Corn Belt greenhouse gas emissions underestimated

Sarah Thamer

University of Minnesota scientists found previous research may have underestimated greenhouse gas emissions in the Midwest’s Corn Belt by nearly 40 percent. 
At the University’s Tall Tower Trace Gas Observatory in UMore Park in Rosemount, researchers used a different method than a United Nations environmental panel utilizes to collect nitrous oxide emission data, leading to dramatic discrepancies in the results from the two methods. 
The University scientists used a top-down approach to measuring the gas by pulling samples of air down the tower from about 200 meters high. The Intergovernmental Panel on Climate Change has used a bottom-up approach, where samples of air are collected near the soil.
“We’ve been disturbed by the fact that we’ve had such a large difference between bottom-up and top-down approaches,” said Tim Griffis, co-author of the July 27 study and a professor in the Department of Soil, Water and Climate.
An IPCC report lists various processes that influence nitrous oxide emissions, but because the report is based on few studies, it still lacks key data, Griffis said.
As a result, the IPCC has underestimated nitrous oxide emissions from rivers and streams by nine-fold, the study found. Rivers have become emission hotspots from agricultural runoff and nitrogen fertilizers, the study said.
“IPCC emission factors on rivers has very large uncertainty,” Griffis said. “It’s one of the most uncertain emission factors that they list.” 
A bottom-up approach for measuring nitrous oxide emissions looks at land use and fertilizer input — a limiting approach in comparison to the top-down approach that the researchers have been using since 2010. 
The top-down approach offers measurements from high altitudes in the atmosphere, allowing researchers to examine emissions on a regional level. In turn, the estimates are much higher than what the IPCC reported.
Nitrous oxide contributes to about 6 percent of long-lived greenhouse gases like carbon dioxide, said Peter Turner, a lead author of the study and a graduate student in the Department of Soil, Water and Climate.
“Nitrous oxide’s global warming potential is about 300 times that of carbon dioxide and 12 times more potent than a molecule of methane,” Turner said.
Anna Henderson, environmental policy planning director for the Minnesota Environmental Quality Board, said agriculture in the U.S. made up 19 percent of Minnesota’s greenhouse gas emissions between 2010 and 2012.
While emissions of nitrous oxide are not new, Turner said he hopes the study can reconcile the differences between bottom-up and top-down emissions by properly accounting for streams. Turner said he hopes streams and rivers will be accounted for in the future to address mitigating their impact on emissions.
Peter Ciborowski, an environment research scientist at the Minnesota Pollution Control Agency, said the study will improve efficient use of nitrogen fertilizer in agriculture — a large contributing factor of nitrous oxide emissions.
“Because we now know that emissions from rivers are an important source, now we can work with land managers and farmers and try to implement better strategies for reducing these emissions,” Griffis said.