By Joshua S Hill | 25 February 2011, 18:19 BST
Researchers led by members of the University of Pittsburgh have extracted a sediment core from the lakebed of Castor Lake in north central Washington which provides a six thousand year climate record of the region.
What they have found is that the traditionally rain-soaked region of the American Pacific Northwest is not going to be seeing a wet spell like the 20th century anytime soon, and that the dry seasons are most likely going to be longer as well.
The sediment core allowed researchers the opportunity to plot the region's drought history since around 4,000 BCE. What they found was that the wet and dry cycles during the past millennium have grown longer.
This deviation is attributed to the irregular pressure and temperature changes brought on by the El Niño and the La Niña climate systems that affect the region.
The researchers also reported that the wet cycle that stretches from the 1940s to the beginning of the next century was the dampest in the record in 350 years.
Lead researcher, Mark Abbot of the University of Pittsburgh, where he is a professor of geology and planetary science, says that these unusually wet years coincide with the introduction of water-use policies.
"Western states happened to build dams and water systems during a period that was unusually wet compared to the past 6,000 years," he said. "Now the cycle has changed and is trending drier, which is actually normal. It will shift back to wet eventually, but probably not to the extremes seen during most of the 20th century."
The analysis of the sediment core showed that the climate of the Pacific Northwest fluctuated relatively evenly between wet and dry periods for thousands of years. They found that;
Measurement of oxygen isotope ratios (red) and grayscale (black) arranged to show drought cycle duration and intensity with 20th century wet period indicated.
However, since approximately 1000 AD, these periods have become longer, shifted less frequently, and saw more extreme events.