Global warming will cause more deaths in summer because of higher temperatures but these will not be offset by fewer deaths in milder winters finds an analysis published online ahead of print in Occupational and Environment Medicine.
The Harvard researchers analysed city-specific weather data related to the deaths of more than 6.5 million people in 50 US cities between 1989 and 2000.
They found that during two-day cold snaps there was a 1.59% increase in deaths because of the extreme temperatures. However, during similar periods of extremely hot weather death rates went up by 5.74%. Deaths did not rise as steeply when temperature fluctuations were less extreme.
Deaths from all causes are known to rise when temperatures go up, and heart attacks and cardiac arrests are more likely when it is very cold. It was anticipated that global warming would increase deaths during hot temperatures but that this would be compensated for by fewer deaths in the winter.
But the authors conclude: 'Our findings suggest that decreases in cold weather as a result of global warming are unlikely to result in decreases in cold-related mortality in the US. Heat-related mortality, in contrast, may increase, particularly if global warming is associated with increased variance of summer temperature.'
While all 50 US cities showed similar rises in deaths when temperatures plummeted, more deaths were seen during extreme temperature rises in cities with milder summers, less air conditioning and higher population density.
The authors suggest that this is because the use of central heating is widespread, whereas fewer people have air-conditioning in their homes.
They say: 'Central heating, which constitutes an important adaptive mechanism against cold, is almost universal in the US and this may explain why the US population seemed fully acclimatised to cold.
'Making air conditioning universally available may reduce heat-related mortality but would, on the other hand, have a perverse effect by enhancing global warming through carbon dioxide emissions from electricity consumption.'