Arctic sea ice
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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
Arctic climate is incredibly complex, varying simultaneously on a number of different timescales for a number of different reasons (Venegas and Mysak, 2000). Against this backdrop of multiple causation and timeframe variability, it is difficult to identify a change in either the extent or thickness of Arctic sea ice that could be attributed to the increase in temperature that has been predicted to result from the burning of fossil fuels. The task is further complicated because many of the records that do exist contain only a few years to a few decades of data, and they yield different trends depending on the period of time studied.
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Extent
Johannessen et al. (1999) analyzed Arctic sea ice extent over the period 1978-1998 and found it to have decreased by about 14 percent. This finding led them to suggest that “the balance of evidence,” as small as it then was, indicates “an ice cover in transition,” and that “if this apparent transformation continues, it may lead to a markedly different ice regime in the Arctic,” as was also suggested by Vinnikov et al. (1999).
Reading Johannessen et al.’s assessment of the situation, one is left with the impression that a relatively consistent and persistent reduction in the area of Arctic sea ice is in progress. However, and according to their own data, that assessment is highly debatable and possibly false. In viewing their plots of sea ice area, for example, it is readily evident that the decline in this parameter did not occur smoothly over the 20-year period of study. In fact, essentially all of the drop it experienced occurred abruptly over a single period of not more than three years (87/88-90/91) and possibly only one year (89/90-90/91). Furthermore, it could be argued from their data that from 1990/91 onward, sea ice area in the Arctic may have actually increased.
Support for this assessment of the data is found in Kwok (2004), who estimated “the time-varying perennial ice zone (PIZ) coverage and construct[s] the annual cycles of multiyear (MY, including second year) ice coverage of the Arctic Ocean using QuikSCAT backscatter, MY fractions from RADARSAT, and the record of ice export from satellite passive microwave observations” for the years 1999-2003. Kwok calculated the coverage of Arctic MY sea ice at the beginning of each year of the study was 3774 x 103 km2 in 2000, 3896 x 103 km2 in 2001, 4475 x 103 km2 in 2002, and 4122 x 103 km2 in 2003, representing an increase in sea ice coverage of 9 percent over a third of a decade.
More questions are raised Parkinson (2000b), who utilized satellite-derived data of sea ice extent to calculate changes in this parameter for the periods 1979-1990 and 1990-1999. He reports that in seven of the nine regions into which he divided the Arctic for his analysis, the “sign of the trend reversed from the 1979-1990 period to the 1990-1999 period,” indicative of the ease with which significant decadal trends are often reversed in this part of the world.
In another study, Belchansky et al. (2004) report that from 1988 to 2001, total January multiyear ice area declined at a mean rate of 1.4 percent per year. In the autumn of 1996, however, they note that “a large multiyear ice recruitment of over 106 km2 fully replenished the previous 8-year decline in total area.” They add that the replenishment “was followed by an accelerated and compensatory decline during the subsequent 4 years.” In addition, they learned that 75 percent of the interannual variation in January multiyear sea area “was explained by linear regression on two atmospheric parameters: the previous winter’s Arctic Oscillation index as a proxy to melt duration and the previous year’s average sea level pressure gradient across the Fram Strait as a proxy to annual ice export.”
Belchansky et al. conclude that their 14-year analysis of multiyear ice dynamics is “insufficient to project long-term trends.” They also conclude it is insufficient to reveal “whether recent declines in multiyear ice area and thickness are indicators of anthropogenic exacerbations to positive feedbacks that will lead the Arctic to an unprecedented future of reduced ice cover, or whether they are simply ephemeral expressions of natural low frequency oscillations.” It should be noted in this regard, however, that low frequency oscillations are what the data actually reveal; and such behavior is not what one would predict from a gradually increasing atmospheric CO2 concentration.
In another study, Heide-Jorgensen and Laidre (2004) examined changes in the fraction of open-water found within various pack-ice microhabitats of Foxe Basin, Hudson Bay, Hudson Strait, Baffin Bay-Davis Strait, northern Baffin Bay, and Lancaster Sound over a 23-year interval (1979-2001) using remotely sensed microwave measurements of sea-ice extent, after which the trends they documented were “related to the relative importance of each wintering microhabitat for eight marine indicator species and potential impacts on winter success and survival were examined.”
Results of the analysis indicate that Foxe Basin, Hudson Bay, and Hudson Strait showed small increasing trends in the fraction of open-water, with the upward trends at all microhabitats studied ranging from 0.2 to 0.7 percent per decade. In Baffin Bay-Davis Straight and northern Baffin Bay, on the other hand, the open-water trend was downward, and at a mean rate for all open-water microhabitats studied of fully 1 percent per decade, while the trend in all Lancaster Sound open-water microhabitats was also downward, in this case at a mean rate of 0.6 percent per decade.
