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.
