Like Ice Melting in a Glass
Images of the shrinking area of Arctic sea ice are now familiar, at least to polar connoisseurs. What’s been happening under the surface is much less well known—but illustrated here. Due to warmer air and warmer water, the ice has been getting thinner, generally faster than its area has been decreasing. We know about the thinning from old submarine sonar readings recently declassified by the US Navy (an unexpected benefit of the Cold War’s end), and from measurements taken by NASA’s Ice Cloud and Land Elevation Satellite (ICESat) since 2003.
In the quarter century following 1980, average autumn and winter Arctic ice thicknesses each dropped over one third.1 Over the same period, September ice extent declined more slowly, about 29% – and September is the time of year ice cover has diminished the most (data source: National Snow and Ice Data Center).
Scientists such as NASA’s Jay Zwally (listen to audio clip, above) worry that the thinning means summertime Arctic sea ice could dwindle and disappear more rapidly than once expected – somewhat like ice cubes in a glass melting faster and faster, the smaller they get. Since Arctic ice mostly reflects the sun’s energy back into space, but exposed ocean mostly absorbs it, retreating ice means faster warming still.
The profiles shown here cartoon the rough pattern (technically, the distribution) of fall Arctic sea ice thickness every five years, within the area of the Arctic Ocean where the Navy has declassified its soundings. The illustration draws on ICESat data restricted to the same area, for comparability. The 1980-2000 thickness estimates are based on multiple years of submarine data in a kind of running average, and so, also for comparability, the 2005 thickness estimates come from the average of three years of data centered on 2005. Data are the same information used to build Figure 2b of NASA scientist Ron Kwok’s recent paper, with a colleague, cited below. The horizontal and vertical scales within the graphic differ.
ICESat: Launched in January 2003, NASA’s Ice, Cloud and land Elevation Satellite (ICESat) carries laser altimeters capable of measuring the height of objects on and above Earth within an accuracy of roughly one inch, from an orbit over 400 miles high. The final laser out of three stopped working in October 2009; together, the lasers have completed just shy of two billion measurements, tracking small and large changes over time in the elevation of Arctic sea ice and of Greenland and Antarctic ice sheets, as well as cloud and aerosol heights, land topography, vegetation, and more.
- Kwok, Ron and Rothrock, A., “Decline in Arctic sea ice thickness from submarine and ICESat records: 1958-2008,” Geophysical Research Letters 36, L15501 (2009) ↩