Global Warming Supercharged by Water Vapor?

Arianne Appel
for National Geographic News
November 10, 2005
Related Photos: A Warming World >>

The latest villain on global warming's most-wanted list is all wet—and a little surprising. Water vapor, experts say, is the culprit behind Europe's rapidly rising temperatures.

Evaporated H2O is a known greenhouse gas—a gas that absorbs and re-emits infrared radiation in Earth's atmosphere, thereby increasing temperatures (see our global warming fast facts).

But only now has a study uncovered evidence that water vapor is a major public enemy in Europe.

According to a team of Swiss scientists, heat from other greenhouse gases is causing more water to evaporate, releasing the vapor into the atmosphere above Europe. That vapor in turn, adds to the greenhouse effect, further warming the region.

Temperatures throughout the Northern Hemisphere have been increasing in recent years. But Europe has been heating up especially quickly, leading to studies, theories, and debate as to why.

In central Europe—Germany, Austria, Poland, Czech Republic, Slovakia, Hungary, Switzerland, and Slovenia—temperatures have risen three times faster than the average for the hemisphere has.

Some scientists have argued that Europe's rising temperatures are due to normal weather-circulation patterns. But the new study's results suggest that large-scale weather patterns are only a minor influence on the temperature increase, said lead researcher Rolf Philipona of the World Radiation Center in Davos Dorf, Switzerland.

"It is an experiment that clearly shows which factors are driving the higher temperatures. It is not the clouds, not the sun, not the aerosols. It is the increased greenhouse gases and the strong water vapor impact," Philipona said.

"We believe strongly that we are observing increased greenhouse effect," said Philipona, whose results were published this week in the journal Geophysical Research Letters.

An increase of greenhouse gases in the atmosphere, from car exhaust, industrial emissions, and other sources, has been observed throughout the planet since about 1960, Philipona said.

Vicious Cycle?

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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