ROME - As large parts of Europe and North America once again bake in an exceptionally hot summer, many people are asking what has now become a perennial question: "Is this global warming?"
The heat has already killed at least 21 people in France, including a 15-month-old baby, prompting fears of a repeat of the European heat wave in 2003 in which at least 15,000 people in France and 20,000 in Italy died.
Large parts of the United States and Canada have also seen record high temperatures this month. "We are cooking," said U.S. meteorologist Dennis Feltgen of the National Weather Service.
Many scientists reckon the globe is warming and will continue to do so due to the "greenhouse effect" caused by emissions from fossil fuels trapping heat in the atmosphere. But they say we should not read too much into a single hot spell.
"As ever, you cannot say any one weather event is caused by global warming ," said Asher Timms of Britain's Tyndall Centre for Climate Change Research. "But globally, it seems that there's quite a shift in our weather patterns."
Sceptics of the global warming theory, which predicts droughts and floods this century unless greenhouse gas emissions are curbed, say the media play up hot summer days for dramatic effect.
Bill O'Keefe, a board member of Washington think tank the George C. Marshall Institute and a consultant to the oil industry, said the record heat could be seen as part of a natural cycle of highs and lows.
"I don't think there is any climatologist or meteorologist that would say you could draw a conclusion about any given year. There have been hotter periods in the past and we will have them in the future," O'Keefe told Reuters.
"If this persisted for a very long time than you might be able to conclude that human activities had an impact."
But many scientists say a warming trend is already clear.
U.S. space agency NASA says 2005 was the warmest globally in more than a century and that the preceding three years were also the warmest since the 1890s.
The U.S. National Climatic Data Center said the first half of 2006 were the warmest six months since records began in 1895.
"NASA's averages for the world and what we produce here are far more informative than looking at the extremes in Britain, France or Italy like the summer of 2003," said Philip Jones, climate research professor at Britain's East Anglia University.
"It's the global averages that count."
"Ten of the last 12 years were the warmest since 1850. The global temperature (since then) rose 0.7 degrees Celsius and most climate models suggest it's going to continue to warm by 2 to 5 degrees Celsius this century," Jones said.
The sceptics say any warming over the last century can be explained by the fact that the planet was coming out of a cold period, known as the "Little Ice Age" and not due to a massive build-up of greenhouse gases after the Industrial Revolution.
Jones said the rate of temperate change in the 20th century was at least three times that in other centuries -- evidence that this was more than a natural cycle.
And although a couple of hot summers do not prove climate change, if global warming is happening, heat waves are inevitable. "Obviously. if you continue to warm the climate you get more extremes at the end of the spectrum," said Jones.
More Sea, Less Snow
There is other evidence, aside from temperature figures, to suggest the Earth is warming , particularly a rise in sea levels, believed to be due to water expanding as it warms and additional melting mountain glaciers adding to sea volumes.
Globally, sea levels are rising by around 1.5 millimetres (0.06 inches) per year and have risen some 20 centimetres (7.8 inches) since the late 1800s, Jones said.
Less snow on mountains, changes in precipitation patterns and an increase in the intensity of hurricanes, possibly due to warmer seas, are also potential evidence of global warming .
Because of the many uncertainties in climate change science, a United Nations body was established in 1988 to collate and check the data and its findings are a major influence on politicians deciding what to do on climate change.
The Intergovernmental Panel on Climate Change will publish its fourth report next year, updating a 2001 paper which said temperatures were set to rise by 1.4 to 5.8 degrees Celsius from 1990 to 2100 with potentially devastating consequences.
Professor Jones, who is working on the report, declined to predict its finding, but suggested it would be one more step in hardening the evidence for global warming.