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May 12, 2011 

How Does Climate Change Contribute to Heavy Rain and Flooding?

Scientists have determined that loading the atmosphere with increasing carbon dioxide and other heat-trapping emissions is raising global temperatures and triggering heavier precipitation events. How is this happening?

Carbon dioxide from burning fossil fuels and destroyed tropical forests accumulates in the atmosphere, trapping heat that would otherwise escape into space, This trapped heat raises the planet’s average temperature. Some of the extra heat evaporates water from the ocean and soil into the atmosphere. Additionally, growing plants transfer water vapor into the atmosphere.

As average global temperatures rise, the warmer atmosphere can also hold more moisture, about 4 percent more per degree Fahrenheit temperature increase. Thus, when storms occur there is more water vapor available in the atmosphere to fall as rain, snow or hail. Worldwide, water vapor over oceans has increased by about 4 percent since 1970 according to the 2007 U.N. Intergovernmental Panel on Climate Change report, its most recent.

It only takes a small change in the amount of water vapor in the atmosphere to have a major effect. That’s because storms can draw upon water vapor from regions 10 to 25 times larger than the specific area where the rain or snow actually falls.

According to the U.S. Global Change Research Program’s (USGCRP) most recent report, scientists have observed less rain falling in light precipitation events and more rain falling in the heaviest precipitation events across the United States. From 1958 to 2007, the amount of rainfall in the heaviest 1 percent of storms increased 31 percent, on average, in the Midwest and 20 percent in the Southeast.

After a heavy precipitation event, there is less water vapor in the atmosphere, and therefore dry spells tend to be longer. In the absence of rain, extra heat exacerbates drying and can contribute to longer and more intense drought periods.

USGCRP has projected that climate change is likely to increase the disparity between light and heavy precipitation events.

What factors are contributing to the Mississippi floods?

Several natural factors, some influenced by human-induced climate change, are triggering the recent heavy rain and flooding along the Mississippi.

The pattern of rains and drought is set up largely by the La Niña—cold tropical Pacific sea surface temperatures—which influences the jet stream and the movement of storms across the United States. These conditions shift precipitation, helping to drive extreme drought in Texas and Oklahoma while also contributing to extremely wet conditions in southern Missouri and the Ohio River Valley.

Additionally, a combination of natural factors and Arctic sea ice melt brought a relatively cold winter to much of the central United States, causing more precipitation to fall as snow, rather than rain, on the upper Midwest. Now that it is spring, that snow is melting and feeding into the Mississippi at the same time heavy rainfalls have occurred.

All these factors helped make April 2011 the 10th wettest on record in the United States, according to the National Climatic Data Center (NCDC) at the National Oceanic and Atmospheric Administration (NOAA), which maintains records going back to 1894. Several states where rainfall drains into the Mississippi also experienced their wettest April on record.

Finally, human-induced climate changes continues to warm the Gulf of Mexico. Higher temperatures increase the amount of water that evaporates from the gulf’s surface as well as the temperature of air that moves over the gulf, increasing the amount of water vapor it can hold. At the same time, shifts in natural ocean currents are also contributing to higher temperatures in and over the gulf.

Natural atmospheric circulation patterns then carry water vapor over the gulf to North America. According to the NOAA’s National Environmental Satellite, Data and Information Service (NESDIS), the Gulf of Mexico was nearly 2 degrees Fahrenheit hotter than average in late April when parts of the Midwest experienced intense downpours.

What is the economic impact of flooding in the United States?

More intense precipitation can have dire consequences for people living in flood-prone regions, especially farmers. The economic impact of flooding caused by heavy precipitation depends on patterns of human settlement in flood-prone areas—how many people and how much property are exposed to flood damage—as well as investments in measures to mitigate flooding, such as constructing drains, levees, ditches and culverts.

Past experience shows that flooding can cause extensive and expensive economic losses. Governments need to update their plans to prevent and mitigate floods to account for the ways in which climate change is driving more intense precipitation events.

According to NOAA, flash flooding and river flooding in the United States caused an average of $2.7 billion in damages annually from 2000 to 2010 (in 2007 dollars). This is solely a measure of damage to property and crops. It does not include deaths or injuries.

Additionally, 16 of the 99 events NCDC tracked since 1980 that each caused more than $1 billion (in 2007 dollars) of economic damage were the result of flooding. Over time, these losses likely will increase, not only due to growing populations and rising property values in flood-prone areas, but also in some measure because climate change makes more intense precipitation events more likely.

 

The Union of Concerned Scientists puts rigorous, independent science to work to solve our planet's most pressing problems. Joining with citizens across the country, we combine technical analysis and effective advocacy to create innovative, practical solutions for a healthy, safe, and sustainable future.

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