Water vapor is one of the most powerful greenhouse gases. As the
oceans warm, thanks to rising levels of CO2, they evaporate water
vapor more freely into the atmosphere. Similarly, as the land surface
warms, it too evaporates more water vapor into the warming atmosphere.
As the atmosphere warms, thanks to rising levels of CO2, it can hold
more water vapor.
“We now think the water vapor feedback is extraordinarily strong,
capable of doubling the warming due to carbon dioxide alone.”
NASA/Goddard Space Flight Center
Public release date: 17-Nov-2008
Contact: Sarah DeWitt
Water vapor confirmed as major player in climate change
Water vapor is known to be Earth’s most abundant greenhouse gas, but
the extent of its contribution to global warming has been debated.
Using recent NASA satellite data, researchers have estimated more
precisely than ever the heat-trapping effect of water in the air,
validating the role of the gas as a critical component of climate
Andrew Dessler and colleagues from Texas A&M University in College
Station confirmed that the heat-amplifying effect of water vapor is
potent enough to double the climate warming caused by increased
levels of carbon dioxide in the atmosphere.
With new observations, the scientists confirmed experimentally what
existing climate models had anticipated theoretically. The research
team used novel data from the Atmospheric Infrared Sounder (AIRS) on
NASA’s Aqua satellite to measure precisely the humidity throughout
the lowest 10 miles of the atmosphere. That information was combined
with global observations of shifts in temperature, allowing
researchers to build a comprehensive picture of the interplay between
water vapor, carbon dioxide, and other atmosphere-warming gases. The
NASA-funded research was published recently in the American
Geophysical Union’s Geophysical Research Letters.
“Everyone agrees that if you add carbon dioxide to the atmosphere,
then warming will result,” Dessler said. “So the real question is,
how much warming?”
The answer can be found by estimating the magnitude of water vapor
feedback. Increasing water vapor leads to warmer temperatures, which
causes more water vapor to be absorbed into the air. Warming and
water absorption increase in a spiraling cycle.
Water vapor feedback can also amplify the warming effect of other
greenhouse gases, such that the warming brought about by increased
carbon dioxide allows more water vapor to enter the atmosphere.
“The difference in an atmosphere with a strong water vapor feedback
and one with a weak feedback is enormous,” Dessler said.
Climate models have estimated the strength of water vapor feedback,
but until now the record of water vapor data was not sophisticated
enough to provide a comprehensive view of at how water vapor responds
to changes in Earth’s surface temperature. That’s because instruments
on the ground and previous space-based could not measure water vapor
at all altitudes in Earth’s troposphere — the layer of the
atmosphere that extends from Earth’s surface to about 10 miles in
AIRS is the first instrument to distinguish differences in the amount
of water vapor at all altitudes within the troposphere. Using data
from AIRS, the team observed how atmospheric water vapor reacted to
shifts in surface temperatures between 2003 and 2008. By determining
how humidity changed with surface temperature, the team could compute
the average global strength of the water vapor feedback.
“This new data set shows that as surface temperature increases, so
does atmospheric humidity,” Dessler said. “Dumping greenhouse gases
into the atmosphere makes the atmosphere more humid. And since water
vapor is itself a greenhouse gas, the increase in humidity amplifies
the warming from carbon dioxide.”
Specifically, the team found that if Earth warms 1.8 degrees
Fahrenheit, the associated increase in water vapor will trap an extra
2 Watts of energy per square meter (about 11 square feet).
“That number may not sound like much, but add up all of that energy
over the entire Earth surface and you find that water vapor is
trapping a lot of energy,” Dessler said. “We now think the water
vapor feedback is extraordinarily strong, capable of doubling the
warming due to carbon dioxide alone.”
Because the new precise observations agree with existing assessments
of water vapor’s impact, researchers are more confident than ever in
model predictions that Earth’s leading greenhouse gas will contribute
to a temperature rise of a few degrees by the end of the century.
“This study confirms that what was predicted by the models is really
happening in the atmosphere,” said Eric Fetzer, an atmospheric
scientist who works with AIRS data at NASA’s Jet Propulsion
Laboratory in Pasadena, Calif. “Water vapor is the big player in the
atmosphere as far as climate is concerned.”