Floods, Droughts Linked to Global Warming

Global warming is responsible for this severe flooding in the Pacific Northwest and California, and we can expect more of the same, and worse, according to a senior scientist with the Commerce Department's National Oceanic and Atmospheric Administration (NOAA).

Observations since the beginning of the 20th century for the United States indicate that intense precipitation events have already increased by about 20 percent, and cold season precipitation has increased by nearly 10 percent, Karl said. An increase in the intensity of precipitation has led to an increased flood potential.

Overall, precipitation totals for the October-December 1996 period exceeded 150% of normal throughout the Northwest. During October, totals averaged 125% of normal throughout the Northwest, following an abnormally wet September. Totals averaged approximately 110% of normal during November. Very heavy precipitation then inundated the Northwest during December, with totals averaging more than 200% of normal recorded throughout the region.

Statewide-average snowpack water content as of January 7, exceeded 150% of normal in California, Idaho, Montana, Nevada, Oregon, Utah and Washington, according to NOAA measurements. The largest abnormalities were observed in Washington, where snowpack water totals exceeded 225% of normal.

What does global warming have to do with the flooding? Karl explains that increased concentrations of greenhouse gases in the atmosphere lead to an increase in mean global temperatures. As the global climate warms, the water cycle is affected because a portion of the heating will go into evaporating larger quantities of water from the earth's surface.

As global temperatures increase, the atmosphere can also support greater amounts of water vapor. This means more flooding with an increase in extreme precipitation events, but also more droughts. Droughts arise where and when the atmosphere is not favorable to rain and snow, and the evaporated moisture is transported to other regions. The additional evaporation from the surface leads to a drying of the soil, and more severe and widespread droughts.

When carbon dioxide concentrations are doubled, the expected frequency and extent of extreme droughts and intense precipitation in the United States increase (more than two inches per day) and Canada (more than one inch per day). Some models show a three to four-fold increase. There is also a distinct increase in wintertime or cold season precipitation.

Recent events, like the flooding in the Northeast last autumn and the flooding this winter, offer examples of the kind of situations that are expected to be associated with an increased risk of global warming, Karl said.

The drought side of the equation is easier to understand in the light of global warming. The World Energy Council, an independent research organization, released a report July 4, 1996 which found that global emissions of carbon dioxide (CO2) increased by 12% between 1990 and 1995. The Council said the trend would continue unless direct action is taken to reduce emissions.

The World Energy Council report said two to three times as many people may die in heat waves annually by the year 2050 as do today, even if they become accustomed to warmer temperatures. It also forecast a substantial spread in the range of malaria and other tropical diseases over the next several years.

The Intergovernmental Panel on Climate Change (IPCC), a large group of climate experts who have been conducting research and reviewing studies over the past several years, has concluded that there has been significant global warming during this century, based on a careful review of all of the available data, including ground-based temperature records, satellite data, balloon-borne temperature profiles, bore hole temperature profiles, and retreat of mountain glaciers.

NOAA offers a web page with flooding data, and further information can also be found at about the National Climatic Data Center.