Ag Weather Forum

An article published by the London, England-based Royal Society of Mathematical, Physical and Engineering Sciences details the connection between increased greenhouse gas buildup and the record flooding in Iowa during spring 2019. The article, "Greenhouse Gases Drove the Increasing Trends in Spring Precipitation Across the Central USA", is published in the Society's March journal. Research authors are Wei Zhang of the Massachusetts Institute of Technology (MIT) and Gabriele Villarini of the University of Iowa.

The article notes the identification of a specific mechanism for heavy moisture inflow into the Midwest. This mechanism is termed either the Midwest Water Hose (MWH) or the Maya Express (ME). The article notes "... This weather type is associated with strong moisture transport accompanied by lifting forced by cold polar air masses, leading to heavy precipitation. The weather setting in MWH leads to (a) strong low-level jet stream over the Midwest, revealed as strong moisture flux transport from the Gulf of Mexico to the Central USA and lifted by cold air from the Arctic. Therefore, the type of rainfall produced by MWH is typically sustained over several days." In the 2019 flooding, "The number of MWH events during January-May 2019 is the highest since 1980, exhibiting a significant upward trend. This weather type contributed to more than 70% of the total precipitation across much of the central USA, a contribution much larger than expected from climatology (approx. 30%). It is, therefore, clear that MWH played a major role in the 2019 flooding event."

Regarding greenhouse gas contribution to the flooding scenario, the article cites previous research which has identified how atmospheric forcing from increased greenhouse gases adds to the atmospheric back-and-forth between air masses to bring about increased precipitation. "The close agreement between observations and climate models lends us confidence to further analyze this weather type in Global Climate Models (GCMs). The ensemble-average frequency of the Midwest Water Hose (MWH) in Coupled Model Intercomparison Project Phase 6 (CMIP6) models exhibits an upward trend significant at the 5% level based on the historical experiments prescribed with all forcing, which include both greenhouse gas (GHG) concentration and natural forcing, consistent with the observed significant upward trend."

The result is rising levels of GHG in the atmosphere make such heavy rainfall and flooding occurrences more likely. The article summarizes that risk level this way: "Furthermore, the risk ratio by comparing the trend probability for the observed trend of this weather type ... is 12.1, suggesting that GHG forcing has increased the probability of the observed trend of this weather type by approximately 12 times ... Based on these results, we can conclude that the upward trend of this weather type that causes heavy precipitation over the central USA can be attributed to GHG concentrations, rather than natural forcing such as volcanic eruptions and solar orbiting changes."

The full article is at this link: https://royalsocietypublishing.org/…

Bryce Anderson can be reached at Bryce.anderson@dtn.com

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