4.4.3.3 Other Evidence of Sea Ice Thickness Change in the Arctic and Antarctic

Haas (2004, and references therein) used ground-based electromagnetic induction measurements to show a decrease of approximately 0.5 m between 1991 and 2001 in the modal thickness (i.e., the most commonly observed thickness) of ice floes in the Arctic Trans-Polar Drift. Their survey of 120 km of ice on 146 floes during four cruises is biased by an absence of ice-free and thin-ice fractions and underestimation of ridged ice, but the data are descriptive of floes that are safe to traverse in summer, and the observed changes are most likely due to thermodynamic forcing.

An emerging new technique, using satellite radar or laser altimetry to estimate ice freeboard from the measured ranges to the ice and sea surface in open leads (and assuming an average floe density and snow depth), offers promise for future monitoring of large-scale sea ice thickness. Laxon et al. (2003) estimated average arctic sea ice thickness over the cold months (October–March) for 1993 to 2001 from satellite-borne radar altimeter measurements. Their data reveal a realistic geographic variation in thickness (increasing from about 2 m near Siberia to 4.5 m off the coasts of Canada and Greenland) and a significant (9%) interannual variability in winter ice thickness, but no indication of a trend over this time.

There are no available data on change in the thickness of antarctic sea ice, much of which is considerably thinner and less ridged than ice in the Arctic Basin.