ALTON, ILLINOIS - MAY 31: Workers ferry supplies through floodwater from the Mississippi River ... [+]
Crops in the American Midwest captured significantly less carbon dioxide last year because flooding forced farmers to delay planting, according to a new study.
The flooding reduced carbon uptake by as much as 1oo million metric tons last June and July, according to the study in AGU Advances, led by researchers from CalTech and funded by NASA. That’s as much CO2 as 21.6 million passenger cars emit in a year.
“Widespread flooding and inundation across the U.S. Midwest during spring and early summer 2019 forced many farmers to delay crop planting,” the scientists write. “We show that the delayed planting resulted in a shift of 16 days in the seasonal cycle of the crop growth and a ~15% lower peak solar‐induced chlorophyll fluorescence value.”
Solar-induced fluorescence, or SIF, is a slight glow given off by plants during photosynthesis. It’s too subtle to be detected by the naked eye, but not too subtle for an instrument aboard the European Space Agency’s Copernicus Sentinel-5 satellite (the same satellite famous for its observations of pollution vanishing during the coronavirus lockdown).
“SIF is the most accurate signal of photosynthesis by far that can be observed from space," Christian Frankenberg, a professor of environmental science and engineering, told CalTech’s Robert Perkins. "And since plants absorb carbon dioxide during photosynthesis, we wanted to see if SIF could track the reductions in crop carbon uptake during the 2019 floods."
The scientists support their SIF observations by showing that they conform to anomalies in atmospheric carbon detected by a NASA satellite and aircraft.
Some of the lost carbon uptake last June and July was offset in August and September by a longer growing season, but overall, carbon uptake on cropland dropped by 60 million metric tons—the equivalent of 13 million passenger vehicles driven for one year or the annual energy use of seven million homes.
At the same time, the flooding increased the carbon uptake of natural vegetation by about 3 percent, or 40 million metric tons of carbon dioxide. That occurred, the scientists suspect, because natural ecosystems are better able to cope with waterlogged soils.
