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;
- 25 percent of droughts tended to persist for 30 years
- The longest drought lasted for 75 years
- Only 19 percent of wet periods lasted longer than 30 years
- The longest wet period was 64 years
However, since approximately 1000 AD, these periods have become longer, shifted less frequently, and saw more extreme events.
- The two driest cycles the researchers found out of the entire 6,000 year period occurred within 400 years of each other. The first in the 1500s and the second during the Great Depression
- The wettest cycle of the past 6,000 years began around the 1650s, and the second began 300 years later in the late 1940s
The changes in the regularity of the cycle were correlated to documented activity of the El Niño and the La Niña climate systems. When El Niño and La Niña became more intense, the wet and dry cycles in the Pacific Northwest became more erratic and lasted for a longer period of time.
The Pittsburgh-led research team produced the climate record from the sediment core by measuring the oxygen isotope rations of the mineral calcite that precipitates from the lake water every summer and builds up in the fine layers on the lake floor (as can be seen in the image of the sediment core). More calcite accumulates in wet years than in the dry years, allowing the scientists to use the calcite as a barometer.
They then reproduced their results by measuring the grayscale of the sediment, measuring the color of mud based on calcite concentrations, with darker mud signifying a drier year. They then compared these results to the Palmer Drought Severity Index which uses meteorological and tree-ring data to determine drought cycles dating back 1,500 years.
The Castor Lake sediment core matched the Palmer Index, and thus expanded it another 4,500 years, suggesting that lakebeds are better records of long-term climate change than tree-rings, if for no other reason than the length of record that they provide.
Source: University of Pittsburgh