When
you open the back of a fine watch, you see layer upon layer of spinning wheels
linked by interlocking cogs, screws and wires. Some of the cogs are so tiny
they're barely visible. Size doesn't matter -- what's important is that the
cogs fit together well so the wheels keep turning smoothly.
For
centuries, scientists have thought of the Earth system as a series of cycles or
interlocking wheels like the ones in a watch. It's a way to make sense of the
movements of water and other essentials back and forth between the air and the
land, ocean and soil or rock beneath them. In today's changing climate, some
cycles are spinning faster or beginning to wobble. There's an urgent need to
understand what is happening to the cogs that keep these cycles turning.
The
minuscule fraction of Earth's water lodged just beneath the land surface is a
tiny cog that links the water cycle to two other fundamental Earth cycles:
energy and carbon. "That linkage is what makes these three gears turn with
a certain harmony," said Dara Entekhabi of the Massachusetts Institute of
Technology, Cambridge. Entekhabi is science team leader for NASA's Soil
Moisture Active Passive mission, scheduled to launch Jan. 29. Developed and
managed by NASA's Jet Propulsion Laboratory in Pasadena, California, SMAP will
provide the most accurate information ever about this small but critical cog.
You
may have learned about the water cycle in school: Water falls from the sky to
the land when it rains or snows, and rises from the land back to the sky when
it heats up and evaporates. Your teacher may not have mentioned that water
vapor isn't the only thing that rises. The heat energy that turned liquid water
into vapor also rises, cooling Earth's surface. In fact, evaporating soil
moisture is the main way that land sheds the solar energy it receives every day
and thus is a major player in the energy cycle. "It's the first process to
kick in when the surface heats up, and it continues as long as there is
moisture in the soil that can evaporate," Entekhabi said. Evaporation gets
rid of nearly half of the solar energy that reaches land, keeping our planet's
temperature comfortable.
If
there's any moisture at all in soil, there's probably a plant growing there.
That's why most evaporation from soil starts with a plant absorbing water
through its roots. Plants need water for photosynthesis, their food-creating
process. During photosynthesis they "sweat" -- or transpire -- water
onto their leaves, where it evaporates.
Besides
using water and energy, plants absorb carbon dioxide from the atmosphere during
photosynthesis. Over land, this is virtually the only natural way for carbon to
be removed from the atmosphere. Soil moisture keeps this vital carbon highway
open by enabling plants to continue growing. "If a plant has access to
water, it happily carries on with photosynthesis," Entekhabi said.
"If not, the plant shuts down, and eventually it wilts and dies."
After
SMAP launches, the new data it will provide are expected to help scientists
answer some long-standing questions about what is likely to happen to these
important Earth cycles in a changing climate. Entekhabi hopes to take advantage
of the synergy available between SMAP and NASA's new Orbiting Carbon
Observatory-2, which measures global carbon dioxide. "We have talked a lot
with the OCO-2 scientists about how we can use simultaneous measurements to
solve the puzzle of how plants respond to soil moisture and how the carbon
cycle and the water cycle are linked," he said. "If we get that linkage
right, we will reduce the uncertainty in future climate projections and know
more about how terrestrial plants are going to act in the future."
SMAP
will be the last of five NASA Earth science launches within 12 months. NASA
monitors Earth's vital signs from land, air and space with a fleet of
satellites and ambitious airborne and ground-based observation campaigns. NASA
develops new ways to observe and study Earth's interconnected natural systems
with long-term data records and computer analysis tools to better see how our
planet is changing. The agency shares this unique knowledge with the global
community and works with institutions in the United States and around the world
that contribute to understanding and protecting our home planet.
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