6,000-year climate record suggests longer droughts, drier climate for Pacific Northwest

February 22, 2011
Measurement of oxygen isotope ratios (red) and grayscale (black) arranged to show drought cycle duration and intensity with 20th century wet period indicated. Credit: Mark Abbott

University of Pittsburgh-led researchers extracted a 6,000-year climate record from a Washington lake that shows that the famously rain-soaked American Pacific Northwest could not only be in for longer dry seasons, but also is unlikely to see a period as wet as the 20th century any time soon. In a recent report in the Proceedings of the National Academy of Sciences, the team linked the longer dry spells to the intensifying El Niño/La Niña climate pattern and concluded that Western states will likely suffer severe water shortages as El Nino/La Nina wields greater influence on the region.

The researchers analyzed a sediment core from Castor Lake in north central Washington to plot the region's drought history since around 4,000 BCE and found that wet and dry cycles during the past millennium have grown longer. The team attributed this recent deviation to the irregular pressure and temperature changes brought on by El Niño/La Niña. At the same time, they reported, the wet cycle stretching from the 1940s to approximately 2000 was the dampest in 350 years.

Lead researcher Mark Abbott, a Pitt professor of geology and planetary science, said those unusually wet years coincide with the period when western U.S. states developed 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."

Abbott worked with his former graduate student, lead author and Pitt alumnus Daniel Nelson, as well as Pitt professor of geology and planetary science Michael Rosenmeier; Nathan Stansell, a Pitt PhD graduate now a postdoctoral researcher at Ohio State University; and Pitt geology and planetary science graduate student Byron Steinman. The team also included Pratigya Polissar, an assistant research professor at Columbia University's Lamont-Doherty Earth Observatory; Joseph Ortiz, associate professor of geology at Kent State University; Bruce Finney, a professor of geology at Idaho State University; and Jon Riedel, a geologist at North Cascades National Park in Washington.

The team produced a climate record from the lake mud by measuring the oxygen isotope ratios of the mineral calcite that precipitates from the lake water every summer and builds up in fine layers on the lake floor. More calcite accumulates in wet years than in dry years. They reproduced their findings by measuring grayscale, or the color of mud based on calcite concentration, with darker mud signifying a drier year.

Duration of dry and wet cycles by percentage over 6,000-year period. Credit: Mark Abbott

The record in the sediment core was then compared to the Palmer Drought Severity Index, which uses meteorological and tree-ring data to determine drought cycles dating back 1,500 years, Abbott explained. The Castor Lake core matched the Palmer Index reconstructed with tree-ring data and expanded on it by 4,500 years, suggesting that lakebeds are better records of long-term climate change, the authors contend.

Analysis of the sediment core revealed that the climate of the Pacific Northwest fluctuated more or less evenly between wet and dry periods for thousands of years, the researchers wrote. Droughts tended to be lengthier with 25 percent of dry periods during the past 6,000 years persisting for 30 years or more and the longest lingering for around 75 years. Wet periods tended to be shorter with only 19 percent lasting more than 30 years and the longest spanning 64 years.

But since around 1000 AD, these periods have become longer, shifted less frequently, and, most importantly, ushered in more extreme conditions, Abbott said. The two driest cycles the researchers detected out of the past 6,000 years occurred within only 400 years of each other—the first in the 1500s and the second during the Great Depression. Wet periods showed a similar pattern shift with five very wet eras crammed into the past 900 years. The wettest cycle of the past 6,000 years began around the 1650s, and the second most sodden began a mere 300 years later, in the 1940s.

The change in cycle regularity Abbott and his colleagues found correlates with documented activity of El Niño/La Niña. When the patterns became more intense, wet and dry cycles in the Pacific Northwest became more erratic and lasted longer, Abbott said.

Explore further: La Nina has a mind of her own

Related Stories

Southwest headed for permanent drought

January 31, 2011

(PhysOrg.com) -- The American Southwest has seen naturally induced dry spells throughout the past, but now human-induced global warming could push the region into a permanent drought in the coming decades, according to Lamont-Doherty ...

Recommended for you


Adjust slider to filter visible comments by rank

Display comments: newest first

2.5 / 5 (11) Feb 22, 2011
And this is due to a NATURAL pattern and NOT the insidious EVIIIILLLLLL meddling of the smart apes and their propensity to burn the black liquefied remains of dead bio-matter??!!!

