According to a frightening new study, global warming will cause more deaths from heat in future summers, but these deaths will not be offset by fewer deaths from cold in the milder winters to come.

When researchers from Harvard analyzed the weather-related deaths of more than 6.5 million people in 50 US cities between 1989 and 2000, they found that two-day cold snaps caused a 1.59% increase in deaths, but that during spells of extremely hot weather death rates went up by an astronomical 5.74%.

Deaths from all causes — not only heat-related deaths — rise when temperatures go up, while heart attack and cardiac arrest become more common when they go down. Experts have long predicted that global warming would mean more deaths during hot weather but that this would be compensated for by fewer deaths from cold.

The researchers, who published their findings online ahead of print in July 2007 issue of Occupational and Environment Medicine, 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 U.S. Heat-related mortality, in contrast, may increase, particularly if global warming is associated with increased variance of summer temperature."

While all U.S. cities showed similar rises in deaths when temperatures dropped, more deaths were seen during heat waves in cities with milder summers, less air conditioning and higher population density.

The researchers suggest that this is because the use of central heating is widespread, while, in contrast, fewer people have air-conditioning in their homes. And, the Harvard researchers declare, "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."