline extreme heat figures from 1980 through 1989 against the most recent decade on record, spanning 2015 to 2024.
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Lopsided Temperature Shifts Across Continents
The expansion of extreme heat is lopsided across the globe, affecting dry and humid heat differently. Data gathered by scientists revealed that periods of severe dry heat grew significantly across slightly more than half of the globe’s terrestrial surface, while humid heat periods lengthened over just under half.
The timing of these extensions varies sharply by region:
- Western U.S., eastern China, northern Africa, and eastern Europe: High temperatures arrived with greater frequency and speed during the 60-day window following the summer season.
- Western Europe, southern Africa, and northwestern India: Extreme heat events pushed backward into the two months prior to summer.
“There are very clear asymmetries in how extreme heat seasons are expanding in different parts of the world,” as stated by the lead author of the new research, NASA Goddard Institute for Space Studies climate scientist Catherine Ivanovich, during a media briefing. “Extreme heat is starting to become something different in a lot of these regions.”
Why Unseasonal Weather Catches Communities Off Guard
Climate simulations tested whether ordinary warming alone could produce this pattern, but models only accounted for changes in the heart of the heat season rather than the lopsided shifts at its edges. Unusually warm conditions outside the core summer months remain rare statistically, making exact causation difficult to pin down.
Regardless of the underlying trigger, shifts in seasonal timing carry severe consequences. Extreme heat poses heightened dangers when it arrives before populations can acclimate or lingers after months of scorching weather. Furthermore, municipal protections like cooling centers and heat alerts are traditionally built around a rigid summer calendar, leaving residents vulnerable when high temperatures strike unexpectedly.
“We can’t confirm what share of the signal is due to climate change using observations alone, [but] it’s certainly the primary component of the story,” Ivanovich said.
Frequently Asked Questions
Are extreme heat seasons expanding everywhere in the world?
No. The study found that extreme heat seasons expanded across just over half of the world’s land area for dry heat and just under half for humid heat, with distinct regional asymmetries.
Which regions experience extreme heat earlier in the year?
According to the research, western Europe, southern Africa, and northwestern India experienced extreme heat events pushing into the two months prior to summer.
What causes the lopsided expansion of extreme heat?
Researchers note that unusually warm conditions outside the core season are rare and difficult to pin down statistically. Climate simulations account for warming in the heart of the heat season, but cannot fully replicate the lopsided shifts at the edges using observations alone.
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