Surely they jest...
5 / 5 (2) Feb 22, 2011
I hate those evil smart apes...
2.5 / 5 (8) Feb 22, 2011
This study can be discarded because the concensus says that drought and intensifying ENSO cycles are being caused by the apes. This is clearly an outlier or an anomaly. If you average this in with all the tree ring studies then it still looks like the apes did it. Sorry, your denialist fantasy that ENSO changes climate over time is wrong, because no matter how much real world evidence you provide, the models all agree.
1 / 5 (3) Feb 22, 2011
Ran out of time on the edit:

Seriously though; This is an interesting article. It seems to indicate that we could be headed for trouble in our future. I wonder what a similar study on the West side of the Pacific would show. Would it be the opposite, or the same? I'd love to see someone do that work and test to see if there is a similar correllation. As I have said before. A core study of only one location doesn't have much value. You need to test the results regionally and against samples in other regions.

I think it would be great to do another core sample somewhat near the Vostok location too. Then you could compare them to check interpretations. It's like our assumptions about the origins of life. Without any other location with life to compare to, there's no way to know if we are right or not.
2.6 / 5 (5) Feb 22, 2011
Not to "rain" on anyone's parade, but weather changes, and we might be in trouble in the future because weather changes.

Lets hope that we get a handle on warming gases and the earth can continue on to it's scheduled cooling trend so that world food production can plummet and American food exports can be used to buy back all that debt that foreign governments own.


1 / 5 (1) Feb 22, 2011
Ah but america, and canada, depend a lot on the prairies for food production.. And that won't be much help once they become deserts.

Therefore we'll both need to import even more foods, which would plunge us into dept, except that no one would be selling it, on account of local needs. Well maybe many 3rd world countries will make their citizens do slave labour on farms in order to sell to 1st world countries..

Oh wait..
5 / 5 (1) Feb 22, 2011
"Famously rain-soaked"?! Yeah, if you live in Seattle. Go over the mountains and people are surprised to find out how much of the state is sagebrush desert.

I'm from Spokane originally. I eventually got to the point where I would tell people I was from North Idaho instead (very close and I did spend a lot of time there as a kid) because if I told them I was from Washington State they'd always say: "Oh, that's where it rains all the time."
4.2 / 5 (5) Feb 22, 2011
So, these guys are saying that there is a close correlation between ENSO cycles and the pattern/duration of wet and dry weather cycles n the PNW.

One would expect, then, if the ENSO activity intensifies -as would be expected from AGW- then these cycles of wet/dry would also intensify.
Seems pretty clear cut -I don't see how you can decouple the effects of GW, just because it appears from this data that the trend has going on longer than just since the industrial revolution- it has to be remembered that human populations(and their carbon-emitting activities) have been on the rise, too, for at least as long.

Not saying that that is a 100% causative factor, but it certainly is a contributing one.
not rated yet Feb 22, 2011
Forest fires imminent.

Summers are already ridiculous in southwestern BC. Last summer I measured over 40 Celsius in the shade on the hottest day. It topped out the 50C thermometer in the direct sun.
1 / 5 (1) Feb 23, 2011
One would expect, then, if the ENSO activity intensifies -as would be expected from AGW

Actually, there are several papers in the past couple years, as well as the most currrent updates on official estimates from the top climate/weather authorities. They all say the same thing; that we do not know what effect global warming will have on ENSO. Historical records are contradictory, in that the strongest/weakest ENSO cycles over a span of 100's of thousands of years, do not coincide regularly with warm/cool periods. Unfortunately, the ENSO cycle seems to change independently of global temperature trends. I believe the current theory is that it is more tied to differences between regions rather than global average conditions. So, while the whole world could be warmer or cooler, it's the relative difference between one region and the surrounding regions that really affect the strength and duration of ENSO. It's still an open question as far as I can tell by reading the research.
1 / 5 (1) Feb 23, 2011
You can look up the NOAA ENSO index record by googling the following and pick the first link:

NOAA Multivariate ENSO Index

I did a neat little 'experiment' by copying the graph into MS Paint and then I used the box tool to copy and paste all the blue parts into one graph and all the red parts into another graph. Try it and see what it looks like.

Also, you should all realize that different organizations define the ENSO cycle differently and depending on which index you are looking at, the strength and durration of event years will be different. Google the following NOAA page and see the section titled "What years are El Nino/La Nina years?":

PSD's El Nino FAQ

That is a VERY good page to browse. Take a few hours with it; It's very good reading. It's a mouthfull, but it is the NOAA/Earth System Laboratory - Physical Science Division FAQ web page.
not rated yet Feb 27, 2011
I can finally move to Washington!

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.