Satellite shows high productivity from U.S. Corn Belt
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.
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.
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