EU: Heatwaves like those that have scorched Europe and the US in recent weeks are becoming more frequent because of global warming, say scientists who have studied decades of weather records and computer models of past, present and future climate.
While it is impossible to attribute any one weather event to climate change, several recent studies suggest that human-generated emissions of heat-trapping gases have produced both higher overall temperatures and greater weather variability, which raise the odds of longer, more intense heatwaves in the future.
Last week, Paul Della-Marta, a researcher at Switzerland's Federal Office of Meteorology and Climatology, presented findings at an international conference on climate science in Gwatt, Switzerland, showing that since 1880 the duration of heatwaves in western Europe has doubled, and the number of unusually hot days in the region has nearly tripled.
In a separate 2004 study, researchers at Britain's Hadley Centre for Climate Prediction and Research produced computer models showing that greenhouse gas emissions had doubled the likelihood of events like the lethal 2003 European heatwave, and that by 2040 it is likely such heatwaves will take place in Europe every other year.
And researchers at the National Climatic Data Centre in Asheville, North Carolina, reported this week that night-time summer temperatures across the US have been unusually high for the past eight years, a record streak.
Richard Heim, a research meteorologist at the centre, said that only the "dust bowl" period of the mid-1930s rivalled the sustained heat levels of recent US summers.
Drew Shindell, an atmospheric physicist at Nasa's Goddard Institute for Space Studies, said the European findings are especially significant because they draw on long-term surface temperature records.
"The European records, being so long, make a convincing case that we're already seeing changes" in the climate, Shindell said. "This is not like 'centuries from now the ice sheets will melt'. This is 'in a few decades it will be dramatically different'. To me, that's alarming."
Kevin Trenberth, chief of the climate analysis branch of the National Centre for Atmospheric Research in Colorado, said, "There are very good reasons to believe that the current US heatwave is at least partly caused by global warming."
Trenberth pointed to a study published in March by the Journal of Geophysical Research that showed that for more than 70 per cent of the land researchers had surveyed worldwide, the number of warm nights each year had increased and the number of cold nights had declined, between 1951 and 2003.
The researchers, led by Hadley Centre scientist LV Alexander, concluded: "This implies a positive shift in the distribution of daily minimum temperature throughout the globe."
Most experts said it is important to pay attention to the high temperatures that have blanketed the US and Europe over the past few years. Last month the US National Oceanic and Atmospheric Administration reported that the first six months of 2006 were the hottest on record in the United States, and last month ranks as England's hottest July since record-keeping began in 1659.
"The trend lines showing so much hot weather in recent years suggests some concern, even if we can't say definitively this is a signal of climate change," said Daniel Esty, a professor of environmental law and policy at Yale University.
Scientists and public health officials said they are particularly worried about an increase in summer night-time temperatures because people tend to recover from excessive heat exposure at night.
Joel Scheraga, national programme director for the US Global Change Research Programme, has suggested that with increasing temperatures and population growth, deaths from extreme heat or cold could as much as triple in major American cities from 1993 to 2050.
However, some climate experts and industry lobbyists question the correlation between global warming and heatwaves.
Konstantin Vinnikov, a senior research scientist at the University of Maryland, said he expected climate change would only have a minor effect on future scorchers.
"These are events that have happened in the past and will happen in the future. Climate trends related to climate change cannot change it too much," Vinnikov said.
Bracewell & Giuliani lobbyist Frank Maisano, who represents US coal-fired power stations, sent an e-mail to reporters this week noting that more than half of the days with temperatures at or above 100 degrees Fahrenheit in the Washington-Baltimore region occurred between 1874 and 1934. - (LA Times-Washington Post service)