In a recent study, Serreze et al. (2003) analyzed the history of satellite passive microwave sea ice records that first became available in October of 1978 to derive trends in Arctic sea ice extent and area over the past quarter-century. The results of their analysis showed a general downward trend in Arctic sea ice "during the passive microwave era" that culminated with record minimums for both sea ice extent and area in 2002. Given that state-of-the-art climate models predict Arctic sea ice extent and area will decline as the climate warms, and that simulations of the past 30 years agree reasonably well with observations in terms of total ice area, climate alarmists consider the past few decades of Arctic sea ice decline to be proof of CO2-induced global warming.
However, as we have indicated in other places throughout our web site (see, for example, Climate Oscillations), earth's climate is forever changing, on one time scale or another; and our understanding of these cycles, including their causes and periodicities, is crucial to the outcome of the global warming debate, for the reality of these phenomena suggest that decadal temperature trends may be nothing more than natural climatic fluctuations. In fact, such may be the case with respect to the recent sea ice trends reported by Serreze et al.; they may simply be a manifestation of natural climate variability operating at decadal time-scales, as is evident from analyses of longer-term climate records from the Arctic.
Polyakov et al. (2002), for example, used long-term Russian observations of surface air temperature from coastal stations, and sea-ice extent and fast-ice thickness from the Kara, Laptev, East Siberian, and Chuckchi seas to gain new insights into trends and variability in the Arctic environment poleward of 62°N over a period of time four times longer than that examined by Serreze et al. Throughout the 125-year Arctic air temperature history they developed, the authors identified "strong intrinsic variability, dominated by multi-decadal fluctuations with a timescale of 60-80 years;" and because of this fact, they found temperature trends in the Arctic to be highly dependent on the particular time period selected for analysis. In fact, they found they could "identify periods when [A]rctic trends were actually smaller or of different sign [our italics] than Northern Hemisphere trends."
Over the bulk of the 20th century, however, when they say "multi-decadal variability had little net effect on computed trends," the temperature histories of the two regions were "similar" and did "not support amplified warming in polar regions predicted by GCMs." Also like the temperature trend, the ice cover trend was "smaller than expected," with fast-ice thickness trends "relatively small, positive or negative in sign at different locations, and not statistically significant at the 95% level." Such trends, in the words of the authors, "do not support the hypothesized polar amplification of global warming."
Similar findings have been reported by Venegas and Mysak (2000), who analyzed century-long records of sea ice concentration and sea level pressure poleward of 40°N latitude. According to these authors, a number of quasi-decadal and interdecadal timescale fluctuations were found to account for a large fraction (60-70%) of the natural climate variability there. Specifically, they identified climate signals with periods of 6-7 years, 9-10 years, 16-20 years, and 30-50 years; and they listed various oceanic processes that could be responsible for these variations.
In a similar vein, Rothrock et al. (1999) and Wadhams and Davis (2000) reported Arctic sea ice thinned nearly by half in recent decades, based upon analyses of submarine sonar data; and once again, at the times of their reports' appearance, the popular media were whipped into a frenzy by the climate-alarmist claim that anthropogenic CO2 emissions were responsible for the thinning. However, in a study designed to rationally evaluate these claims, Holloway and Sou (2002) demonstrated "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."
Based on a number of different data-fed model runs, the latter authors report 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 indicate "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." The authors' analysis suggests that the earlier inferred rapid thinning of Arctic sea ice was, as they put it, "unlikely," due to problems arising from "undersampling." They also report 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." Thus, natural variability at decadal scales once again appears to erase another climate-alarmist global warming "fingerprint."
Lastly, we note the study of McConnell et al. (2000), who appropriately interpret short-term (1978-1988) trends in ice-sheet elevation in southern Greenland that were derived from "a physically based model of firn densification and records of annual snow accumulation reconstructed from 12 ice cores at high elevation" with longer ice core records. According to the authors, "the decadal-scale changes in ice-sheet elevation that occurred during 1978-88 are typical over the last few centuries and well within the natural variability of accumulation-driven elevation change." In addition, they note that "accurate detection of any long-term mass imbalance of the ice sheets and assessment of likely causes will require multi-decadal time series of surface elevation in conjunction with widely distributed ice-core-derived accumulation measurements collected over the time period of interest." Put more simply, it will take several decades of several different types of measurements, including snow accumulation, even to be sure that there is a real long-term change -- as opposed to natural cyclical variations -- occurring in the mass balance of the Greenland Ice Sheet. And then comes the job of trying to decide what the cause of such a change might be … if there is one.
In light of the results of these several analyses, the next time you hear people getting excited about Arctic sea ice thinning or the Greenland Ice Sheet shrinking, tell them to check back in a decade or two; for there's a lot of natural variability to be overcome before we can be sure of anything even remotely related to potential impacts of anthropogenic CO2 emissions in the Arctic.
References
Holloway, G. and Sou, T. 2002. Has Arctic Sea Ice Rapidly Thinned? Journal of Climate 15: 1691-1701.
McConnell, J.R., Arthern, R.J., Mosley-Thompson, E., Davis, C.H., Bales, R.C., Thomas, R., Burkhart, J.F. and Kyne, J.D. 2000. Changes in Greenland ice sheet elevation attributed primarily to snow accumulation variability. Nature 406: 877-879.
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.
Rothrock, D.A., Yu, Y. and Maykut, G.A. 1999. Thinning of the Arctic sea ice cover. Geophysics Research Letters 26: 3469-3472.
Serreze, M.C., Maslanik, J.A., Scambos, T.A., Fetterer, F., Stroeve, J., Knowles, K., Fowler, C., Drobot, S., Barry, R.G. and Haran, T.M. 2003. A record minimum arctic sea ice extent and area in 2002. Geophysical Research Letters 30: 10.1029/2002GL016406.
Venegas, S.A. and Mysak, L.A. 2000. Is there a dominant timescale of natural climate variability in the Arctic? Journal of Climate 13: 3412-3434.
Wadhams, P. and Davis, N.R. 2000. Further evidence of ice thinning in the Arctic Ocean. Geophysical Research Letters 27: 3973-3975.