Climate Clues in the Ice

Image in title graphic courtesy of Jet Propulsion Laboratory.
By Laurie J. Schmidt
For the past five years, Drew Rothrock, Principal Research Scientist at the University of Washington's Applied Physics Laboratory, has been studying measurements of sea ice draft (sea ice thickness below the water surface) in the Arctic Ocean. Rothrock and colleagues compared sea ice draft data collected on U.S. Navy submarine cruises between 1993 and 1997 with data collected on cruises in the same region between 1958 and 1976. Since changes in sea ice correspond to climate change patterns, Rothrock said he had expected to find that the ice had thinned by maybe 45 to 50 centimeters (18 to 20 inches), but he found that mean ice draft at the end of the melt season in most of the deep-water portion of the Arctic Ocean had actually decreased by about 1.3 meters (3.5 feet).

(Graph derived from Rothrock et al.: Thinning of the Arctic Sea-Ice Cover, 1999)
First, Rothrock and his co-workers looked at the data from three SCICEX cruises. These data, available from NASA's National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC), showed a perennial ice cover with drafts of between 1 and 3 meters, a considerably thinner layer than prior studies had shown. Next, he segmented the data into two groups, based on the period during which they were collected. He limited his comparison to data collected in summer and autumn on the early cruises (1958 to 1976), since the SCICEX data are from the end of summer. Next, he identified 29 points where the earlier submarine tracks either crossed or were closely parallel to the 1990s cruise tracks.

Sea ice thickness refers to the height of the ice from keel to ridge. Sea ice draft refers to the height of the ice from keel to water surface.

Arctic sea ice is a habitat for polar bears. It is essential for catching prey and rearing young. (Image courtesy of Jet Propulsion Laboratory).
Arctic sea ice cover is an essential ingredient of the Earth's climate system; ocean currents can influence climate by redistributing heat, especially in coastal regions. Scientists fear that continued melting of sea ice could weaken the North Atlantic Current, the northward continuation of the Gulf Stream. The Gulf Stream transports 25 times more water than all the Earth's rivers, and a diversion could result in extremely cold winters in the North Atlantic regions, especially in northern Europe.

This false color map of the North Pole shows the percentage of the surface that was covered by ice in August 1985. Regions colored purple and red show where there is greater coverage, while greens and blues indicate less ice. (Image courtesy of the National Snow and Ice Data Center).
What is causing these dramatic changes in sea ice thickness? Rothrock said he doesn't have a definitive answer, and he is cautious about how he frames his findings. "Whether a change of the observed magnitude in ice thickness over several decades is part of a natural cycle of climate variability, or represents a mix of human influences on the climate system and natural climate variability, is presently unknown from the perspective of these observations alone," he said.

Yearly fluctuations of the North Atlantic Oscillation (NAO) affect weather. When the NAO sustains either a positive or negative mode for a long period, climate is affected. In the positive mode, high pressure over the Tropical Atlantic increases, and low pressure over Greenland deepens. In the negative mode, high pressure weakens over the Tropical Atlantic and the low pressure in the North Atlantic moves south. Wet and dry weather patterns result. (Image courtesy of NOAA).
Perhaps the major obstacle that Rothrock and his colleagues are up against in defining a cause for thinning sea ice draft is insufficient data. "There is not yet a good temporal record of ice draft over the Arctic Ocean; what we have constructed here is better thought of as two climatologies, one for 1958 to 1976 and another for 1993 to 1997," said Rothrock.

Benjamin Franklin's map of the Gulf Stream. (Image courtesy of the Smithsonian Institution's Ocean Planet).
Parkinson and groups at other research centers are using Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave Imager (SSM/I) data to examine trends in the length of the sea ice season and in the extent of the Arctic ice. They have found a net decrease in Arctic ice extent of 2.9 percent per decade since 1978.
Mark Anderson, Associate Professor in the Department of Geosciences at the University of Nebraska, and graduate student Sheldon Drobot are using SMMR and SSM/I data to examine the onset of spring melt in the Arctic to detect possible signs of climate changes. "Passive microwave data are ideal for deriving snow melt onset dates, because brightness temperatures change rapidly during melt onset," said Drobot. "By identifying these periods of rapid change in the microwave data, the occurrence and variability of melt onset can be monitored. We're looking for trends, because they will tell us whether a climate signal exists."

Although sea ice has increased in some regions, the Arctic has witnessed a net decrease in sea ice. (Image courtesy of the National Snow and Ice Data Center's State of the Cryosphere).
More comprehensive data might provide specific information to illuminate why Arctic ice is thinning. Because Earth's climate is determined by interactions among the sun, atmosphere, oceans, and lands, it is likely that the answer lies in a conglomeration of factors. Scientists are only beginning to understand the complexities, but Rothrock's findings underscore the need for more research.
"Whether ice volume has reached a minimum in a multi-decadal cycle or will continue the observed decline, this large thinning of the ice cover is a major climatic signal that needs to be accounted for in a successful theory of climate variability," said Rothrock.
References
Rothrock, D.A., Y. Yu, and G.A. Maykut, 1999: Thinning of the Arctic Sea-Ice Cover, Geophysical Research Letters, 26(23), pp. 3469-3472.
For more information
NASA National Snow and Ice Data Center (NSIDC) Distributed Active Archive Center (NSIDC DAAC)
About the remote sensing data used | ||
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Sensor | Scanning Multichannel Microwave Radiometer (SMMR) Special Sensor Microwave Imager (SMM/I) |
|
Parameter | sea ice thinning |
|
DAAC | NASA National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC) |
Page Last Updated: Jul 21, 2020 at 6:40 PM EDT