A NASA Space Sleuth Hunts the Trail of Earth's Water
For the first time, NASA scientists have used a shrewd
spaceborne detective to track the origin and movement of
water vapor throughout Earth's atmosphere. This perspective
is vital to improve the understanding of Earth's water cycle
and its role in weather and climate.
NASA's newest detective in the mysteries of atmospheric water
vapor is the Tropospheric Emission Spectrometer instrument on
the Aura satellite. A team of scientists from NASA's Jet Propulsion
Laboratory, Pasadena, Calif., and the University of Colorado, Boulder,
used the instrument's observations of heavy and light water vapor to
retrace the "history" of water over oceans and continents, from ice
and liquid to vapor and back again. Heavy water vapor molecules have
more neutrons than lighter ones do.
Image right: This view depicts the distribution of "heavy" and "light" water vapor molecules over Earth's tropics. Red illustrates heavy water vapor, which indicates recent evaporation or plant "exhalation." Blue and purple show lighter water vapor that has undergone significant condensation. The data was obtained Oct. 7, 2006, by the Tropospheric Emission Spectrometer on NASA's Aura satellite. Image credit: NASA/JPL + Browse version of image
By analyzing the distribution of the heavy and light molecules, the team
was able to deduce the sources and processes that cycle water vapor, the
most abundant greenhouse gas in Earth's atmosphere.
The team found that tropical rainfall evaporation and water "exhaled" by
forests are key sources of moisture in the tropical atmosphere. They noted
that more water than they had expected is transported over land rather than
ocean into the lower troposphere (Earth's lowermost atmosphere), especially
over the Amazon River basin and tropical Africa.
"One might expect most of the water to come directly from the wet ocean,"
said study co-author Dr. David Noone of the University of Colorado. "Instead,
it appears that thunderstorm activity over the tropical continents plays a
key role in keeping the troposphere hydrated."
The team found that in the tropics and regions of tropical rain clouds, rainfall
evaporation significantly adds moisture to the lower troposphere, with typically
20 percent and up to 50 percent of rain there evaporating before it reaches the
ground. The atmosphere retains this water, which can be used to make clouds. The
strength and location of this evaporation give scientists new insight into how
water in Earth's atmosphere helps move energy from Earth's surface upwards. The
main role of the atmosphere in Earth's climate system is to take energy deposited
by the sun and dispose of it back into space.
The team also found evidence that water transported upwards by thunderstorm activity
over land originates from both plant "exhalation" in large forests and evaporation over
nearby oceans. The balance between these two different sources tells us how vegetation
interacts with climate and helps maintain regional rainfall levels.
"This link between vegetation, hydrology and climate has implications for how societies
choose to manage their ecological resources," said Noone. "Our measurements provide a
baseline against which future changes in vegetation-climate interactions can be measured."
The details of this journey are critical for understanding clouds and climate, as
well as changes in precipitation patterns and water resources, Noone explained. "Our
study measures the conditions under which precipitation and evaporation occur, providing
insights into the processes responsible. Better knowledge of these processes ultimately
leads to a clearer understanding of the factors that drive the global water cycle and
its importance in climate and global climate change."
Noone and his co-authors said there has been a general lack of information on the way
water moves around in Earth's atmosphere -- where it comes from and where it ends up.
"Since we measure the history of water, so to speak, we can tell the difference
between air masses that have undergone extensive condensation from those that are more
dominated by evaporation from the ocean surface," said study co-author Dr. John Worden of JPL.
"These results also lay the groundwork for research to help interpret the isotopic
measurements that scientists use to study Earth's climate in the past," added JPL
co-author Dr. Kevin Bowman.
Study results appear in the February 1 issue of the journal Nature.
For more on the Tropospheric Emission Spectrometer, visit: http://www.nasa.gov/aura
JPL is managed for NASA by the California Institute of Technology in Pasadena.
Media contacts: Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
Adriana Bailey/Jim Scott 303-492-6289/3114
University of Colorado, Boulder, Colo.