With respect to the context of these open-water declines, Heide-Jorgensen and Laidre report that “increasing trends in sea ice coverage in Baffin Bay and Davis Strait (resulting in declining open-water) were as high as 7.5 percent per decade between 1979-1999 (Parkinson et al., 1999; Deser et al., 2000; Parkinson, 2000a,b; Parkinson and Cavalieri, 2002) and comparable significant increases have been detected back to 1953 (Stern and Heide-Jorgensen, 2003).” They additionally note that “similar trends in sea ice have also been detected locally along the West Greenland coast, with slightly lower increases of 2.8 percent per decade (Stern and Heide-Jorgensen, 2003).”
Cavalieri et al. (2003) extended prior satellite-derived Arctic sea ice records several years back in time by bridging the gap between Nimbus 7 and earlier Nimbus 5 satellite datasets via comparisons with National Ice Center digital sea ice data. For the newly extended period of 1972-2002, they determined that Arctic sea ice extent had declined at a mean rate of 0.30 ± 0.03 x 106 km2 per decade; while for the shortened period from 1979-2002, they found a mean rate of decline of 0.36 ± 0.05 x 106 km2 per decade, or at a rate that was 20 percent greater than the full-period rate. In addition Serreze et al. (2002) determined that the downward trend in Arctic sea ice extent during the passive microwave era culminated with a record minimum value in 2002.
These results could readily be construed to indicate an increasingly greater rate of Arctic sea ice melting during the latter part of the twentieth century. However, the results of these studies are not the end of the story. As Grumet et al. (2001) have described the situation, recent trends in Arctic sea ice cover “can be viewed out of context because their brevity does not account for interdecadal variability, nor are the records sufficiently long to clearly establish a climate trend.”
In an effort to overcome this “short-sightedness,” Grumet et al. developed a 1,000-year record of spring sea ice conditions in the Arctic region of Baffin Bay based on sea-salt records from an ice core obtained from the Penny Ice Cap on Baffin Island. In doing so, they determined that after a period of reduced sea ice during the eleventh through fourteenth centuries, enhanced sea ice conditions prevailed during the following 600 years. For the final (twentieth) century of this period, however, they report that “despite warmer temperatures during the turn of the century, sea-ice conditions in the Baffin Bay/Labrador Sea region, at least during the last 50 years, are within ‘Little Ice Age’ variability,” suggesting that sea ice extent there has not yet emerged from the range of conditions characteristic of the Little Ice Age.
In an adjacent sector of the Arctic, this latter period of time was also studied by Comiso et al. (2001), who used satellite imagery to analyze and quantify a number of attributes of the Odden ice tongue—a winter ice-cover phenomenon that occurs in the Greenland Sea with a length of about 1,300 km and an aerial coverage of as much as 330,000 square kilometers—over the period 1979-1998. By utilizing surface air temperature data from Jan Mayen Island, which is located within the region of study, they were able to infer the behavior of this phenomenon over the past 75 years. Trend analyses revealed that the ice tongue has exhibited no statistically significant change in any of the parameters studied over the past 20 years; but the proxy reconstruction of the Odden ice tongue for the past 75 years revealed the ice phenomenon to have been “a relatively smaller feature several decades ago,” due to the warmer temperatures that prevailed at that time.
In another study of Arctic climate variability, Omstedt and Chen (2001) obtained a proxy record of the annual maximum extent of sea ice in the region of the Baltic Sea over the period 1720-1997. In analyzing this record, they found that a significant decline in sea ice occurred around 1877. In addition, they reported finding greater variability in sea ice extent in the colder 1720-1877 period than in the warmer 1878-1997 period.
Also at work in the Baltic Sea region, Jevrejeva (2001) reconstructed an even longer record of sea ice duration (and, therefore, extent) by examining historical data for the observed time of ice break-up between 1529 and 1990 in the northern port of Riga, Latvia. The long date-of-ice-break-up time series was best described by a fifth-order polynomial, which identified four distinct periods of climatic transition: (1) 1530-1640, warming with a tendency toward earlier ice break-up of nine days/century, (2) 1640-1770, cooling with a tendency toward later ice break-up of five days/century, (3) 1770-1920, warming with a tendency toward earlier ice break-up of 15 days/century, and (4) 1920-1990, cooling with a tendency toward later ice break-up of 12 days/century.
