Dangerously hot days are set to become more scorching by 2100 because of climate change with the U.S. Midwest or the Mediterranean region sizzling well above 40 degrees Celsius (104F), Dutch scientists said on Wednesday.

They said the likely jump in temperatures on the hottest summer days would far outpace the average of year-round global warming this century projected by the U.N. Climate Panel. Heatwaves can be a big threat to human health.

"The extremes warm faster than the averages," said Rob van Dorland, a spokesman for the Royal Netherlands Meteorological Institute which wrote the study with scientists at Utrecht University.

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If world temperatures rose on average by perhaps 3 Celsius (5.4F) by 2100, the temperature on the most sweltering day of the year could leap by up to 8 Celsius (14.4F), he said.

Computer projections indicated that temperatures would hit baking peaks above 50 Celsius (122.00F) in parts of Australia, India, the Middle East, North Africa, the Sahel and equatorial and subtropical South America by the 2100.

"In much of the United States, in southern Europe and in the populated regions of Australia, values far exceeding 40 Celsius are reached," the scientists wrote in the journal of the American Geophysical Union.

"Such temperatures, if lasting for some days, are life-threatening and receive relatively little attention in the climate change debate," they wrote.

A heatwave in Europe in 2003 killed 15,000 people in France and almost 3,000 in Italy, mostly elderly people who are often most at risk. The Dutch report did not examine likely impacts on public health, or food production.

The U.N. Climate Panel projects that rising temperatures will cause more heatwaves, droughts, floods, contribute to spread disease and melt glaciers, pushing up world sea levels.

Dorland said it was unclear why extreme temperatures would rise so fast -- a theory was that warmer soils dry out, reducing the amount of heat they can absorb from the sun. "If the soil is drier, then solar energy goes more to heating the air," he said.

And a shift in wind patterns could also bring more sweltering conditions, for instance around the Mediterranean. "In the Mediterranean area we expect more southeasterly winds in summer...that means more sun and less cloud," he said.