Flooding in Midwest in 2019 led to higher atmospheric carbon dioxide levels after delay in crop growing: Study
The March-June flooding event in the Midwestern states in the United States last year led to a delayed growing season for crops in the region and this, in turn, led to a reduction of 100 million metric tons of net carbon uptake during the months of June and July, according to a new study.
The Midwest region, which stretches from Kansas and Nebraska in the west to Ohio in the east is also known as the Corn Belt which accounts for roughly 40% of world corn and soybean production. Previous studies have shown that the region accounts for a high rate of photosynthesis, resulting in higher net carbon uptake by Midwest cropland compared to nearby forests as recorded by tower-based atmospheric carbon dioxide measurements.
The new study was conducted by Yi Yin along with other researchers at the Division of Geological and Planetary Sciences, part of the California Institute of Technology. It was published on March 31 in the Advancing Earth and Space Science (AGU) Advances journal.
For reference, the massive California wildfires of 2018 launched an estimated 12.4 million metric tons of carbon into the atmosphere. Though part of the carbon uptake deficit during the floods was compensated for later in the growing season, the authors of the study say that the combined effects likely resulted in a 15% reduction in crop productivity relative to 2018.
Carbon uptake was measured using satellite data with researchers using a novel marker of photosynthesis known as solar-induced fluorescence to measure the reduced carbon uptake due to the delay in the growing season. Independent observations of atmospheric carbon dioxide levels were used to confirm the reduction.
"We were able to show that it's possible to monitor the impacts of floods on crop growth on a daily basis in near real-time from space, which is critical to future ecological forecasting and mitigation," said Yin regarding the study.
The Midwest saw record rainfalls during the spring and early summer months of 2019. From April to June, the National Oceanic and Atmospheric Administration reported that 12-month precipitation measurements had hit all-time highs. The resulting floods damaged homes and infrastructure and impacted agricultural productivity.
Solar-induced fluorescence (SIF) is the very faint flow emitted by plants -- a small amount of the sunlight used as plants convert carbon dioxide and sunlight into oxygen and energy during photosynthesis. It is far too dim for us to see with bare eyes, but it can be measured through a process called satellite spectrophotometry.
The research team quantified SIF using measurements from a European Space Agency (ESA) satellite-borne instrument to track the growth of crops with unprecedented detail. They found that the seasonal cycle of the 2019 crop growth was delayed by around two weeks and the maximum seasonal photosynthesis was reduced by about 15%. The stunted growing season was estimated to have led to a reduction in carbon uptake by plants of around 100 million metric tons from June to July 2019.
"SIF is the most accurate signal of photosynthesis by far that can be observed from space," says Christian Frankenberg, professor of environmental science and engineering at Caltech. "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 team also analyzed atmospheric carbon dioxide measurements from NASA's Orbiting Carbon Observatory-2 (OCO-2) satellite as well as from aircraft from NASA's Atmospheric Carbon and Transport America (ACT-America) project and found that SIF-based estimates of reduced uptake are consistent with elevated atmospheric carbon dioxide when both quantities were connected by atmospheric transport models.
"This study illuminates our ability to monitor the ecosystem and its impact on atmospheric CO2 in near real-time from space. These new tools allow for global sensing of biospheric uptake of carbon dioxide," says Paul Wennberg, the R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering, director of the Ronald and Maxine Linde Center for Global Environmental Science, and founding member of the Orbiting Carbon Observatory project. Wennberg is also the principal investigator of the Resnick Sustainability Institute's Climate Science Research Initiative at Caltech.