Cities are hotter than the surrounding countryside because they receive more heat through traffic pollution.

Tarmac and concrete store up heat efficiently during daytime and release it gradually over the course of the night, whereas fields and forests give up their stored heat quickly.

This effect, called the "urban heat island," will greatly intensify if the gloomiest scenarios about global warming come true, according to Richard Betts of the Met Office's Hadley Centre for Climate Prediction.

A doubling of carbon dioxide (CO2) in the atmosphere could triple the intensity of the heat island effect, according to his study, reported in next Saturday's New Scientist.

In the case of London, there have been only 20 nights in the past 30 years when the minimum temperature failed to fall below 20 C (68 F).

Under Betts' scenario, this would happen six times more often, equivalent to an extra three nights per year, with the risk of inflicting "quite significant effects" on human health.

A prolonged heatwave in Europe last August caused at least 20,000 deaths, especially among elderly people.

Global warming is the term used for a gradual heating of the Earth's surface by "greenhouse" gases -- carbon pollution from oil and other fossil fuels that traps the Sun's heat instead of letting it radiate safely back out into space.

CO2 concentrations stand at 379 parts per million (ppm), according to measurements taken in March at a US observatory on Hawaii, which put the year-on-year increase at three ppm. This compares with the yardstick of 280 ppm of pre-industrial times.

SPACE.WIRE