Satellite Shows High Productivity From The United States Corn Belt, But Drought Risks Still Remain

April Flowers for – Your Universe Online

During the Northern Hemisphere’s growing season, data returned from satellite sensors shows that the Midwest has more photosynthetic activity than any other region of the planet, according to a recent NASA report.

School children are taught that plants convert light to energy through the process of photosynthesis. What is less known, however, is that chlorophyll also emits a fraction of the absorbed light as fluorescent glow, which is invisible to the naked eye. The research team has found that the glow is an excellent indicator of the gross productivity of a given region.

Prior research by Joanna Joiner, of NASA’s Goddard Space Flight Center, demonstrated that the fluorescent glow of plants could be extracted from the data of existing satellites, which were designed and built for other purposes. For the current study, Joiner worked with Luis Guanter of the Freie Universität Berlin, and Christian Frankenberg of NASA’s Jet Propulsion Laboratory to estimate the photosynthesis from agriculture for the first time using the satellite data.

“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,” Frankenberg said.

On an annual basis, the tropics are the most productive. During the Northern Hemisphere’s growing season, however, the research team noticed that the US Corn Belt had a significant advantage. “Areas all over the world are not as productive as this area,” said Frankenberg.

They analyzed data from the Global Ozone Monitoring Experiment 2 (GOME-2) on Metop-A, a European meteorological satellite, finding that fluorescence in the Corn Belt — from Ohio to Nebraska and Kansas — peaks in July. During this peak, the levels are 40 percent higher than those observed in the Amazon.

The researchers confirmed these findings by comparing them with ground-based measurements from carbon flux towers and yield statistics. Satellite measurements have a resolution of more than 1,158 square miles, while the resolution of ground based measurements is approximately 0.4 square miles. The study shows that even with the course resolution, the satellite method was able to estimate the photosynthetic activity occurring inside plants at the molecular level for areas with relatively homogenous vegetation like the Corn Belt.

The new method still faces challenges in estimating the productivity of fragmented agricultural areas. These areas are not properly sampled by current satellite instruments, but future missions with better resolution could help—such as the upcoming NASA Orbiting Carbon Observatory-2 mission, which is scheduled to launch in July 2014. The findings, published in the Proceedings of the National Academy of Sciences, could also help researchers improve the computer models used to simulate Earth’s carbon cycle. Guanter found that a strong underestimation of crop photosynthesis in the models—by 40 to 60 percent.

Drought Risks Remain

Perhaps these findings could also help with another Corn Belt challenge. A new study from Columbia University’s The Earth Institute reveals that increasing heat is expected to extend dry conditions to more cities and farmland by the end of the century. The findings, published in Climate Dynamics, demonstrate that higher evaporation rates may play an important role in future drought conditions, despite the fact that most studies focus on rainfall projections. The researchers say that evaporation, caused by warmer temperatures wringing more moisture from the soil, will affect even those regions predicted to have an increase in rainfall.

In one of the first studies to model the effects of both changing rainfall and evaporation rates on future drought, the research team used the latest climate simulations to estimate that 12 percent of land will be subject to drought by 2100 through rainfall alone. If higher evaporation rates from the added energy and humidity in the atmosphere is considered, the percentage of land affected by drought rises to 30. The study, which excludes Antarctica, demonstrates that even areas expected to get more rain, including important wheat, corn and rice belts in the western United States and southeastern China, will be at risk of drought because of an increase in evaporative drying.

“We know from basic physics that warmer temperatures will help to dry things out,” said Benjamin Cook, a climate scientist with joint appointments at Columbia University’s Lamont-Doherty Earth Observatory and the NASA Goddard Institute for Space Studies. “Even if precipitation changes in the future are uncertain, there are good reasons to be concerned about water resources.”

The latest International Panel on Climate Change’s (IPCC) climate report cautions that soil moisture is expected to decline worldwide, leading to a greater risk of agricultural drought in already dry regions. Consistent with the current study, the IPCC report predicts a strong change of evaporation rates in the Mediterranean, southwestern United States and southern African regions.

The researchers used two drought metric formulations to analyze projections of both rainfall and evaporative demand from the climate model simulations completed for the IPCC’s 2013 climate report. Comparison of the two metrics shows an agreement that increased evaporative drying will probably tip marginally wet regions at mid-latitudes like the Great Plains and a swath of southeastern China into aridity, which would not be the case if rainfall were the only consideration. Dry zones in Central America, the Amazon and southern Africa will grow larger, while the summer aridity of Greece, Turkey, Italy and Spain is expected to extend northward into continental Europe.

Currently, when one area’s crop yields are temporarily lowered by bad weather, other regions are typically able to compensate. According to the study, however, the warmer weather of the future could simultaneously wither crops in multiple regions. “If rain increases slightly but temperatures also increase, drought is a potential consequence.”

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