Researchers are predicting that the most extreme heatwaves could more than double in size by 2050 if greenhouse gas targets aren’t met.
In turn, that could mean more illness due to heat stress and even more demand on electricity to power air conditioning, they have warned.
Brad Lyon, Associate Research Professor at the University of Maine and lead author of the new paper, said: "By mid-century, in a middle greenhouse emissions scenario, the average size of heat waves could increase by 50%.
"Under high greenhouse gas concentrations, the average size could increase by 80% and the more extreme heatwaves could more than double in size. As the physical size of these affected regions increases, more people will be exposed to heat stress."
He added:"Larger heatwaves would also increase electrical loads and peak energy demand on the grid as more people and businesses turn on air conditioning in response."
The study, published in the journal Environmental Research Letters, looked at how heatwaves move around - changing in size and strength over their lifetime. It is the first study to look at their size.
Prof Lyon said: "What is new in our study is the way we calculated them, which allowed us to consider size as a new heatwave dimension.
"Heatwave size is another dimension of extreme heat that people don't necessarily think of. It's a different vantage point from which to view them and assess their impacts."
The study comes after figures from climate experts at Berkeley Earth in California found that 29 countries across the world experienced their hottest temperatures on record between May and August this year.
A total of 396 heat records were broken in the northern hemisphere with Germany, France and the Netherlands experiencing the most days of record heats.
Figures from climate experts found that monthly highs were recorded in 1,200 separate instances north of the equator.