OSLO (Reuters) - People in cities around the Mediterranean including Athens, Rome and Marseilles are likely to suffer most in Europe from ever more scorching heatwaves this century caused by climate change, scientists said on Sunday.

The number of heatwaves was likely to surge to almost 3 each summer from 2071-2100 in the Mediterranean region from just one every third year from 1961-1990, it said. Most other parts of Europe would suffer far less.

The number of Mediterranean summer days with temperatures above 105 Fahrenheit (40.6C), a threshold in the United States for public health warnings, would rise to about 16 a year from 1.6 in the same period.

Heat-related health problems would be felt most by people living near the coast or in low-lying river valleys, according to scientists in Switzerland and the United States writing in the journal Nature Geoscience about health and heat projections.

“Some of the most densely populated European regions, such as the urban areas of Athens, Bucharest, Marseilles, Milan, Rome and Naples, would experience the severest changes in health indicators,” they wrote.

About 40,000 people died in an extreme heatwave in Europe in 2003. But Erich Fischer, lead author of the study at the Institute for Atmospheric and Climate Science at ETH Zurich, said it was uncertain how deadly future heatwaves would be.

Air pollution might aggravate health risks for people with respiratory or heart problems in hotter temperatures, he said. And he said the study did not consider that cities can act as “heat islands” -- often warmer than surrounding countryside.

On the other hand, improved weather forecasts can help ensure that people at risk -- especially the elderly and the very young -- stay in the shade and drink more on hot days. And air conditioning might become more efficient and widely used.

“People living in Arizona show that you can adapt to heat,” Fischer told Reuters. In such hot climates, people avoid straining themselves outdoors at the hottest part of the day.

He said the study was the first to pinpoint areas of Europe where rising temperatures would coincide with rising humidity, high night-time temperatures and long-lasting heatwaves -- all factors that can aggravate health problems.

Global warming will mean more moisture in the air from the Mediterranean, for instance, making it harder for people to sweat away excess heat. High night-time temperatures can make sleep harder.

“We see the strongest increases in the number of these days with dangerous health conditions ... all along the coast of the Mediterranean and in low-altitude river basins, such as the Po or the Danube,” he said.

The study defines a heatwave as at least 6 days in a row with temperatures among the hottest 10 percent of those recorded in the region for those dates. That means that a heatwave in Greece is hotter than one in Scandinavia.