Arctic sea ice thinned dramatically between the winters of 2004 and 2008, with thin seasonal ice replacing thick older ice as the dominant type for the first time on record. The new results, based on data from a NASA Earth-orbiting spacecraft, provide further evidence for the rapid, ongoing transformation of the Arctic’s ice cover.
Scientists from NASA and the University of Washington in Seattle conducted the most comprehensive survey to date using observations from NASA’s Ice, Cloud and land Elevation Satellite (ICESat) to make the first basin-wide estimate of the thickness and volume of the Arctic Ocean’s ice cover. Ron Kwok of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., led the research team, which published its findings July 7 in the Journal of Geophysical Research-Oceans.
The Arctic ice cap grows each winter as the sun sets for several months and intense cold ensues. In the summer, wind and ocean currents cause some of the ice naturally to flow out of the Arctic, while much of it melts in place. But not all of the Arctic ice melts each summer; the thicker, older ice is more likely to survive. Seasonal sea ice usually reaches about 6 feet in thickness, while multi-year ice averages 9 feet.
ICESat measures the distances to the top of the snow cover and to the sea surface. The difference between the two quantities gives the total “freeboard” measurement—i.e., the amount of ice above the water line relative to the local sea level.
Buoyancy causes a fraction (about 10%) of sea ice to stick out above the sea surface. By knowing the density of the ice and applying Archimedes’ Principle—an object immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object—the total thickness of the ice can be calculated.
Using ICESat measurements, scientists found that overall Arctic sea ice thinned about 7 inches a year, for a total of 2.2 feet over four winters. The total area covered by the thicker, older multi-yea" ice that has survived one or more summers shrank by 42%.
Previously, scientists relied only on measurements of area to determine how much of the Arctic Ocean is covered in ice, but ICESat makes it possible to monitor ice thickness and volume changes over the entire Arctic Ocean for the first time. The results give scientists a better understanding of the regional distribution of ice and provide better insight into what is happening in the Arctic.
Ice volume allows us to calculate annual ice production and gives us an inventory of the freshwater and total ice mass stored in Arctic sea ice. Even in years when the overall extent of sea ice remains stable or grows slightly, the thickness and volume of the ice cover is continuing to decline, making the ice more vulnerable to continued shrinkage. Our data will help scientists better understand how fast the volume of Arctic ice is decreasing and how soon we might see a nearly ice-free Arctic in the summer.—Ron Kwok
In recent years, the amount of ice replaced in the winter has not been sufficient to offset summer ice losses. The result is more open water in summer, which then absorbs more heat, warming the ocean and further melting the ice. Between 2004 and 2008, multi-year ice cover shrank 595,000 square miles—nearly the size of Alaska’s land area.
During the study period, the relative contributions of the two ice types to the total volume of the Arctic’s ice cover were reversed. In 2003, 62% of the Arctic’s total ice volume was stored in multi-year ice, with 38% stored in first-year seasonal ice. By 2008, 68% of the total ice volume was first-year ice, with 32% multi-year.
One of the main things that has been missing from information about what is happening with sea ice is comprehensive data about ice thickness. US Navy submarines provide a long-term, high-resolution record of ice thickness over only parts of the Arctic. The submarine data agree with the ICESat measurements, giving us great confidence in satellites as a way of monitoring thickness across the whole Arctic Basin.—Jay Zwally, study co-author and ICESat project scientist at NASA’s Goddard Space Flight Center
The research team attributes the changes in the overall thickness and volume of Arctic Ocean sea ice to the recent warming and anomalies in patterns of sea ice circulation.
To sum up, the primary changes in the overall thickness and volume of the Arctic Ocean sea ice are attributable to the thinning of the MY [multiyear] ice cover and the decline in MY ice coverage. Over the winter record, there is a net loss of 57% of MY sea volume (6300 km3) and a 33% reduction in MY coverage of the Arctic Ocean relative to their values in winter of 2004. These are dramatic changes. At the same time, the thickness of the FY [first-year] ice cover has not changed significantly. Over this short record, there is a reversal in the volumetric and areal contributions of the two ice types to the total volume and area of the Arctic Ocean ice cover. Examining the ice export together with the decline in MY ice coverage suggest that the near-zero replenishment of the MY ice cover, an imbalance in the cycle of replenishment and ice export after the summers of 2005 and 2007, has played a significant role in the loss of Arctic sea ice volume over the ICESat record. Changes in MY ice export, by itself, do not explain the record minimums in recent years.—Kwok et al. (2009)
Kwok, R., G. F. Cunningham, M. Wensnahan, I. Rigor, H. J. Zwally, and D. Yi (2009) Thinning and volume loss of the Arctic Ocean sea ice cover: 2003–2008. J. Geophys. Res., 114, C07005, doi: 10.1029/2009JC005312
Data visualization of Arctic sea ice thickness, as measured by ICESat, shows the decline of the thickest ice (white, 4 to 5 meters thick) and increase in thinner ice (deep blue, 0 to 1 meter) from 2003 to 2008. Credit: NASA Goddard’s Scientific Visualization Studio
Data visualization of ice thickness, as measured by ICESat, shows the yearly growth (winter) and retreat (fall) of ice in the Arctic Ocean. Credit: NASA Goddard’s Scientific Visualization Studio