Climate scientists surer than ever: Humans to blame

By Sandi Doughton
The Seattle Times

The distinction between “likely” and “very likely” might seem slight to most people, but to climate scientists, it’s huge.

In 2001, an international panel charged with reviewing all the evidence for and against global warming said emissions from power plants, cars and industries were likely contributing to rising temperatures around the world.

Today in Paris, the influential group went further, declaring it very likely that human activity is to blame.

“This says we are 90 percent sure the bulk of global warming is caused by [man-made] greenhouse gases,” said Thomas Ackerman, director of the University of Washington’s Joint Institute for the Study of the Atmosphere and Oceans. “We’ll leave a little wiggle room because we’re scientists and that’s the way we think, but 90 percent is about as good as it gets.”

Eleven of the hottest years on record have occurred in the past 12 years, and the convergence of melting glaciers, vanishing permafrost and rising sea levels cannot be explained by natural climate variation, according to a draft of the report that concludes: “Warming of the climate system is unequivocal.”

Intergovernmental Panel on Climate Change:

The draft says the future holds parched conditions for Africa, much of South and Central America and the American Southwest, while winter rains will intensify, replacing snow across more northern latitudes — including the Pacific Northwest. Cold snaps will become rare and stronger rainstorms more common.

The new report is the fourth from the Intergovernmental Panel on Climate Change, established in 1988 by the United Nations and the World Meteorological Organization. As the most forceful consensus yet on the perils of global warming, it could sway governments and industries. And it comes at a time when the issue is receiving unprecedented attention in the United States.

Though his administration has so far opposed emission limits, President Bush acknowledged “the serious challenge of global climate change” in his State of the Union address. Congress convened hearings this week and is considering several bills to cap emissions. Hollywood has taken notice, with two Oscar nominations for former Vice President Al Gore’s movie, “An Inconvenient Truth.”

States and cities, including Seattle, are enacting their own measures to scale back carbon dioxide, the primary heat-trapping gas. And last week, the leaders of 10 major corporations, including General Electric and oil giant BP America, called for mandatory emission limits.

Based on more than five years of additional data and study, the new report warns that even if emissions level off, warming and sea-level rise will continue for centuries. The world is already experiencing droughts, drenching rains, tidal surges, heat waves and more intense hurricanes that “more likely than not” are the result of global warming, the draft says.

Scientists say complex computer models have accurately predicted most of the climate change so far, increasing their confidence in future projections. They’ve also been able to tease out evidence of global warming on smaller scales than ever before, showing that every continent except Antarctica has heated up.

Carbon-dioxide levels in the atmosphere are higher than any time in the past 650,000 years, according to studies of ice cores, and average global temperatures have increased about 1.3 degrees F since the dawn of the industrial age. Sea level rose nearly 7 inches during the 20th century, and the rate of rise has more than doubled in the past decade.

But based on a new understanding of the way the oceans expand as they absorb heat, the report is expected to scale back earlier predictions that sea level could rise nearly 3 feet by the end of this century, flooding islands and coastal areas. The most probable sea-level rise remains the same, about 16 inches, but the worst-case estimate is now just under 2 feet.

Some scientists are pushing for a rewrite of that section of the report, arguing that it doesn’t take into account the recent collapse of a major ice shelf in Antarctica and the accelerating loss of ice from Greenland.

The planet could well get hot enough to melt the Greenland ice sheet, raising water levels by 23 feet, but the change will occur very gradually, over a thousand years or more, the draft says.

The draft predicts temperature increases of 2 to 11 degrees by the end of the century, depending on the level of greenhouse emissions. The scientists say the most likely temperature increase will be between 3 and 7 degrees — more than enough to seriously disrupt global weather patterns and ocean circulation.

For the first time, the report also points out that as oceans absorb carbon dioxide, they are becoming more acidic. A team of Seattle-based scientists reported in 2004 that the change imperils the tiny shellfish that are vital to the marine food chain.

Though additional warming is now inevitable, if emissions were significantly scaled back and alternative energy sources phased in soon, the outlook could improve within 30 to 50 years, said Chris Bretherton, director of the UW Program on Climate Change.

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