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Arctic Sea Ice (Thickness) - Summary
Based on analyses of submarine sonar data, Rothrock et al. (1999) and Wadhams and Davis (2000) suggested that Arctic sea ice had thinned by nearly 50% over the prior few decades.  Their reports were widely cited by both the Intergovernmental Panel on Climate Change (IPCC: Houghton et al., 2001) and by the popular media, who at the time of the publications' appearance were whipped into a frenzy by climate-alarmist claims that the sea-ice thinning was caused by global warming that had been induced by anthropogenic CO2 emissions.

So how has this contention withstood the test of time, albeit of but a few short years' duration?

The first hint of an answer appeared essentially simultaneously with the publication of the ominous reports, when Johannessen et al. (1999) used surface-based measurements to derive variations in area-averaged Arctic sea-ice thickness from 1978 to 1991.  They, too, detected a downward trend in this parameter.  However, in carefully scrutinizing their data, it can be seen that the decline in sea-ice thickness did not occur smoothly over the period of study.  In fact, essentially all of the drop occurred rather abruptly over a single period of not more than three years (1987/88-1990/91) and possibly only one year (1989/90-1990/91).

The next chapter in the complex detective story occurred two years later, when Winsor (2001) analyzed a much more comprehensive set of Arctic sea-ice data that had been obtained from six submarine cruises conducted between 1991 and 1997 that had covered the central Arctic Basin from 76 N to 90 N, as well as two areas that had been particularly densely sampled, one centered around the North Pole (>87 N) and one in the central part of the Beaufort Sea (centered at approximately 76 N, 145W).  The transect data across the entire Arctic Basin revealed that the mean Arctic sea-ice thickness had remained "almost constant" over the period of study.  Data from the North Pole also showed little variability, and a linear regression of the data revealed a "slight increasing trend for the whole period."  As for the Beaufort Sea region, annual variability in sea ice thickness was greater than at the North Pole; but once again, in Winsor's words, "no significant trend" in mean sea-ice thickness was found.  Combining the North Pole results with the results of an earlier study, Winsor thus concluded that "mean ice thickness has remained on a near-constant level around the North Pole from 1986 to 1997."

The following year, Holloway and Sou (2002) explored "how observations, theory, and modeling work together to clarify perceived changes to Arctic sea ice," incorporating data from "the atmosphere, rivers, and ocean along with dynamics expressed in an ocean-ice-snow model."  On the basis of a number of different data-fed model runs, they found that for the last half of the past century, "no linear trend [in Arctic sea ice volume] over 50 years is appropriate," noting that their results indicated "increasing volume to the mid-1960s, decadal variability without significant trend from the mid-1960s to the mid-1980s, then a loss of volume from the mid-1980s to the mid-1990s."  The net effect of this behavior, in their words, was that "the volume estimated in 2000 is close to the volume estimated in 1950."  Hence, they suggested that the initial inferred rapid thinning of Arctic sea ice was, as they put it, "unlikely," due to problems arising from under sampling.  They also reported that "varying winds that readily redistribute Arctic ice create a recurring pattern whereby ice shifts between the central Arctic and peripheral regions, especially in the Canadian sector," and that the "timing and tracks of the submarine surveys missed this dominant mode of variability."

In the same year, Polyakov et al. (2002) employed newly available long-term Russian fast-ice data obtained from the Kara, Laptev, East Siberian and Chuckchi Seas to investigate trends and variability in the Arctic environment poleward of 62N.  This study revealed that fast-ice thickness trends in the different seas were "relatively small, positive or negative in sign at different locations, and not statistically significant at the 95% level."  A year later, these results were reconfirmed by Polyakov et al. (2003), who reported that the available fast-ice records "do not show a significant trend," while noting that "in the Kara and Chukchi Seas trends are positive, and in the Laptev and East Siberian Seas trends are negative," but stating that "these trends are not statistically significant at the 95% confidence level."

The following year, Laxon et al. (2003) used an eight-year time series (1993-2001) of Arctic sea-ice thickness data derived from measurements of ice freeboard made by radar altimeters carried aboard ERS-1 and 2 satellites to determine the mean thickness and variability of Arctic sea ice between latitudes 65 and 81.5N, which region covers the entire circumference of the Arctic Ocean, including the Beaufort, Chukchi, East Siberian, Kara, Laptev, Barents and Greenland Seas.  These real-world observations served a number of purposes: (1) they revealed "an interannual variability in ice thickness at higher frequency, and of greater amplitude, than simulated by regional Arctic models," (2) they undermined "the conclusion from numerical models that changes in ice thickness occur on much longer timescales than changes in ice extent," and (3) they showed that "sea ice mass can change by up to 16% within one year," which finding "contrasts with the concept of a slowly dwindling ice pack, produced by greenhouse warming."  Laxon et al. thus concluded that "errors are present in current simulations of Arctic sea ice," stating in their closing sentence that "until models properly reproduce the observed high-frequency, and thermodynamically driven, variability in sea ice thickness, simulations of both recent, and future, changes in Arctic ice cover will be open to question."

Finally, and most recently, based on monthly fields of ice motion obtained from the International Arctic Buoy Program, 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."  This analysis indicated, in their words, 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 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]."

These observations suggest that 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 claimed 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 likely be several in decades to come, totally irrespective of past or future changes in the air's CO2 content.  Whether any portion of possible past sea ice thinning was due to global warming is consequently still impossible to know, for temporal variability in Arctic sea-ice behavior is simply 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. (2003) noted 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)."

In view of these several observations, it will likely be a number of years before anything definitive can be said about CO2-induced global warming on the basis of the thickness of Arctic sea-ice, other than that its impact on sea-ice thickness is much too small to be detected at the present time.

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.

Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Xiaosu, D., Maskell, K. and Johnson, C.A. (Eds.).  2001.  Climate Change 2001: The Scientific Basis.  Cambridge University Press, Cambridge, UK.

Johannessen, O.M., Shalina, E.V. and Miles M.W.  1999.  Satellite evidence for an Arctic sea ice cover in transformation.  Science 286: 1937-1939.

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

Laxon, S., Peacock, N. and Smith, D.  2003.  High interannual variability of sea ice thickness in the Arctic region.  Nature 425: 947-950.

Pfirman, S., Haxby, W.F., Colony, R. and Rigor, I.  2004.  Variability in Arctic sea ice drift.  Geophysical Research Letters 31: 10.1029/2004GL020063.

Polyakov, I.V., Alekseev, G.V., Bekryaev, R.V., Bhatt, U., Colony, R.L., Johnson, M.A., Karklin, V.P., Makshtas, A.P., Walsh, D. and Yulin A.V.  2002.  Observationally based assessment of polar amplification of global warming.  Geophysical Research Letters 29: 10.1029/2001GL011111.

Polyakov, I.V., Alekseev, G.V., Bekryaev, R.V., Bhatt, U.S., Colony, R., Johnson, M.A., Karklin, V.P., Walsh, D. and Yulin, A.V.  2003.  Long-term ice variability in Arctic marginal seas.  Journal of Climate 16: 2078-2085.

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.