‘Megadroughts’ May Recur in Rio Grande Area; example 50 year drought from 122 to 172 AD

‘Megadroughts’ May Recur in Rio Grande Area
By John Fleck - Albuquerque Journal Staff Writer
Nov 9, 2011

Tree rings from the headwaters of the Rio Grande show a 50-year drought from 122 to 172 AD, suggesting that “megadroughts” may be a recurring feature of the region’s climate, according to new research by a University of Arizona team.

Scientists have long known about similar drought a thousand years later that has been linked to the end of the early Native American culture of Chaco Canyon and the Four Corners area. But it was never clear whether that drought was unique.

The new evidence, being published in the journal Geophysical Research Letters, comes from a grove of bristlecone pine found by University of Arizona graduate student Cody Routson in the mountains near Summitville, Colo. He initially spotted the trees through binoculars during the summer of 2007, and returned in later years to take samples used to show the trees’ year-by-year growth.

The oldest living trees were 1,000 years old, Routson said, and dead trees allowed the team to extend the record back another 1,000 years.

The resulting data provides the most detailed record of climate in the San Juan and Rio Grande basins – the river basins that provide much of New Mexico’s water supply – over the last 2,000 years.

Scientists have long known about the medieval drought, but never knew whether it was unique, said Columbia University climate researcher Richard Seager. Routson’s data shows it was not, Seager said in a telephone interview.

The 50-year drought was embedded in a much longer dry period that lasted about 400 years, Routson and his colleagues concluded.

What caused the drought remains a mystery. Routson and his colleagues are looking at large-scale climate patterns involving the oceans for clues, as well as things like continental dust.

But whatever the cause, the new evidence should serve as a reminder to residents of the arid Southwest that long spells much drier than what we have become accustomed to are the norm, said University of Arizona climate scientist Jonathan Overpeck, Routson’s adviser and a co-author of the paper. Overpeck first publicly discussed the research during an April Senate field hearing in Santa Fe organized by Sen. Jeff Bingaman, D-N.M.

“Whether it’s climate change, natural variability or both working together as I anticipate, those who depend on Rio Grande for water should have plans for getting by on a lot less water,” Overpeck said.