Ancient Dry Spells Offer Clues About the Future of Drought
12.05.11
Related briefing materials may be found
http://www.nasa.gov/topics/earth/features/ancient-dry-briefing.html">here.
As parts of Central America and the U.S. Southwest endure some of the worst droughts to hit those areas in decades, scientists have unearthed new evidence about ancient dry spells that suggest the future could bring even more serious water shortages. Three researchers speaking at the annual meeting of the American Geophysical Union in San Francisco on Dec. 5, 2011, presented new findings about the past and future of drought.
(Video available at link)Pre-Columbian Collapse
Ben Cook, a climatologist affiliated with NASA's Goddard Institute for Space Studies (GISS) and Columbia University's Lamont-Doherty Earth Observatory in New York City, highlighted new research that indicates the ancient Meso-American civilizations of the Mayans and Aztecs likely amplified droughts in the Yucatán Peninsula and southern and central Mexico by clearing rainforests to make room for pastures and farmland.
Converting forest to farmland can increase the reflectivity, or albedo, of the land surface in ways that affect precipitation patterns. "Farmland and pastures absorb slightly less energy from the sun than the rainforest because their surfaces tend to be lighter and more reflective," explained Cook. "This means that there’s less energy available for convection and precipitation."
New climate modeling shows that widespread deforestation in pre-Columbian Central America corresponded with decreased levels of precipitation. This image shows how much precipitation declined from normal across the region between 800 C.E. and 950 C.E. It was during this period of time that the Mayan civilization reached its peak population and abruptly collapsed. (Credit: Ben Cook, NASA’s Goddard Institute for Space Studies)
Cook and colleagues used a high-resolution climate model developed at GISS to run simulations that compared how patterns of vegetation cover during pre-Columbian (before 1492 C.E.) and post-Columbian periods affected precipitation and drought in Central America. The pre-Columbian era saw widespread deforestation on the Yucatán Peninsula and throughout southern and central Mexico. During the post-Columbian period, forests regenerated as native populations declined and farmlands and pastures were abandoned.
Cook's simulations include input from a newly published land-cover reconstruction that is one of the most complete and accurate records of human vegetation changes available. The results are unmistakable: Precipitation levels declined by a considerable amount -- generally 10 to 20 percent -- when deforestation was widespread. Precipitation records from stalagmites, a type of cave formation affected by moisture levels that paleoclimatologists use to deduce past climate trends, in the Yucatán agree well with Cook's model results.
The effect is most noticeable over the Yucatán Peninsula and southern Mexico, areas that overlapped with the centers of the Mayan and Aztec civilizations and had high levels of deforestation and the most densely concentrated populations. Rainfall levels declined, for example, by as much as 20 percent over parts of the Yucatán Peninsula between 800 C.E. and 950 C.E.
Cook's study supports previous research that suggests drought, amplified by deforestation, was a key factor in the rapid collapse of the Mayan empire around 950 C.E. In 2010, Robert Oglesby, a climate modeler based at the University of Nebraska, published a study in the Journal of Geophysical Research that showed that deforestation likely contributed to the Mayan collapse. Though Oglesby and Cook's modeling reached similar conclusions, Cook had access to a more accurate and reliable record of vegetation changes.
During the peak of Mayan civilization between 800 C.E. and 950 C.E., the land cover reconstruction Cook based his modeling on indicates that the Maya had left only a tiny percentage of the forests on the Yucatán Peninsula intact. By the period between 1500 C.E. and 1650 C.E., in contrast, after the arrival of Europeans had decimated native populations, natural vegetation covered nearly all of the Yucatán. In modern times, deforestation has altered some areas near the coast, but a large majority of the peninsula’s forests remain intact.
"I wouldn't argue that deforestation causes drought or that it's entirely responsible for the decline of the Maya, but our results do show that deforestation can bias the climate toward drought and that about half of the dryness in the pre-Colonial period was the result of deforestation," Cook said.
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