In the summer of 2008, NOAA’s CSI team investigated the origin of devastating floods in the Midwest. The previous winter had brought heavy snowfall and rain to the Upper Midwest. Soils were saturated, and by the end of March, NOAA was already predicting high flood potential in three basins: The Mississippi, the Ohio, and the lower Missouri. Then came the rain. Between June 1 and 13, rivers and creeks in 10 Midwestern states swelled with rainfall that exceeded historic averages for those two weeks by six inches or more. Fresh from the work implicating La Niña in the tornadoes of February, CSI looked for evidence that the recent strong La Niña was also involved in the setup to the deadly floods.
On June 13, 2008, Iowa’s Cedar River crested at its highest level ever recorded, flooding 10 square miles of the city of Cedar Rapids, Iowa. Image courtesy of Don Becker, USGS.
La Niña had an alibi this time. Historically, La Niña conditions sent wet weather south and east of the regions rained out in 2008. In fact, a severe drought hit the Midwest during 1988, coincident with a strong La Niña. Though they looked at multiple lines of evidence, the team could find no evidence linking the flooding to La Niña.
Then, the team considered climate change. Published climate model runs indicated that extreme rain events fit within a much larger emerging pattern of changes in the water cycle that are related to global warming. Although climate models do not predict an increase in mean precipitation over the Midwest, observations and projections suggest that rainfall patterns are changing: the number of medium rain events is decreasing, while the numbers of light drizzles and heavy downpours are on the rise.
Publications by the United Nations’ Intergovernmental Panel on Climate Change and the U.S. Global Change Research Program both note an increase in heavy precipitation events over North America during the last 50 years—conditions that are consistent with higher temperatures and more water vapor in the atmosphere. This, in turn, is consistent with the observed increase in greenhouse gases. Thus, evidence compiled by CSI suggests that human-caused global warming was a factor in the Midwest flooding disaster.
Public interest in the attribution of extreme weather events is growing. “Increasingly, decision-makers and the media want to know how well we understand the causes of regional and seasonal climate variation and trends,” Hoerling said. “If an attribution study suggests, for example, that La Niña winters have a higher-than-normal likelihood of extreme snowfall events, communities can use that information to get ready for those conditions.”
Veteran journalist Ann Schrader said Hoerling’s assessment is spot-on. Schrader has written dozens of weather stories during her 30 years at the Denver Post newspaper. “Water resource managers, emergency planners, insurance companies…they all want attribution for extreme events,” she said. “These groups are being asked to commit precious resources and they want to understand the risks posed by climate. They are also frequently asking the general public to comply with orders or requests for certain behaviors,” Schrader said. “Attribution helps. It adds validity, and helps a great deal in explaining the science behind weather forecasting.”
NOAA’s Climate Attribution Site. Accessed Oct 19, 2009.
Climate Change Science Program Synthesis and Assessment Product 1.3, 2008: Reanalysis of Historical Climate Data for Key Atmospheric Features Implications for Attribution of Causes of Observed Change. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research [Randall Dole, Martin Hoerling, and Siegfried Schubert (eds.)]. National Oceanic and Atmospheric Administration, National Climatic Data Center, Asheville, NC, 156 pp.
Climate Change Science Program Synthesis and Assessment Product 3.3, 2008: Weather and Climate Extremes in a Changing Climate. [Thomas Karl, Gerald Meehl, Christopher Miller, Susan Hassol, Anne Waple. William Murray (eds.)] Department of Commerce, NOAA’s National Climate Data Center, Washington, D.C., USA, 164 pp.
IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
The Relation of El Niño–Southern Oscillation (ENSO) to Winter Tornado Outbreaks A. R. Cook and J. T. Schaefer, Monthly Weather Review, DOI: 10.1175/2007MWR2171.1