An analysis of sediments from two Pacific Northwest lakes could shed new light on the frequency and timing of drought in the western United States.

The findings were published yesterday in the journal Proceedings of the National Academy of Sciences.

Researchers examined sediment cores drilled from two small Washington state lakes, Castor and Lime, to reconstruct a record of the Pacific Northwest climate over the past 1,500 years. Changes in the ratio of two different forms, or isotopes, of oxygen in minerals in the cores allowed the scientists to track changes in the amount of water in the lakes over time.

Then they compared that information with similar climate records produced by analyzing patterns of growth from tree rings.

"The tree-ring data from the Pacific Northwest, central Washington in particular, suggests that during the Medieval Climate Anomaly, conditions were drier, and during the Little Ice Age, conditions were wetter," said lead author Byron Steinman, a postdoctoral fellow in paleoclimatology at Pennsylvania State University who began studying Castor Lake as a graduate student with his University of Pittsburgh adviser, Mark Abbott. "That was the first thing we noticed when we compared the data from Castor Lake and the tree-ring record -- we saw the opposite longer-term pattern."

The lake sediments contained evidence that during the unusually warm conditions of the Medieval Climate Anomaly, A.D. 950 to 1250, precipitation in the Pacific Northwest was above average. And during the Little Ice Age, A.D. 1450 to 1850, the region was drier than normal.

One possibility to explain the difference, Steinman said, is that the lake cores and the tree rings record the climate in different seasons. The lake sediment is better at capturing precipitation during winter months, while tree growth occurs in spring and summer.

"Perhaps there were both wetter winters and hotter and drier summers during the Medieval Climate Anomaly," he said.

Heading toward a 'La Niña-like future'

What sounds like a fairly wonky debate about the relative merits of tree rings versus muck dredged up from the bottom of lakes has broader implications, said Penn State climatologist Michael Mann, one of Steinman's co-authors.

"What is important about this study is that it does seem to reinforce this picture of a La Niña-like medieval period," Mann said.

During that period, there is some evidence that high solar activity and an absence of major volcanic eruptions combined to warm much of the Northern Hemisphere.

Today, it's human influences, including rising greenhouse gas levels in the atmosphere, that are exerting a warming effect. But the new study suggests that man-made warming may also push the world to more La Niña-like conditions, Mann said.

"If we have a La Niña-like future, then there are a number of impacts that will actually be worse than what is currently projected" by the Intergovernmental Panel on Climate Change, he said.

La Niña is known to exacerbate Atlantic Ocean hurricane activity and increase the likelihood of drought in the desert Southwest, for example. Scientists believe two back-to-back La Niña events that began in 2010 and ended earlier this year intensified the historic drought that gripped much of Texas last year.

But some experts who did not contribute to the new study questioned its conclusions.

Critics question results

Richard Seager, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory, said sediment cores like those drilled from Castor and Lime lakes can be a useful check on reconstructions of past climate that rely on tree-ring data.

Seager said the new analysis did not appear to use the most recent compilation of tree-ring records from the western United States. Those records show a north-south dipole between the Pacific Northwest and the Southwest, Seager said. In other words, when the Pacific Northwest is wet, the Southwest is usually dry -- and vice versa.

The idea of a conflict between the tree-ring and lake sediment records appears to be overblown, he said. "I just don't think that's a fair criticism," Seager said.

Greg Pederson, a research scientist at the U.S. Geological Survey's Northern Rocky Mountain Research Center, said it was "not entirely appropriate" for the authors of the Proceedings of the National Academy of Sciences paper to compare tree-ring analyses that reconstruct summer drought in the Pacific Northwest and Northern Rockies with the lake data, which record winter precipitation in the same areas.

That's an apples-to-oranges comparison, he said, because the two data sets examine different seasons.

Pederson said previous research and observations in the region show that precipitation can vary greatly between summer and winter. A dry winter won't necessarily lead to a dry summer, and a wet winter doesn't always flow into a wet summer.

As for the suggestion that La Niña-like conditions during the Medieval Climate Anomaly augur a more La Niña-like future, Seager said it's not clear that all factors that produce strong climate warming create a La Niña effect.

Natural warming produced by higher solar output changes the energy reaching Earth from the sun, while warming produced by greenhouse gases alters the amount of the sun's radiation that Earth reflects back into space.

"It's not clear that the climate system will respond the same way to both of those," Seager said.