Richard Seager, pictured above outside his office building at the Lamont-Doherty Earth Observatory, will discuss the causes of persistent drought in the American West on October 18 at the New York Academy of Sciences.

Droughts that last several years are a recurring feature of the American West. They are also potentially costly natural disasters with impacts ranging from declining agricultural production, reduced water availability, increased forest fires, variable river flows, and declining fisheries. The Dust Bowl drought of the 1930s, which was memorialized in some of the greatest works of American culture, was a searing example of how droughts can also affect the social, economic and political fabric of the country.

Richard Seager, a senior scientist at Columbia University's Lamont-Doherty Earth Observatory, will visit the New York Academy of Science on October 18 to discuss the hydrological and climate history of the American West from 800 A.D. to the present. He will also present his and colleagues' analyses of the past climate housed in such records as tree rings and lake sediment and explain what climate model simulations tell us about the past and future climate of the American West.

Only in recent years have meteorologists, climatologists and oceanographers begun to unravel the causes of persistent droughts in the west, placing the blame on subtle but recurrent changes in sea surface temperatures (SSTs) in the tropical Pacific that arise as part of the ocean-atmosphere phenomenon known as the El Niño Southern Oscillation. During periods of persistently cool waters in the eastern equatorial Pacific, atmospheric temperatures and circulation changes in a manner that, over North America, effectively suppresses precipitation.

Droughts over North America are, however, not isolated events, but arise as part of a global reorganization of the Earth's rain belts. When drought strikes the American West it also strikes southern South America, parts of Europe and central Asia. At the same time, land areas in the Tropics see consistently more rainfall.

Ed Cook and researchers at Lamont-Doherty's Tree Ring Laboratory used more than 20,000 tree samples from the United States, Mexico, and parts of Canada to reconstruct the history of drought across North America over the last 2005 years. Click on the above picture to view an animation of their reconstructed record.

Even though the droughts that struck the American West had devastating consequences for Native Americans, settlers and animal communities in the mid-nineteenth century and on small farmers during the Dust Bowl years, these droughts were modest compared to those that scientists believe occurred during the Medieval period. At that time, the American West experienced severe droughts with alarming regularity over the span of several hundred years. Once more, the evidence points to the tropical Pacific Ocean as the cause, with SSTs there apparently varying in response to a period of relatively high solar irradiance and reduced volcanic activity on Earth. If so, then the causes and impacts of the Medieval mega-droughts in the West have important implications for the present-day West in a world also under the influence of human-induced global warming.

Seager is a senior research scientist at Lamont-Doherty Earth Observatory, an affiliate of The Earth Institute at Columbia University, who specializes in studying the ways that coupling between the atmosphere and ocean causes climate variability around the world on seasonal timescales to periods spanning tens of thousands of years. He was educated in England until moving to New York in 1983 and earned his Ph.D. from Columbia in 1990. Seager's presentation will take place from 6:00 to 7:00 p.m. on Tuesday October 18 at the headquarters of the New York Academy of Science (directions available at http://www.nyas.org/about/directions.asp). The event is free for members of NYAS and affiliates of the Environmental Sciences Section and costs $20 for non-members or $10 for non-member students. A reception will follow.