TCS Daily


Arctic Sea Ice -- Is it Disappearing?

By George Taylor - March 7, 2005 12:00 AM

It has long been recognized by climate modelers that CO2-induced global warming should be most noticeable in the polar regions. In most of the world, there is enough of the earth's dominant greenhouse gas -- water vapor -- to absorb the heat that radiates from the surface. Water vapor is scarcest where the air is coldest, however, because cold air can "hold" much less water vapor than warm air can. Since temperatures are coldest in the polar regions, the driest air on earth occurs there.

In those regions, the other greenhouse gases, especially CO2 and methane, take on a potentially much stronger role, and we would expect that increases in those gases would have their biggest impacts in those regions. As some scientists have said, the Arctic is "the canary in the coal mine" when it comes to global warming, and we should be watching there for early warnings of impending climate change.

Sure enough, we're hearing plenty of media reports from American media and abroad suggesting that big changes are already happening. Here are a few examples:

  • "Since the industrial revolution, human activity has released ever-increasing amounts of carbon dioxide and other greenhouses gases into the atmosphere, leading to gradual but unmistakable changes in climate throughout the world--especially at the higher latitudes. Average surface temperatures in the Arctic Circle have risen by more than half a degree Celsius (0.9 degrees Fahrenheit) per decade since 1981. The extent of Arctic sea ice cover has decreased by 7 - 9 percent per decade. And the three smallest extents of summer ice ever seen there have all occurred since 2002. According to the latest forecasts, the Arctic could be ice-free in the summer by the end of this century." Earth Policy Institute, February 26, 2005

  • "The Arctic sea ice has receded by about 40 percent since 1979. By the end of this century the region could be ice free during the summer months, according to Michael Oppenheimer, a geoscientist at Princeton University in New Jersey." National Geographic News, February 25, 2005

  • "Up to 60% of Arctic sea ice could vanish in about half a century if climate change continues, a scientist warned today. Over the last 20 years, the Arctic's perennial sea ice sheet has reduced by about 15%, said Dr Mark New of the University of Oxford. 'That could go up to 50% or 60%, say by the 2050s, if current trends continue,' Dr New told an international conference in Exeter, Devon, on avoiding dangerous climate change." PA News, February 2, 2005

  • "Global warming will hit the Arctic harder and faster than the rest of the world and could cause the extinction of polar bears and other Arctic wildlife within 20 years, conservationists warn. 'If we don't act immediately the Arctic will soon become unrecognizable,' said Tonje Folkestad, climate change officer with WWF's International Arctic Program." Environmental News Service, February 1, 2005

Let's see what science, rather than the press, tells us about sea ice in the Arctic.

Sea Ice in the Sub-Arctic, According to Scientific Journals

Grumet et al. (2001) created a record of sea ice conditions in the Baffin Bay region of the Canadian Artic going back 1,000 years. They concluded that the 11th through 14th centuries saw reduced sea ice, but that ice extent was greater over the next six centuries. The last century has shown that "sea-ice conditions in the Baffin Bay/Labrador Sea region, at least during the last 50 years, are within 'Little Ice Age' variability," despite several periods of warmer temperatures. The authors added an interesting statement, as well, stating that the sea ice cover history of the Arctic "can be viewed out of context because their brevity does not account for interdecadal [decade to decade] variability, nor are the records sufficiently long to clearly establish a climate trend."


For an area in the Greenland Sea, Comiso et al. (2001), used satellite images to assess the size and character of the Odden ice tongue, a 1,300 km long feature, from 1979 to 1998. They were also able to infer its character back to the early 1920s using temperature measurements. The authors stated that there has been no statistically significant change in any of the parameters studied over the past 20 years. However, the proxy record several decades further into the past reveals that the ice tongue was "a relatively smaller feature several decades ago," apparently as a result of warmer temperatures.


Omstedt and Chen (2001) identified a proxy record of the annual maximum coverage of Baltic sea from 1720 through 1997. They stated that there was a sharp decline in sea ice in about 1877. There was also greater variability in sea ice extent in the first 150 years of the record, which was colder, than in the warmer period of the last 100 years.


Jevrejeva (2001)
reported on a longer Baltic sea ice data set from 1529 to 1990 for the port of Riga, Latvia. The time series included four climate eras: (1) 1530-1640, with warming accompanied by earlier ice break-up (by 9 days/century); (2) 1640-1770, a cooler period with later ice break-up (5 days/century); (3) 1770-1920, with warming and a tendency toward earlier ice break-up (15 days/century); and (4) 1920-1990, a cooling period with later ice breakup (by 12 days/century).


