Global Warming Supercharged by Water Vapor?

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Lonnie Thompson, a climatologist at Ohio State University, said, "In the climate community, there has been debate as to whether water vapor is a slave to temperature."

"This research indicates that small changes in temperature, driven by greenhouse gases, put more water vapor into the atmosphere, which drives up the temperature more," said Thompson, who studies ice cores and glacier retreat in the tropics.

Under normal conditions, much of the heat that is emitted from the Earth's surface, called long-wave radiation, goes into the atmosphere and back out to space. But water vapor, carbon dioxide, and other greenhouse gases in the atmosphere absorb some of that heat, Thompson said.

With an increased amount of water vapor in the atmosphere, more long-wave radiation is trapped, then emitted back to Earth, Thompson said. "So you have more energy to heat the Earth's surface."

By plotting recent climate data and geographical data, the researchers found that the increase in greenhouse gases in Europe has caused a major disruption in the natural cycle of water evaporating from the surface of the Earth.

The water cycle—in which water evaporates, rises into the atmosphere, and eventually returns to Earth in the form of precipitation—has been disrupted to the point where the water vapor itself is helping to fuel the temperature increase, Philipona said.

The Atlantic Coast: A Clue

The team reached its conclusion through a complex process of elimination.

They identified the various factors that influence temperature change, including cloud cover, air circulation, and greenhouse gases. Then the researchers cast each factor as an input in an equation whose result is temperature change.

The scientists examined climate change data for Europe, paying close attention to differences in temperature changes throughout the continent.

The researchers then plotted the average monthly temperatures for the years 1995 to 2002 for different areas of Europe, including the Alps and central Europe. They made similar graphs of monthly changes in humidity for the same areas.

While Europe's overall temperature has increased recently, not all regions have increased to the same degree. Some areas have even experienced a temperature decrease.

The team noted that air currents from the Atlantic Ocean in the west typically bring warm, humid air onto the continent, helping to warm the coast.

Even so, the greatest temperature increases were not near the Atlantic coast but farther east—in fact, some temperatures along the coast had actually decreased.

Finally, they concluded that what was different in Germany and Poland was the greater amount of water vapor being released into the atmosphere by forests and crops.

The increased humidity had driven the temperature up, Philipona said.

The scientists calculated that 70 percent of the recent increase in temperatures in central Europe is due to water vapor, and 30 percent is due to other greenhouse gases.

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