Pacific Northwest precipitation: a look 1,500 years into the past

Castor Lake crop.JPGView full sizeMud cores from Castor Lake provide a look back in time to reconstruct the past 1500 years of precipitation history of the Pacific Northwest.

For Oregonians, accurately predicting precipitation is a serious topic. With minimal rain in summer, melting winter snow feeds our rivers for agriculture, drinking water and salmon production.

A research team set out to track precipitation history by getting a little muddy. Their

reconstructs the Northwest's precipitation over the past 1,500 years based on sediment from the region's lake bottoms.

One preliminary interpretation of the findings is that our understanding of the

could be incomplete -- and a wetter, cooler future might be on its way. Today's study is published in the journal


Forecasting the weather is a complicated task. 10-day forecasts have been shown to perform

. Projecting global climate farther out presents an even greater challenge. As researchers develop new climate models, they can first test them by predicting past weather and checking against what actually happened.

However, scientists have no way to know temperature or precipitation exactly before human records. Instead, they rely on



and lake sediment that tell the story indirectly.

Examining a Core.JPGView full sizeResearchers examine a mud core pulled from a lake bottom and cut in half. Visible stripes of sediment can help determine annual deposition patterns.

In the new study, researchers collected mud from Castor Lake and Lime Lake in northern Washington. By analyzing the chemical signature of oxygen in the mud layers, they estimated annual precipitation back to 500 A.D.

The surprise came when they compared results to a precipitation study based on tree rings. The mud record shows a noticeably different pattern over the past millennium, particularly from 900-1300 A.D. Scientists believe the American West was bone dry at this time, called the

, but the new study suggests precipitation was plentiful.

This disagreement doesn't mean either study is wrong, however. The authors of the new study propose a seasonal explanation for the difference. Tree rings grow mostly during the summer, and mud settles out best on calm, icy lakes in winter. So researchers might best figure past precipitation using both proxy types, said

, who led the study with

at the University of Pittsburgh. "Neither alone is as useful as the two systems combined."

If the Medieval Climate Anomaly was indeed a period of high precipitation in the Northwest, this could impact our understanding of the El Niño/La Niña weather cycle.

How warming or cooling of the atmosphere might cause El Niño/La Niña is not well understood. However, once these phases begin, their effects are well documented. The Northwest and the Southwest typically experience opposite precipitation patterns. During El Niño, the Northwest is warm and dry while the Southwest is cool and wet, and during La Niña, the opposite is true.

Based on solar output and the history of volcanic eruptions at the time, current climate models predict the Medieval Climate Anomaly was more like El Niño. However, if precipitation was high in the Northwest, it could be better categorized as La Niña-like.

The composition of the atmosphere today has a few features in common with what scientists think the atmosphere was like leading up to the Medieval Climate Anomaly, although the role of human-caused greenhouse gas emissions complicates the comparison.

If the Medieval Climate Anomaly was more like La Niña, it could mean current predictions of an El Niño-like warm and dry future for us are wrong, said

, a co-author of the study.

"That would actually be a good thing in terms of fresh water resources in the Pacific Northwest, but a very bad thing for the desert southwest," where water resources are already strained and which has the opposite response to the El Niño cycle.

The present data and climate model are still too preliminary to make confident forecasts of that magnitude. "It's difficult to answer" what this really means for the future, said Dr. Steinman.

Studying lakes as a proxy for past climate and precipitation needs to be performed cautiously. The University of Oregon's

, who was not involved in this study, explained that interpreting precipitation from lake sediment records is complex. Other conclusions are possible.

He pointed out that while their results don't completely contradict other climate proxies, there is still very little agreement, and these disparities need to be reconciled.

His own research has shown that

during the Medieval Climate Anomaly, in agreement with conventional understanding that this period was hot and dry. Other existing records show that area glaciers advanced considerably in the 1800s, indicating plentiful snow and rain. However, the new study suggests low winter precipitation.

In order to confirm the results of this study, the team plans to collect sediment cores from additional lakes in the area and repeat the analysis.