Darby Team Finds Climate Evidence in Arctic Sea Sediments
A team of scientists led by Old Dominion University geological oceanographer Dennis Darby reports this week in an article slated for publication in the journal Nature Geoscience that it has identified for the first time a clear 1,500-year cycle in the Arctic Oscillation (AO), the surface atmosphere pressure pattern in the far north that greatly influences weather in the Northern Hemisphere.
The researchers' findings indicate that this natural cycle could be forcing some of the Arctic ice melting and unusual weather experienced recently in the heavily populated northern half of the globe. In a worst-case scenario that Darby described in an interview, the cyclical pressure pattern could combine with manmade climate change to exacerbate severe weather and flooding trends.
In the near term, according to Darby, "The implications are that the severe winters of 2009-11 (experienced in northern Europe and elsewhere) might be tame compared to what is possible, based on past swings in the AO."
From a 20-meter-deep core from offshore of Alaska in 1,300 meters water depth, the researchers could detect varying amounts of sand grains ice-rafted from Russia over the last 8,000 years. These ice-rafted sand grains that originated thousands of miles away from this Alaskan coast and could have only gotten there under particular AO conditions.
Darby and his colleagues were able to show through geochemical analysis that some of the iron grains among 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 gotten to the Alaskan coast by drifting in ice. Furthermore, the ice floes would only move from the Kara to offshore Alaska during a strong positive AO.
When the AO index is positive, surface pressure is low in the polar region. This helps the middle latitude jet stream to 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.
Darby's co-authors for the Nature Geoscience article, "1,500-year Cycle in the Arctic Oscillation Identified in Holocene Arctic Sea-ice Drift," are Joseph Ortiz, a geologist from Kent State University; Chester Grosch, a physical oceanographer and computer scientist from ODU; and Steven Lund, a geophysicist from the University of Southern California.
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.
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 oscillations 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 CO2.
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."
This leads Darby to his near-term forecast: "Thus the AO is potentially capable of much larger swings to positive or negative phases. If so, then these much stronger highs and lows should have greater impact on weather and climate, causing much colder Northern Hemisphere winters during a strong negative AO and milder winters during strong positive AO than seen in the last several hundred years. Such AO positive swings could add to anthropogenic warming causing even more rapid ice melt than has been seen in the last decade."
One of the most expensive research instruments on the ODU campus, a $1.2 million electron probe microanalyzer, enables Darby to sift through climate clues dating back 100,000 years or so. His goal is to chart natural climate cycles, and to determine how those cycles may be better understood separate from random climate changes that scientists believe are caused by man-made atmospheric pollution.
An electron microprobe works similarly to a scanning electron microscope. A sample is bombarded with an electron beam, and X-rays are emitted at wavelengths characteristic to the elements being analyzed. This enables the prevalent elements present within small sample volumes (typically 10-30 cubic micrometers or less) to be determined.
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.
The top of a cylindrical core sample is made up of recently deposited sediments on the sea bottom; deeper down in the sample are sediments from times past. There are numerous ways to date a grain of sand depending on where it shows up in the vertical sample. Darby has worked with some samples that were deep enough (more than 300 m below the sea-floor) to have sediments dating back more than 40 million years.
Analysis of core samples, therefore, can help identify long-standing climate patterns. These revelations are of heightened importance now because of concerns about climate change and global warming.
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.
The focus on the Arctic by climate scientists has guaranteed Darby a steady stream of research grants, one of them being the $500,000 contributed by the National Science Foundation (NSF) in 2009 to assist in the university's purchase of the new microprobe. Another of his NSF grants - $433,000 for a three-year project extending through July of 2014 - is for analysis of core samples from the bottom of the Denmark Strait off Greenland. The project is titled "Do Holocene Variations in Arctic Sea Ice and Greenland Icebergs Drifting Through Denmark Strait Reflect Natural Cycles?"
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. 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."