On the other hand, in a study of the Nordic Seas (the Greenland, Iceland, Norwegian, Barents, and Western Kara Seas), Vinje (2001) determined that “the extent of ice in the Nordic Seas measured in April has decreased by 33% over the past 135 years.” He notes, however, that “nearly half of this reduction is observed over the period 1860-1900,” and we note, in this regard, that the first half of this sea-ice decline occurred over a period of time when the atmosphere’s CO2 concentration rose by only 7 ppm, whereas the second half of the sea-ice decline occurred over a period of time when the air’s CO2 concentration rose by more than 70 ppm. If the historical rise in the air’s CO2 content has been responsible for the historical decrease in sea-ice extent, its impact over the last century has declined to less than a tenth of what its impact was over the preceding four decades. This in turn suggests that the increase in the air’s CO2 content over the past 135 years has likely had nothing to do with the concomitant decline in sea-ice cover.
In a similar study of the Kara, Laptev, East Siberian, and Chuckchi Seas, based on newly available long-term Russian observations, Polyakov et al. (2002) found “smaller than expected” trends in sea ice cover that, in their words, “do not support the hypothesized polar amplification of global warming.” Likewise, in a study published the following year, Polyakov et al. (2003) report that “over the entire Siberian marginal-ice zone the century-long trend is only -0.5% per decade,” while “in the Kara, Laptev, East Siberian, and Chukchi Seas the ice extent trends are not large either: -1.1%, -0.4%, +0.3%, and -1.0% per decade, respectively.” Moreover, they say “these trends, except for the Chukchi Sea, are not statistically significant.”
Divine and Dick (2006) used historical April through August ice observations made in the Nordic Seas—comprised of the Iceland, Greenland, Norwegian, and Barents Seas, extending from 30°W to 70°E—to construct time series of ice-edge position anomalies spanning the period 1750-2002, which they analyzed for evidence of long-term trend and oscillatory behavior. The authors report that “evidence was found of oscillations in ice cover with periods of about 60 to 80 years and 20 to 30 years, superimposed on a continuous negative trend,” which observations are indicative of a “persistent ice retreat since the second half of the 19th century” that began well before anthropogenic CO2 emissions could have had much effect on earth’s climate.
Noting that the last cold period observed in the Arctic occurred at the end of the 1960s, the two Norwegian researchers say their results suggest that “the Arctic ice pack is now at the periodical apogee of the low-frequency variability,” and that “this could explain the strong negative trend in ice extent during the last decades as a possible superposition of natural low frequency variability and greenhouse gas induced warming of the last decades.” However, as they immediately caution, “a similar shrinkage of ice cover was observed in the 1920s-1930s, during the previous warm phase of the low frequency oscillation, when any anthropogenic influence is believed to have still been negligible.” They suggest, therefore, “that during decades to come … the retreat of ice cover may change to an expansion.”
In light of this litany of findings, it is difficult to accept the claim that Northern Hemispheric sea ice is rapidly disintegrating in response to CO2-induced global warming. Rather, the oscillatory behavior observed in so many of the sea ice studies suggests, in the words of Parkinson (2000b), “the possibility of close connections between the sea ice cover and major oscillatory patterns in the atmosphere and oceans,” including connections with: “(1) the North Atlantic Oscillation (e.g., Hurrell and van Loon, 1997; Johannessen et al., 1999; Kwok and Rothrock, 1999; Deser et al., 2000; Kwok, 2000, Vinje, 2001) and the spatially broader Arctic Oscillation (e.g., Deser et al., 2000; Wang and Ikeda, 2000); (2) the Arctic Ocean Oscillation (Polyakov et al., 1999; Proshutinsky et al., 1999); (3) a ‘see-saw’ in winter temperatures between Greenland and northern Europe (Rogers and van Loon, 1979); and (4) an interdecadal Arctic climate cycle (Mysak et al., 1990; Mysak and Power, 1992).” The likelihood that Arctic sea ice trends are the product of such natural oscillations, Parkinson continues, “provides a strong rationale for considerable caution when extrapolating into the future the widely reported decreases in the Arctic ice cover over the past few decades or when attributing the decreases primarily to global warming,” a caution with which we heartily agree.
One final study of note is that of Bamber et al. (2004), who used high-accuracy ice-surface elevation measurements (Krabill et al., 2000) of the largest ice cap in the Eurasian Arctic—Austfonna, on the island of Nordaustlandet in northeastern Svalbard—to evaluate ice cap elevation changes between 1996 and 2002. They determined that the central and highest-altitude area of the ice cap, which comprises 15 percent of its total area, “increased in elevation by an average of 50 cm per year between 1996 and 2002,” while “to the northeast of this region, thickening of about 10 cm per year was also observed.” They further note that the highest of these growth rates represents “as much as a 40% increase in accumulation rate (Pinglot et al., 2001).”
