Background
A few years back, Rothrock et al. (1999) and Wadhams and Davis (2000) analyzed submarine sonar data that suggested Arctic sea ice had thinned by almost half in the preceding few decades [but see also Winsor (2001)], leading climate alarmists to suggest that this phenomenon was due to global warming caused by anthropogenic CO2 emissions.  Subsequently, Holloway and Sou (2002)] demonstrated that much of the thickness reduction observed by the former investigators was the result of a wind-driven redistribution of ice (dynamics) and not a warming-induced melting (thermodynamics).  Later still, however, Dumas et al. (2003) reemphasized the role of thermal-driven changes, suggesting it "may be more important for longer-term trends associated with climate warming."

What was done
Working with monthly fields of ice motion obtained from the International Arctic Buoy Program in an attempt to better clarify this issue, Pfirman et al. (2004) analyzed Arctic sea-ice drift dynamics from 1979-1997 using a Lagrangian perspective that "shows the complexities of ice drift response to variations in atmospheric conditions."

What was learned
Pfirman et al.'s analysis indicated that "large amounts of sea ice form over shallow Arctic shelves, are transported across the central basin and are exported primarily through Fram Strait and, to lesser degrees, the Barents Sea and Canadian Archipelago," consistent with the observations of several other investigators.  They also determined that within the central Arctic, ice travel times averaged 4.0 years from 1984-85 through 1988-89, but only 3.0 years from 1990-91 through 1996-97.  This enhanced rate of export or surge of old ice to Fram Strait from the Beaufort Gyre over the latter period decreased the fraction of thick, ridged ice within the central basin of the Arctic, and was deemed by Pfirman et al. to be responsible for some of the sea ice thinning observed between the 1980s and 1990s.  They also note that the rapid change in ice dynamics that occurred between 1988 and 1990 was "in response to a weakening of the Beaufort high pressure system and a strengthening of the European Arctic low (a shift from lower North Atlantic Oscillation/Arctic Oscillation to higher NAO/OA index) [Walsh et al., 1996; Proshutinsky and Johnson, 1997; Kwok, 2000; Zhang et al., 2000; Rigor et al., 2002]."

What it means
Much of the reported thinning of Arctic sea ice that occurred in the 1990s [if real, as per Winsor (2001)] was not the result of CO2-induced global warming, as suggested at the time by many climate alarmists.  Rather, it was a natural consequence of changes in ice dynamics caused by an atmospheric regime shift, of which there have been several in decades past and will be several in decades to come, totally irrespective of past or future changes in the air's CO2 content.

Whether any of the thinning was due to the thermodynamics of global warming, as per the possibility mentioned by Dumas et al., is still impossible to know, for temporal variability in Arctic sea-ice behavior is too great to allow such a small and slowly-developing signal to be detected yet.  In describing an earlier regime shift, for example, Dumas et al. note that "a sharp decrease in ice thickness of roughly 0.6 m over 4 years (1970-74) [was] followed by an abrupt increase of roughly 0.8 m over 2 years (1974-76)."

Hence, the jury is still out on the effects of possible global warming on Arctic sea-ice thickness, although it has brought in a solid partial verdict in the case of atmospheric-induced ice dynamics.

References
Dumas, J.A., Flato, G.M. and Weaver, A.J.  2003.  The impact of varying atmospheric forcing on the thickness of arctic multi-year sea ice.  Geophysical Research Letters 30: 10.1029/2003GL017433.

Holloway, G. and Sou, T.  2002.  Has Arctic Sea Ice Rapidly Thinned?  Journal of Climate 15: 1691-1701.

Kwok, R.  2000.  Recent changes in Arctic Ocean sea ice motion associated with the North Atlantic Oscillation.  Geophysical Research Letters 27: 775-778.

Proshutinsky, A.Y. and Johnson, M.A.  1997.  Two circulation regimes of the wind driven Arctic Ocean.  Journal of Geophysical Research 102: 12,493-12,514.

Rigor, I.G., Wallace, J.M. and Colony, R.L.  2002.  Response of sea ice to the Arctic oscillation.  Journal of Climate 15: 2648-2663.

Rothrock, D.A., Yu, Y. and Maykut, G.A.  1999.  Thinning of the Arctic sea ice cover.  Geophysics Research Letters 26: 3469-3472.

Wadhams, P. and Davis, N.R.  2000.  Further evidence of ice thinning in the Arctic Ocean.  Geophysical Research Letters 27: 3973-3975.

Walsh, J.E., Chapman, W.L. and Shy, T.L.  1996.  Recent decrease of sea level pressure in the central Arctic.  Journal of Climate 9: 480-486.

Winsor, P.  2001.  Arctic sea ice thickness remained constant during the 1990s.  Geophysical Research Letters 28: 1039-1041.

Zhang, J.L., Rothrock, D. and Steele, M.  2000.  Recent changes in Arctic sea ice: The interplay between ice dynamics and thermodynamics.  Journal of Climate 13: 3099-3114.