Researchers take samples aboard the U.S. Coast Guard Cutter Healy in the Arctic.
Credit: Joseph Ortiz, Kent State University
Date: December 4, 2012
A team of scientists supported by the National Science Foundation (NSF) has identified for the first time a clear 1,500-year cycle in the far North's surface atmosphere pressure pattern. Called the Arctic Oscillation (AO), the cycle greatly influences weather in the Northern Hemisphere.
Lead researcher Dennis Darby, a geological oceanographer at Virginia's Old Dominion University, used the findings to describe a worst-case scenario in which the cyclical pressure pattern could combine with man-made climate change to exacerbate severe weather and flooding trends.
The findings were published Nov. 11 on Nature Geoscience's website. Darby coauthored the paper with a team of scientists from Old Dominion and Kent State universities and the University of Southern California (USC).
Coauthors are Joseph Ortiz, a geological oceanographer from Kent State; Chester Grosch, a physical oceanographer and computer scientist from ODU and Steven Lund, a geophysicist from USC.
William Wiseman, a program director in the Arctic Natural Sciences Program in NSF's Office of Polar Programs, said the new research is innovative in its approach to separating human influences on climate from naturally occurring events.
"Separating the effects of human contributions to climate variability from those due to natural variability is never easy," he said. "Darby and his colleagues, using clever analyses of sediment data, have noted an important long-term variation in sediment transport that is consistent with variability in the Arctic climate on similar time scales. This work adds one more piece of information to our understanding of a very complex system."
Working from a 20-meter-long sediment core raised offshore of Alaska from waters 1,300 meters deep, the researchers could detect varying amounts of iron-rich sand grains ice-rafted from Russia over the last 8,000 years. The core was originally recovered from the flank of Barrow Canyon by an NSF-funded oceanographic cruise on which researchers Lloyd Keigwin, Julie Brigham-Grette and Neil Driscoll were co-investigators.
Darby and his colleagues were able to show through geochemical analysis that some of these Russian grains came from the Kara Sea, which is off the northern Russia landmass east of the northern tip of Finland. This is more than 3,000 miles from the core sample site, and the authors say Kara iron grains could have only arrived at the Alaskan coast by drifting in ice. Furthermore, the ice floes would only move from the Kara to offshore Alaska during strong positive AO conditions.
When the AO index is positive, surface pressure is low in the polar region. This helps the mid-latitude jet stream blow strongly and consistently from west to east, thus keeping cold Arctic air locked in the polar region. When the AO index is negative, there tends to be high pressure in the polar region, weaker zonal winds and greater movement of frigid polar air into the populated areas of the middle latitudes.
Measurements taken by instruments in modern times clearly show relatively short-term fluctuations in the AO, with profound impacts on weather and climate. "But how the AO varies during the Holocene (roughly the last 12,000 years) is not well understood," the authors write in Nature Geoscience.
Darby said that time-series analysis of the researchers' geochemical record reveals a 1,500-year cycle that is similar to what other researchers have proposed in recent decades, based on scattered findings in paleoclimate records. But he and his colleagues are the first to find a high-resolution indicator of the Arctic record that resolves multidecadal-through-millennial-scale AO cycles, he said.
"Our record is the longest record to date to reconstruct the AO and documents that there is millennial scale variability in the AO," Ortiz said. "The sedimentation rate at our site is also sufficient to statistically differentiate between a 1,000-year cycle and a 1,500-year cycle, which helps us to understand the dynamics of the response of the climate system to external forcing during the Holocene geological period."
The 1,500-year cycle is distinct from a 1,000-year cycle found in a similarly analyzed record of total solar irradiance, the authors write, suggesting that the longer cycle arises from either internal oscillation of the climate system or as an indirect response to low-latitude solar forcing.
"The AO can remain in a rather strong negative or positive mode for many decades," the research team writes in the Nature Geoscience article. "When it is positive as suggested by the upswing in the Kara series during the last 200 years, then the additional warmth due to the entrapped Arctic cold air masses during winters could exacerbate the mid-latitude signature of anthropogenic global warming resulting from increased atmospheric carbon dioxide. When the AO is strongly negative as seen in the winters of 2009-11, the Northern Hemisphere experiences prolonged intervals of colder than normal conditions. Because the maximum amplitudes of the AO as recorded in the Kara (iron) grain record in recent decades is less than a third of the amplitude in the past, the full range of variability in the AO is not likely recorded in the instrumental records of the last few decades."
Darby does his detective work by analyzing sediments, mostly from core samples that have been collected when researchers drill a hollow tube into the floor of the Arctic Ocean or nearby seas. The work is made possible by an iron-grain chemical fingerprinting technique he developed that enables him to determine the landmass where the grains originated. This provides evidence about winds and currents--and therefore the overall weather patterns--that brought the grain to its resting place.
Even if natural cycles are responsible for some recent warming trends, this doesn't let humans off the hook for polluting the atmosphere, Darby said. Human influence may combine with natural cycles to increase global warming.
Darby's research is not directly involved in weighing human contributions to climate change, such as increases in carbon dioxide in the atmosphere brought on by combustion.
"We're looking for natural conditions that are helping to cause this global warming and sea level rise," Darby said. "There seems to be a natural pacing to climate change. If you don't know what changes are naturally occurring over the long haul, you don't know how to deal with conditions over the short term."
Peter West, NSF (703) 292-7530 firstname.lastname@example.org
William J. Wiseman, NSF (703) 292-4750 email@example.com
Dennis Darby, Old Dominion University 757-683-4701 firstname.lastname@example.org