Europe's summer of 2003 seared itself into the record books as the hottest, deadliest summer the continent has endured in at least 500 years. Temperatures in Paris topped 104 degrees. Even nightfall brought little or no relief.
Now, a new analysis from researchers at the Hadley Center for Climate Prediction and Research and Oxford University in Britain suggests more than half of the risk that the heat wave would occur can be traced to human influence on climate.
If concentrations of heat-trapping "greenhouse" gases from power plants and factories continue to increase, even at a modest pace, they say, by 2040 more than half of Europe's summers are likely top those record temperatures of 2003. By 2100, the summer of 2003 could even stand as an unusually cool one.
The study is hitting the streets on the eve of international talks on climate change scheduled to begin Dec. 7 in Buenos Aires. Over two weeks, delegates hope to nail down their countries' individual plans for implementing the 1997 Kyoto Protocol, which takes effect in February following Russia's recent ratification of the agreement. But talks also will discuss efforts to adapt to climate change, as well as explore ways to bring more countries - including the United States - back into the process.
Shortly after taking office, the Bush administration rejected the Kyoto process, which set specific targets and timetables for reducing carbon-dioxide emissions. Instead, the White House elected to try to reduce the economy's "greenhouse gas intensity" - a measure of the amount of greenhouse gases emitted for a given level of economic growth, which can allow emissions to rise.
It is on the issue of adaptation - including the question of who pays to help poorer countries adjust to changes - that the Hadley study is expected to create the most buzz in Buenos Aires.
As similar "attribution" studies are conducted and refined, and climate models improve in their ability to capture changes over smaller patches of the planet and shorter time periods, "very soon, it might be possible to sue for damage caused by climate change," suggests Daithi Stone, an Oxford University climate researcher who was part of the team reporting the results.
Others doubt that direct suits will be possible, since it will be difficult to establish at any given time whether methane from Australia or carbon dioxide from North America holds sway over climate. Instead, liability for climate-change-related damage or adaptation costs may be reflected more immediately in insurance premiums, decisions to provide coverage at all in some areas, and in shareholder actions against corporate CEOs, says John Stanton, an attorney with the National Environmental Trust, an environmental organization in Washington, D.C.
Still, the research, published in Thursday's edition of the journal Nature, represents a breakthrough, according to Swiss climate researchers Christoph Schar and Gerd Jendritzky. They see the Hadley team's work as "the first successful attempt to detect man-made influence on a specific extreme climate event."
In the past, the best climate scientists could say about current extreme weather was that events were "consistent" with forecasts from sophisticated computer simulations of climate change. Even in the US, federal estimates of the effects of global warming have focused on the future, indicating an increased likelihood that extreme summer heat waves could strike cities in the Midwest and East Coast. But no study so far has been able to tie a recent extreme climate event to human influence, researchers say.
The new study, led by the Hadley Center's Peter Stott, does not argue for an ironclad cause-effect relationship between the heat wave and humanity's contribution to global warming. The researchers note that the atmosphere is so chaotic that no simple cause-effect link can be drawn.
But they say it is possible instead to estimate the added risk for unusually hot summers because of human influence on climate. Using simulations of 20th-century climate with and without the Industrial Revolution's CO2, sulfate aerosols, and other gases, and accounting for changes in solar radiation, volcanic eruptions, and other natural factors that prod the climate to change, they not only noted an increase in average temperatures during July and August in Europe over the past 50 years. The results allowed them to calculate the added risk for extremely hot European summers from human-influenced climate change.
What Dr. Stone found surprising, he says, is the strength both of the human signature and of the statistical measure of confidence in the results - essentially 90 percent. "We've got a good handle on the sources of uncertainty in the model runs," Stone says. "And when we take these into account, they don't change the result substantially."