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.
