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March 15, 2004
Satellite Finds Warming
“Relative” to Humidity
A NASA-funded study found some climate
models might be overestimating the amount of
water vapor entering the atmosphere as the Earth
warms. Since water vapor is the most important
heat-trapping greenhouse gas in our atmosphere,
some climate forecasts may be overestimating
future temperature increases.
In response to human emissions of greenhouse
gases, like carbon dioxide, the Earth warms, more
water evaporates from the ocean, and the amount
of water vapor in the atmosphere increases. Since
water vapor is also a greenhouse gas, this leads
to a further increase in the surface temperature.
This effect is known as “positive water
vapor feedback.” Its existence and size
have been contentiously argued for several years.
Ken Minschwaner, a physicist at the New Mexico
Institute of Mining and Technology, Socorro,
N.M., and Andrew Dessler, a researcher with the
University of Maryland, College Park, and
NASA’s Goddard Space Flight Center,
Greenbelt, Md, did the study. It is in the March
15 issue of the American Meteorological
Society’s Journal of Climate. The
researchers used data on water vapor in the upper
troposphere (10-14 km or 6-9 miles altitude) from
NASA’s Upper Atmosphere Research Satellite
(UARS).
Their work verified water vapor is increasing
in the atmosphere as the surface warms. They
found the increases in water vapor were not as
high as many climate-forecasting computer models
have assumed. “Our study confirms the
existence of a positive water vapor feedback in
the atmosphere, but it may be weaker than we
expected,” Minschwaner said.
“One of the responsibilities of science
is making good predictions of the future climate,
because that’s what policy makers use to
make their decisions,” Dessler said.
“This study is another incremental step
toward improving those climate
predictions,” he added.
According to Dessler, the size of the positive
water vapor feedback is a key debate within
climate science circles. Some climate scientists
have claimed atmospheric water vapor will not
increase in response to global warming, and may
even decrease. General circulation models, the
primary tool scientists use to predict the future
of our climate, forecast the atmosphere will
experience a significant increase in water vapor.
NASA’s UARS satellite was used to
measure water vapor on a global scale and with
unprecedented accuracy in the upper troposphere.
Humidity levels in this part of the atmosphere,
especially in the tropics, are important for
global climate, because this is where the water
vapor has the strongest impact as a greenhouse
gas.
UARS recorded both specific and relative
humidity in the upper troposphere. Specific
humidity refers to the actual amount of water
vapor in the air. Relative humidity relates to
the saturation point, the amount of water vapor
in the air divided by the maximum amount of water
the air is capable of holding at a given
temperature. As air temperatures rise, warm air
can hold more water, and the saturation point of
the air also increases.
In most computer models relative humidity
tends to remain fixed at current levels. Models
that include water vapor feedback with constant
relative humidity predict the Earth’s
surface will warm nearly twice as much over the
next 100 years as models that contain no water
vapor feedback.
Using the UARS data to actually quantify both
specific humidity and relative humidity, the
researchers found, while water vapor does
increase with temperature in the upper
troposphere, the feedback effect is not as strong
as models have predicted. “The increases in
water vapor with warmer temperatures are not
large enough to maintain a constant relative
humidity,” Minschwaner said. These new
findings will be useful for testing and improving
global climate models.
NASA’s Earth Science Enterprise is
dedicated to understanding the Earth as an
integrated system and applying Earth system
science to improve prediction of climate, weather
and natural hazards using the unique vantage
point of space. NASA plans to launch the Aura
satellite in June 2004. Along with the Terra and
Aqua satellites already in operation, Aura will
monitor changes in Earth’s atmosphere.
For information about NASA and agency programs
on the Internet, visit:
http://www.nasa.gov
For more information about the research and
images on the Internet, visit:
http://www.gsfc.nasa.gov/topstory/2004/ 0315humidity.html
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Contacts:
Elvia H. Thompson
Headquarters, Washington
Phone: 202/358-1696
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
Phone: 607/273-2561
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Water Vapor and Climate
Change
There is no more important greenhouse gas than
water vapor. As one of the fundamental parts of
Earth’s atmosphere, water vapor affects
global warming in both positive and negative
terms, and offers a trail for scientists to
follow towards a better understanding about how
the planet functions as a whole. NASA’s
Aqua satellite is dedicated to tracking
Earth’s water, and plans are underway to
launch Aura this June, another satellite for
better understanding Earth’s atmosphere. By
applying integrated analytic tools to the study
of climate and climate change, experts hope to
learn more specifically how water vapor and other
greenhouse gasses move and function throughout
the atmosphere. Credit: NASA/Goddard Space Flight
Center Scientific Visualization Studio
Positive Water Vapor
Feedback
This diagram shows the mechanisms behind a
positive water vapor feedback loop. Increases in
carbon dioxide, a greenhouse gas, cause a rise
global air temperatures. Due to increased
evaporation and since warmer air holds more
water, water vapor levels in the atmosphere rise,
which further increases greenhouse warming. The
cycle reinforces itself. The background is a
sunset through altocumulus clouds. Credit: NASA
and NOAA Historic NWS Collection
High-Resolution Image
Water Vapor Measurements from HALOE
(1992-1997)
This animated graph shows measurements of the
water vapor amounts in the upper troposphere and
stratosphere from the Halogen Occultation
Experiment (HALOE) instrument on the Upper
Atmosphere Research Satellite. HALOE retrieved
specific humidity, which refers to the actual
amount of water vapor in the air. The
relationship between HALOE water vapor and
changes in temperature were used to confirm a
positive water vapor feedback in the upper
troposphere. Credit: NASA/Goddard Space Flight
Center Scientific Visualization Studio
Water Vapor Measurements from MLS
(September 1994)
This map shows the monthly mean water vapor in
the upper troposphere and lower stratosphere from
the Microwave Limb Sounder (MLS) on the Upper
Atmosphere Research Satellite. MLS retrieved
relative humidity, which points to the amount of
water vapor in the air relative to the maximum
amount that it can hold (the saturation point).
Relative humidities increase from red to blue,
showing areas where tropical convection is
moistening the upper troposphere. Credit: Ken
Minschwaner, New Mexico Institute of Mining and
Technology
Satellite Water Vapor Images on TV
News
Television weather forecasters have made satellite images of water vapor popular lately. This image is from the NOAA GOES-12 satellite. The lighter gray shades indicate water vapor. The lightest areas are likely where precipitation would be falling, and the black areas show drier air. Credit: NASA
GOES-2 Interactive Water Vapor Weather Satellite Images
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