Extreme heat is bleeding into spring and fall — and it's harder on the body for it

Summer heat is no longer staying in summer. A new study finds that extreme heat seasons have expanded significantly across more than half of global land areas — not by making summers hotter in the middle, but by pushing dangerous heat earlier into spring and later into fall, the times of year when people and communities are least equipped to handle it.

We found that over more than half of global land areas, extreme heat seasons are expanding significantly.— Casey (Catherine) Ivanovich, climate scientist, Columbia University and lead author

The expansion is not uniform. In eastern China, northern Africa, and eastern Europe, extreme heat events became more common more rapidly in the two months after the historical heat season ends. In western Europe, southern Africa, and northwestern India, the bigger shift is in the opposite direction — heat arriving in the two months before the traditional season begins, according to USA Today.

For the United States, the pattern tilts toward fall. In much of the country, the extreme heat season is growing more into autumn than into spring — a timing that creates a dangerous overlap with other seasonal crises.

In much of the U.S., we're seeing a larger expansion of the extreme heat season into the fall than the spring. This can unfortunately increase the overlap of extreme heat with other seasonal hazards, such as the wildfire season in the West and the Atlantic hurricane season in the East.— Casey Ivanovich, Columbia University

When multiple hazards hit at once or in quick succession, Ivanovich said, they are much more dangerous than if they occurred in isolation. A heat wave layered on top of a wildfire or a hurricane compounds the strain on emergency systems, infrastructure, and human bodies already under stress.

The body's vulnerability to heat is not constant — it depends heavily on timing. Spring heat waves arrive before people have acclimated to warm weather or set up cooling strategies like air conditioning. Fall heat waves are dangerous in a different way: they follow months of repeated heat exposure that can leave the body already depleted. The Columbia Climate School, which is affiliated with the NASA Goddard Institute for Space Studies, noted both dynamics in a statement accompanying the study.

Heat waves in the spring can catch us off guard before our bodies have acclimated to the heat and before we're ready to utilize cooling strategies like installing air conditioning. In the fall, heat waves can be more dangerous if we've already experienced repeated extreme heat events that leave our bodies stressed.— Casey Ivanovich, Columbia University

The preparedness gap is institutional as well as physical. Cooling centers, heat alerts, and public health resources are typically organized around a summer calendar. Heat arriving outside that window can fall through the cracks of systems not designed to catch it.

The study also broke down heat into two distinct types — dry and humid — because they pose different risks and require different responses. Dry heat is harder on crops and ecosystems; humid heat is more dangerous to people because it limits the body's ability to cool itself through sweating. The analysis found that dry-heat seasons had expanded significantly across just over half of global land area, while humid-heat seasons had expanded across just under half.

While recent papers have explored changing average temperatures and how the start and end of the summer season are changing, our study is the first to our knowledge to explicitly measure the change in the timing of extreme heat.— Casey Ivanovich, Columbia University

On the question of cause, Ivanovich was careful but direct: the researchers cannot confirm the precise share of the signal attributable to climate change using observations alone, but rising temperatures associated with climate change are, according to the study, likely the primary driver pushing extreme heat into new times of year. Future research, she said, will work to quantify the roles of climate change, land use change, and natural climate variability separately.

Separate analysis from Climate Central found that carbon pollution drove risky heat across the globe and in the U.S. during the summer, adding real-time context to the study's longer-term findings.

Why it matters — As extreme heat spreads into spring and fall, the systems communities rely on — cooling centers, heat alerts, emergency response — are built around a summer calendar that no longer matches the reality of when dangerous heat arrives.

Reported by usatoday.com, climatecentral.org