A "heat season" used to be a straightforward concept: put away the sweaters, toughen up for a couple of harsh seasons, and then chill. That's a theory that's quietly falling apart. A new study published this week in the journal AGU Advances, led by NASA climate scientist Catherine Ivanovich, with colleagues at NASA's Goddard Institute for Space Studies and New York University, reveals that dangerously hot days are not just confined to the calendar time that everyone expects. They are creeping into spring in some places and running late into autumn in others and the trend is not just a matter of increasing average temperatures.

The research team scanned 45 years of temperature data on six continents that are inhabited, comparing the 80s decade with the latest decade of 2015-24. They measured extremes in two ways, plain dry-bulb heat (the number on a thermometer) and wet bulb globe temperature (which also includes humidity and a better indication of what the human body actually suffers). In about 50% of the land surface of the world, the time window of exposure to extreme heat has increased significantly since the base decade for dry heat, and in less than 50% for humid heat.

The discovery is not only alarming because of the heat spreading beyond summer. It's the asymmetry. Now, in the western U.S. and eastern China, northern Africa and eastern Europe, punishing heat is emerging in the traditional cooling down months - and it's a surprise. The opposite is occurring in western Europe, southern Africa and northwestern India: extreme heat is coming earlier, before people can acclimatize and before the cooling centers and heat-alert systems can be activated.

The story of Phoenix is a dramatic one with a before and after. The city has seen the number of extreme heat days increase by a nearly twofold increase from 183 extreme heat days in the 1980s baseline decade to 338 in the 2015-24 decade. No extremes of the 1980s happened after the heat season ended. By 2015-2024, six percent did. In the past March, Phoenix recorded nine days in a row above 100 degrees F, compared to just one day in the historical record.

Those statistics aren't abstract; they represent the cost of human life. In 2024 alone, there were 608 heat-related deaths in Phoenix's home county of Maricopa. Almost half of the incidents happened in the summer, when everyone is on holiday and the fire service's attention is diverted, but the remainder were spread throughout the rest of the year. It's that scattering that is the danger Ivanovich's team is warning of: heat outside its normal range is more damaging to the body because it hasn't been exposed to it throughout the year; it's also more detrimental to public health systems, which are often staffed and funded to operate on a summer-only schedule.

This isn't just a scholarly "what if.It is not a ‘what if’ academic curiosity. Researchers at the Indian Institute of Technology, Gandhinagar, in India, and also with Stanford and Purdue, have independently found that the amount of monsoon-season humidity will amplify the country's window of "uncompensated heat stress," when the body can no longer cool itself by sweating, well beyond the pre-monsoon summer peak. In a parallel study of North Africa and Middle East, regional climate models simulations to the end of the century suggest the hot-day season will increase in length, mostly in spring, to half of the calendar year. Each region had different mechanisms and different regional characteristics but the same message: a lengthening of heat seasons which is not uniform.

Most importantly, the Columbia/NASA group did a test to see if what they saw could be explained by simple global warming, that is, by a warming in which temperatures rise in the same way everywhere on the globe. It did a good job of explaining the increased intensity that formed the heart of each region's traditional heat season; but it failed to explain how the edges were growing rather unevenly. That suggests that regional drivers also exist, such as changes in rainfall, land use, and atmospheric circulation, in addition to the overall warming.

The researchers are wary of going too far, since by definition extreme heat events happen relatively infrequently, and extreme heat without its typical season of the year is even more rare. What they next need to do is simulate the same comparison in a series of climate models that create numerous climate variants of the same climate, thus expanding their set of extreme events.

The economic side of the story is as important as the other: heat's growing footprint is already reflected in the balance sheets. The London School of Economics' Grantham Research Institute estimated that one heatwave during June this year, in Britain, cost the economy over £1.15 billion in lost GDP, as a result of lower productivity in terms of hours worked and lost working time, and the well documented drop in effort of those who work outdoors and in physically demanding jobs.

If there is no policy action, a separate UK think tank estimates that cumulative losses due to heatwaves could cost £25 billion by 2030. With a model that assumes a fast-rising temperature, Austrian economists calculate that real GDP losses could be around 0.7 percent in three years billions of euros of lost production, mostly in construction, agriculture and manufacturing. As of 2020, heat exposure is estimated to cost the U.S. economy about $100 billion each year, and this could swell to $500 billion per year by 2050, some researchers project. None of these estimates are quite yet taking into consideration a heat season that is not behaving as it did before, indicating the true cost is likely being under estimated in insurance pricing, agricultural planning and budgets for urban infrastructure.

There is also a compounding-hazard issue that few policymakers have taken into account. The combination of those two events brings even more risk than either alone, as when extreme heat infiltrates the wildfire season in the American West, or the hurricane season in the Southeast. Emergency planning around a single hazard plan is now under threat from multiple simultaneous threats.