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Is the recent heat wave a clue to global warming?
San Francisco Chronicle


July 31, 2006

Thermometers have spiked throughout much of the United States, Canada and Europe, and scientists are predicting more intense, longer and more frequent heat waves in the future.

While leading climate scientists have been reluctant to link regional heat waves with rising temperatures in the world's atmosphere and oceans, they say the recent weather patterns are consistent with computer projections for global warming.



In the United States, the first six months of 2006 were the hottest recorded in more than a century, according to the National Oceanic and Atmospheric Administration's National Climatic Data Center. Canada reported the hottest winter and spring since it started keeping track about a half-century ago, while England, Germany and France are sweltering, and the Netherlands is recording the hottest month since temperatures were first measured 300 years ago.

"The current heat waves throughout much of North America and Europe are consistent with the predictions of our global climate models," said physicist John Harte, a professor and researcher in the University of California's Energy and Resources Group and the Ecosystem Sciences Division.

"In the future, we can expect more intense, more long-lasting, and more frequent heat waves as a consequence of global warming. If you warm the planet as a whole, as we've been doing, it's likely that any particular heat wave is going to be hotter with global warming, and any hurricane will be more intense. You warm the whole, and you warm the parts," Harte said.

Temperature trends for the rest of the world won't be known until the end of the year.

Every hot spell can't be blamed on global climate change, scientists say. But the planet's dramatic warming over the past 50 years has made matters worse, and the continuing rate of discharge from carbon dioxide, methane and other heat-trapping gases will keep raising the world's temperatures throughout the rest of the century and beyond, they say.

"But you can't predict where those parts will be," said Harte.

On a larger scale, the current pattern of warming is consistent with what scientists have projected would happen in global climate models, as an oversupply of greenhouse gases from industrial and traffic sources trap some of the heat that once radiated into space.

The warming has been the greatest in the Arctic regions, particularly Alaska, Siberia and Scandinavia, as melting ice and snow reflect less sunlight back into the atmosphere and expose more land to heat and warmth. Antarctica also has warmed. Within the United States, the warming is greater in the West than in the East.

"This is expected," said James Hansen, director of the National Aeronautics and Space Administration's Goddard Institute for Space Studies. The subtropics, which include the American Southwest and the Mediterranean regions, become hotter and drier with increasing greenhouse gases, he said.

"Weather will fluctuate a lot from year to year. But the situation this year is of the nature of the expected trend. So get used to it," Hansen said.

While most global-climate modelers say they can' t yet correctly predict patterns of regional climate change, they say high latitudes should warm more than low latitudes, and the land should warm more than the ocean, Hansen said. The higher temperatures being recorded around the world are consistent with those predictions, he said.

During the past century, global surface temperatures have increased 1.2 degrees Fahrenheit, according to weather records. But within the century, the past 25 to 30 years have been even hotter, leading scientists to calculate that over a century the rate would be 3.2 degrees, with some of the highest increases occurring in the high latitudes, according to NOAA's National Climatic Data Center.

Three degrees might not seem like much, but it represents more than the annual average temperature difference between San Francisco and Santa Barbara, said Christopher Field, director of the Department of Global Ecology at the Carnegie Institution of Washington's branch at Stanford University.

Most of the world's leading climate experts agree that the current warming is not part of Earth's natural fluctuations. They base conclusions on studies that assess carbon dioxide and temperature levels from borings of Antarctic ice cores carrying trapped gaseous bubbles. The cores date 650,000 years, covering six periods of ice ages and warming periods.

Carbon dioxide in the atmosphere has increased 35 percent since before the Industrial Revolution, from about 278 parts per million to 378.9 parts per million in 2005, the highest level on record.

Harte, in collaboration with Margaret Torn, a staff scientist at the Lawrence Berkeley National Laboratory, added insight to global climate models in May in a paper based on the ice-core data and published by the American Geophysical Union.

They concluded that temperatures by the end of the century will be even hotter than the models currently predict because of the heretofore uncalculated feedback effects of global warming brought about by increasing amounts of greenhouse gases.

If emissions from the burning of fossil fuels continue as expected, and carbon dioxide levels reach 560 parts per million by 2060 as projected, temperatures could increase by 12 degrees Fahrenheit above pre-industrial global temperatures, the study found. Current models project a rise of 8 degrees.

"These findings add to the sense of urgency that we do something about the problem," said Harte. "The predicted warmer future is not inevitable."

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