 By KEAY DAVIDSON San Francisco Chronicle July 26, 2006
But this week - as many people flee to air-conditioned theaters to watch Al Gore's global warming film, "An Inconvenient Truth" - the latest sweltering weather is starting to look to many like a calling card of global warming.
Some of the nation's top climate experts also believe the heat wave is caused at least partly by global climate change. Others, however, disagree and say it's still too early to blame the current weather on the planet's changing climate. How hot is it? The first six months of 2006 were the warmest of any year in the United States since record keeping began in 1895, according to the National Oceanic and Atmospheric Administration's National Climatic Data Center. In Northern California, the weather has been hot enough to drain power supplies, dry up streams and contribute to several deaths. "I think there are very good reasons to believe that the current U.S. heat wave is at least partly caused by global warming," Kevin Trenberth, one of the nation's top global-warming computer modelers, wrote in an e-mail. In recent years, studies by several scientific teams show that "the frequency of cold nights dropped everywhere, and warm nights increased everywhere" around the world, said Trenberth, a scientist for the Climate Analysis Section of the National Center for Atmospheric Research in Boulder, Colo. "Heat waves have also increased most places around the world." A noted atmospheric scientist and climate modeler, Govindasamy Bala of Lawrence Livermore National Laboratory, agreed. "It is true that the current heat wave could have occurred by chance. But I believe that the likelihood of such occurrences increases due to global warming," Bala said. Yet there are doubters - for example, James O'Brien, Florida's state climatologist. O'Brien criticized colleagues who he thinks are too quick to link short-term and long-term weather. He recalled that in 1988, "we had a big Midwest heat wave ... which (NASA scientist) Jim Hansen told the U.S. Senate was due to global warming." Instead, O'Brien said, the heat wave was caused by high sea-surface temperatures in the tropical Pacific. Likewise, he said, during another recent heat wave, "they said that many people died in Chicago due to this global warming. In fact, it was due to old, poor people not being advised about (how to survive) the heat wave." Also cautious is Philip Klotzbach, an atmospheric scientist at Colorado State University: "Heat waves have happened for many years (i.e., the Dust Bowl in the 1930s), so to say that this one particular event is caused by global warming is really impossible," he wrote in an e-mail. Chris Field, director of the Department of Global Ecology at the Carnegie Institution of Washington's branch at Stanford University, said scientists can't attribute singular weather events to global warming. But many studies conclude that heat waves tend to get hotter as the planet warms. "This week's heat wave might or might not have occurred without global warming, but it is a good bet that heat waves will be hotter and more frequent in the warmer world," Field said. Michael Mann, a leading global warming expert at Pennsylvania State University, agreed, saying climate change is "stacking the deck" and making heat waves more likely. "As we see more and more such record-breaking extremes," Mann said, "we can increasingly implicate climate change for the shift. This holds for heat waves, droughts and intense tropical storms." One thing that scientists tend to agree on is an expectation of more extreme weather as global warming continues. "What is worrisome," said Claudia Tebaldi, a climate statistician who works at the Boulder research center, "is that climate models all agree on the intensification of heat waves in the future."
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