Key Facts

Jefferson City, the capital of Missouri, has experienced two catastrophic floods in the past 20 years.⁴ In coming decades, scientists expect more precipitation in winter and spring, when the risk of flooding is already high.⁴

• Heavy downpours in Jefferson City now occur about twice as often as they did a century ago.³
• If our carbon emissions continue to rise at today's rates, spring rainfall is projected to increase 25 percent or more by the end of this century.^9,10
• The so-called 100-year flood is likely to become more frequent,⁴ and heavy downpours that now occur once every 20 years are projected to occur every four to 15 years in many parts of the United States by the end of this century.⁵

Details

Jefferson City sits on the south bank of the Missouri River. Besides housing state government, it is a crossroads for water and ground transportation.

The city's climate—like that of other communities across the Midwest—has changed measurably over the past half-century.² Heavy downpours now occur about twice as often as they did a century ago.³

On average in the United States, the amount of rain falling during the heaviest 1 percent of rainstorms has increased nearly 20 percent during the past 50 years—almost three times the rate of increase in total precipitation.^4,5 The Midwest saw an even larger average increase of 31 percent, surpassed only by the Northeast (at 67 percent).⁴ Scientists attribute the rise in heavy precipitation to climate change that has already occurred over the past half-century.⁶

With more-frequent heavy downpours comes greater risk of flooding.⁴ Missouri has already suffered two record-breaking floods in the past two decades.

The Great Flood of 1993 affected 500 miles of the Mississippi and Missouri river system, including Jefferson City.⁴ The flood halted major east-west traffic from St. Louis to Kansas City, MO, and as far north as Chicago—disrupting one-quarter of all U.S. freight for about six weeks.⁷

When another catastrophic flood struck the region in 2008, the president declared all but five of Missouri's counties storm- or flood-related disaster zones, leading to $180 million in federal disaster assistance.⁸

What the Future Holds

Scientists expect more intense storms to occur in the Midwest throughout the year, and more precipitation to fall in winter and spring.⁴ If our carbon emissions continue to rise at current rates, spring rainfall in Jefferson City is projected to increase 25 percent or more by the end of this century.^9,10

If we do nothing to change our emissions, the frequency of these intense downpours is projected to double by the end of the century.^9,10 If we make significant efforts to reduce our emissions, heavy rainfalls are projected to occur 50 percent more often⁹—and to drop 10-25 percent more precipitation than such rainstorms do today.⁵ In St. Louis, scientists expect the frequency of rainfalls of more than two inches (roughly five centimeters) to increase by more than 40 percent over the next few decades.⁹

Flood risk in the Midwest is already high in winter and spring, so these changes are expected to lead to more frequent flooding—risking damage to homes, businesses, and infrastructure such as roads and bridges; higher costs for insurance and emergency management; and threats to public health.⁴ Scientists project that the number of high-flow days on Midwest rivers are likely to more than triple by the end of the century if we continue on today's emission track.¹¹

The so-called 100-year flood is likely to become more frequent.⁴ Because transportation planners use such events to determine infrastructure needs, future plans based on the past are likely to become less reliable—and planners will need to develop models that reflect the effects of climate change.⁷

Paradoxically, flooding and dry days are related. Climate models project not only more intense rainstorms but also longer periods with little precipitation in between.¹²

Changing rain patterns and rising temperatures could put crops in the Midwest at risk. Wet springs are likely to delay planting, for example.⁴ And in hotter weather, plants typically require more rainfall simply to retain the same amount of moisture. Yet the region can expect 20 percent less rain in summer, with more of it falling as downpours.⁹

Heavy downpours can also endanger public health by overwhelming water treatment plants, and causing raw sewage to spill into rivers if sewer lines are not separate from storm drains.⁴ Tackling sewer overflows can cost cities billions of dollars over multiple decades.⁹

Credits

Endnotes

1. Photograph courtesy of the Missouri Highway and Transportation Department. Online at http://mo.water.usgs.gov/ Reports/ 1993-Flood/ images/ scan14.jpg. Accessed April 5, 2010. ↑
2. DeGaetano, A.T. 2002. Trends in twentieth-century temperature extremes across the United States. Journal of Climate 15:3188-3205. ↑
3. Kunkel, K., K. Andsager, and D. Easterling. 1999. Long-term trends in extreme precipitation events over the conterminous United States and Canada. Journal of Climate 12:2515-2527. ↑
4. U.S. Global Change Research Program. 2009. Global climate change impacts in the United States. Edited by T.R. Karl, J.M. Melillo, and T.C. Peterson. Cambridge University Press. ↑
5. Kunkel, K.E., P.D. Bromirski, H.E. Brooks, T. Cavazos, A.V. Douglas, D.R. Easterling, K.A. Emanuel, P.Ya. Groisman, G.J. Holland, T.R. Knutson, J.P. Kossin, P.D. Komar, D.H. Levinson, and R.L. Smith, 2008: Observed changes in weather and climate extremes. In: Weather and climate extremes in a changing climate: Regions of focus—North America, Hawaii, Caribbean, and U.S. Pacific Islands. Edited by Karl, T.R., G.A. Meehl, C.D. Miller, S.J. Hassol, A.M. Waple, and W.L. Murray. Synthesis and assessment product 3.3. Washington, DC: U.S. Climate Change Science Program, pp. 35-80. ↑
6. Trenberth, K.E., P.D. Jones, P. Ambenje, R. Bojariu, D. Easterling, A. Klein Tank, D. Parker, F. Rahimzadeh, J.A. Renwick, M. Rusticucci, B. Soden, and P. Zhai. 2007. Observations: Surface and atmospheric climate change. 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. Edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller. Cambridge University Press. ↑
7. National Research Council. 2008: Potential impacts of climate change on U.S. transportation. Special report 290. Washington, DC: Transportation Research Board. Online at http://onlinepubs.trb.org/ onlinepubs/ sr/ sr290.pdf. Accessed April 5, 2010. ↑
8. Federal Emergency Management Agency. 2009. Federal assistance from seven disaster declarations tops $180 million. Online at http://www.fema.gov/ news/ newsrelease.fema? id=47799. Accessed April 24, 2010. ↑
9. Union of Concerned Scientists. 2009. Confronting climate change in the U.S. Midwest: Missouri. Cambridge, MA. Online at www.ucsusa.org/mwclimate. ↑
10. The emissions scenarios referred to here—from the Intergovernmental Panel on Climate Change—are the high-emissions path known as A1FI, and the low-emissions path known as B1. Emissions over the past several years have followed the high-emissions path. ↑
11. Cherkauer, K.A., and T. Sinha. 2010. Hydrologic impacts of projected future climate change in the Lake Michigan region. In: Journal of Great Lakes Research. pp. 33-50 ↑
12. Gutowski, W.J., G.C. Hegerl, G.J. Holland, T.R. Knutson, L.O. Mearns, R.J. Stouffer, P.J. Webster, M.F. Wehner, F.W. Zwiers, 2008: Causes of observed changes in extremes and projections of future changes. In: Weather and climate extremes in a changing climate: Regions of Focus—North America, Hawaii, Caribbean, and U.S. Pacific Islands. Edited by T.R. Karl, G.A. Meehl, C.D. Miller, S.J. Hassol, A.M. Waple, and W.L. Murray. Washington, DC: U.S. Climate Change Science Program. ↑