Based on the ancillary sea-ice and meteorological data they analyzed, Bamber et al. concluded that the best explanation for the dramatic increase in ice cap growth over the six-year study period was a large increase in precipitation caused by a concomitant reduction in sea-ice cover in this sector of the Arctic. Their way of characterizing this phenomenon is simply to say that it represents the transference of ice from the top of the sea (in this case, the Barents Sea) to the top of the adjacent land (in this case, the Austfonna ice cap). And as what has been observed to date is only the beginning of the phenomenon, which will become even stronger in the absence of nearby sea-ice, “projected changes in Arctic sea-ice cover,” as they say in the concluding sentence of their paper, “will have a significant impact on the mass-balance of land ice around the Arctic Basin over at least the next 50 years.” Which result, we might add, may be just the opposite of that forecast by the IPCC.
Thickness
Based on analyses of submarine sonar data, Rothrock et al. (1999) suggested that Arctic sea ice in the mid 1990s had thinned by about 42 percent of the average 1958-1977 thickness. The IPCC reports the Rothrock finding but then reports that other more recent studies found “the reduction in ice thickness was not gradual, but occurred abruptly before 1991,” and acknowledges that “ice thickness varies considerably from year to year at a given location and so the rather sparse temporal sampling provided by submarine data makes inferences regarding long term change difficult” (IPCC 2007, p. 353). Johannessen et al. (1999), for example, found that 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).
Two years after Johannessen et al., Winsor (2001) analyzed a more comprehensive set of Arctic sea-ice data 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 at the North Pole (>87° N) and one in the central part of the Beaufort Sea (centered at approximately 76° N, 145°W). 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 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 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.” They suggest 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 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.”
In the same year, Polyakov et al. (2002) employed newly available long-term Russian landfast-ice data obtained from the Kara, Laptev, East Siberian, and Chuckchi Seas to investigate trends and variability in the Arctic environment poleward of 62°N. 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.”
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 81.5°N, 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 (1) revealed “an interannual variability in ice thickness at higher frequency, and of greater amplitude, than simulated by regional Arctic models,” (2) undermined “the conclusion from numerical models that changes in ice thickness occur on much longer timescales than changes in ice extent,” and (3) 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. 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.”
Pfirman et al. (2004) analyzed Arctic sea-ice drift dynamics from 1979-1997, based on monthly fields of ice motion obtained from the International Arctic Buoy Program, using a Lagrangian perspective that “shows the complexities of ice drift response to variations in atmospheric conditions.” This 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 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].”
Lastly, in a paper on landfast ice in Canada’s Hudson Bay, Gagnon and Gough (2006) cite nine different studies of sea-ice cover, duration, and thickness in the Northern Hemisphere, noting that the Hudson Bay region “has been omitted from those studies with the exception of Parkinson et al. (1999).” For 13 stations located on the shores of Hudson Bay (seven) and surrounding nearby lakes (six), Gagnon and Gough then analyzed long-term weekly measurements of ice thickness and associated weather conditions that began and ended, in the mean, in 1963 and 1993, respectively.
Results of the study revealed that a “statistically significant thickening of the ice cover over time was detected on the western side of Hudson Bay, while a slight thinning lacking statistical significance was observed on the eastern side.” This asymmetry, in their words, was “related to the variability of air temperature, snow depth, and the dates of ice freeze-up and break-up,” with “increasing maximum ice thickness at a number of stations” being “correlated to earlier freeze-up due to negative temperature trends in autumn,” and with high snow accumulation being associated with low ice thickness, “because the snow cover insulates the ice surface, reducing heat conduction and thereby ice growth.” Noting that their findings “are in contrast to the projections from general circulation models, and to the reduction in sea-ice extent and thickness observed in other regions of the Arctic,” Gagnon and Gough say “this contradiction must be addressed in regional climate change impact assessments.”
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. 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, 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).”
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 too small to be detected at the present time.
References
Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html
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.
Gagnon, A.S. and Gough, W.A. 2006. East-west asymmetry in long-term trends of landfast ice thickness in the Hudson Bay region, Canada. Climate Research 32: 177-186.
Holloway, G. and Sou, T. 2002. Has Arctic Sea Ice Rapidly Thinned? Journal of Climate 15: 1691-1701.
IPCC. 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (Eds.) Cambridge University Press, Cambridge, United Kingdom and New York, NY.
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.
Parkinson, C.L., Cavalieri, D.J., Gloersen, P., Zwally, J. and Comiso, J.C. 1999. Arctic sea ice extent, areas, and trends, 1978-1996. Journal of Geophysical Research 104: 20,837-20,856.
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.
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.
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