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Data from
satellite sensors show that during the Northern Hemisphere's growing season, the
Midwest region of the United States boasts more photosynthetic activity than any
other spot on Earth, according to NASA and university scientists.
Healthy plants convert light to energy via photosynthesis, but chlorophyll
also emits a fraction of absorbed light as fluorescent glow that is invisible to
the naked eye. The magnitude of the glow is an excellent indicator of the amount
of photosynthesis, or gross productivity, of plants in a given region.
Research in 2013 led by Joanna Joiner, of NASA's Goddard Space Flight Center
in Greenbelt, Md., demonstrated that fluorescence from plants could be teased
out of data from existing satellites, which were designed and built for other
purposes. The new research led by Luis Guanter of the Freie Universität Berlin,
used the data for the first time to estimate photosynthesis from agriculture.
Results were published March 25 in Proceedings of
the National Academy of Sciences.
According to co-author Christian Frankenberg of NASA's Jet Propulsion
Laboratory in Pasadena, Calif., "The paper shows that fluorescence is a much
better proxy for agricultural productivity than anything we've had before. This
can go a long way regarding monitoring – and maybe even predicting – regional
crop yields."
The magnitude of fluorescence portrayed in this
visualization prompted researchers to take a closer look at the productivity of
the U.S. Corn Belt. The glow represents fluorescence measured from land plants
in early July, over a period from 2007 to 2011.
Image Credit:
NASA's Goddard Space Flight
Center
Guanter,
Joiner and Frankenberg launched their collaboration at a 2012 workshop, hosted
by the Keck Institute for Space Studies at the California Institute of
Technology in Pasadena, to explore measurements of photosynthesis from space.
The team noticed that on an annual basis, the tropics are the most productive.
But during the Northern Hemisphere's growing season, the U.S. Corn Belt "really
stands out," Frankenberg said. "Areas all over the world are not as productive
as this area."
The researchers set out to describe the phenomenon observed by carefully
interpreting the data from the Global Ozone Monitoring Experiment 2 (GOME-2) on
Metop-A, a European meteorological satellite. Data showed that fluorescence from
the Corn Belt, which extends from Ohio to Nebraska and Kansas, peaks in July at
levels 40 percent greater than those observed in the Amazon.
Comparison with ground-based measurements from carbon flux towers and yield
statistics confirmed the results.
The match between ground-based measurements and satellite measurements was a
"pleasant surprise," said Joiner, a co-author on the paper. Ground-based
measurements have a resolution of about 0.4 square miles (1 square kilometer),
while the satellite measurements currently have a resolution of more than 1,158
square miles (3,000 square kilometers). The study confirms that even with coarse
resolution, the satellite method could estimate the photosynthetic activity
occurring inside plants at the molecular level for areas with relatively
homogenous vegetation like the Corn Belt.
Unlike most vegetation, crops are made to perform.
Frequent access to nutrients and irrigation results in healthy, productive
plants – the glow of which can be detected by satellites.
Image Credit:
Wikimedia commons/Illustratedjc
Challenges
remain in estimating the productivity of fragmented agricultural areas, not
properly sampled by current space-borne instruments. That's where missions with
better resolution could help, such as NASA's Orbiting Carbon Observatory-2 – a
mission planned for launch in July 2014 that will also measure solar-induced
fluorescence.
The research could also help scientists improve the computer models that
simulate Earth's carbon cycle, as Guanter found a strong underestimation of crop
photosynthesis in models. The analysis revealed that carbon cycle models – which
scientists use to understand how carbon cycles through the ocean, land and
atmosphere over time – underestimate the productivity of the Corn Belt by 40 to
60 percent.
Unlike most vegetation, food crops are managed to maximize productivity. They
usually have access to abundant nutrients and are irrigated. The Corn Belt, for
example, receives water from the Mississippi River. Accounting for irrigation is
currently a challenge for models, which is one reason why they underestimate
agricultural productivity.
"If we don't take into account irrigation and other human influences in the
agricultural areas, we're not going to correctly estimate the amount of carbon
taken up by vegetation, particularly corn," Joiner said. "Corn plants are very
productive in terms of assimilating carbon dioxide from the atmosphere. This
needs to be accounted for going forward in trying to predict how much of the
atmospheric carbon dioxide will be taken up by crops in a changing climate."
According to Frankenberg, the remote sensing-based techniques now available
could be a powerful monitoring tool for food security, especially data from
OCO-2 and in combination with data from other upcoming satellites, such as
NASA's Soil Moisture Active Passive, scheduled for launch later this year.
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.
NASA
Guillermo Gonzalo Sánchez Achutegui
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