Moving Poleward


Laxon, et al (2003) were motivated by a "mismatch between the observed variability and that predicted by models." The actual variability and model predictions appear to be very different, and this casts doubts on the models' ability to predict future changes. Unfortunately, the "sparseness of sea ice thickness observations" in the Arctic means that the "regional and interannual variability of sea ice thickness is entirely based on models of the Arctic." Since too few observations exist, scientists resort to very imperfect models, which perform poorly. In conclusion, "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."


Okay, so if we need to be cautious about models perhaps we should just see what the data say.


Polyakov, et al (2002) studied sea ice cover over the Kara, Laptev, East Siberian and Chukchi Seas north of Russia. Sea ice cover trends were "smaller than expected" and "do not support the hypothesized polar amplification of global warming." In a later report, Polyakov et al (2003b) stated that "long-term ice thickness and extent trends are small and generally not statistically significant"; in fact, "over the entire Siberian marginal-ice zone the century-long trend is only 0.5% per decade," or 5% per century."


A number of researchers have suggested that inflows of Atlantic water into the Arctic profoundly affect temperatures and sea ice trends in the latter ocean. Polyakov, et al (2004) are among these. The first sentence of their paper states "Exchanges between the Arctic and North Atlantic Ocean have a profound influence on the circulation and thermodynamics of each basin." The authors attributed most of the variability to multidecadal variations on time scales of 50-80 years, with warm periods in the 1930s-40s and in recent decades, and cool periods in the 1960s-70s and early in the twentieth century. These are associated with changes in ice extent and thickness (as well as air and sea temperature and ocean salinity). The most likely causative factor involves changes in atmospheric circulation, including but not limited to the Arctic Oscillation.


It is tempting to employ satellite data to estimate sea ice trends (see, for example, Parkinson, et al, 1999; and Parkinson, 2000) because satellites give us broad coverage (and prevent us from having to trek to the Arctic to make measurements!). Satellite data are limited to only the last several decades, however. According to Schmith and Hansen (2003), trend studies of Arctic sea ice conditions "should be regarded with some care" since the period of satellite observations coincided with but one phase of a clear multidecadal oscillation (alternating climate periods several decades long). Instead, the authors studied sea ice observations for the period 1820-2000 for waters off Greenland. One parameter which shows multidecadal variability is the correlation between ice export and the North Atlantic Oscillation (NAO); see trends below. In recent decades there has been a strong correlation between the two, as there was in the 1930s-40s. During the 1960s-70s and from about 1870-1920 there were much lower correlations. This "casts doubt on the hypothesis of enhanced greenhouse effect being the cause" for recent NAO-sea ice correlations, according to the authors.

 



North Atlantic Oscillation, 19-year average

 

Rigor, et al (2002) suggest that the Arctic Oscillation (AO) affects surface air temperatures and sea ice thickness over the Arctic in a profound way. Ice thickness responds primarily to surface winds changes caused by the AO, whose long-term trends are shown below. Positive AO values (as have been observed in recent years) correspond to higher wind speeds (and generally thinner ice).

 


Arctic Oscillation, 19-year average

 
Parkinson (2000) seems to have identified decadal or longer trends as well. The analysis described in that paper divided the Arctic into nine regions. In seven of the nine the sign of the trend "reversed from the 1979-1990 period to the 1990-1999 period," which is another reason to be cautious when evaluating relatively short data sets.


Holloway and Sou (2002) used data from "the atmosphere, rivers and ocean along with dynamics expressed in an ocean-ice-snow model." The authors warn against using any linear trend longer than 50 years due to multidecadal variability, which included "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. They also suggest that changes in wind patterns play a large role in ice thickness changes and that "Arctic sea ice volume has decreased more slowly than was hitherto reported." In fact, "the volume estimated in 2000 is close to the volume estimated in 1950."


Interdecadal Variability


Again and again we see terms "decadal," interdecadal" or "multi-decadal" in describing Arctic sea ice conditions. You have seen the similarity of the NAO and AO and can view the long-term variability. Note that in the NAO and AO charts the year 1970 (a starting point for many of the time series being mentioned) occurred at a time of minimum NAO and AO value.


Now consider a data set from the Pacific -- the Pacific Decadal Oscillation (PDO), an index which correlates strongly with ocean conditions in the tropical Pacific - positive values correlating with El Nino conditions, negative with La Nina. Below are annual values of the PDO. In the following chart I have plotted the NAO, AO and PDO together, with 19-year smoothing to show long-term trends.


And then take a look at the final chart, showing surface air temperature in the Arctic, from Polyakov, et al (2002). Notice how closely the temperatures match the NAO-AO-PDO chart: negative values of the latter match up with cooler temperatures (such as in the 1960s and 70s), while positive values of the indices correlate with warmer Arctic temperatures -- in recent years, and in the 1930s-40s.

 


North Atlantic, Arctic, and Pacific Decadal Oscillations, 19-year average

 

 

 

Surface air temperature anomalies in the Arctic, from Polyakov, et al (2002)

 

 

What This Tells Us

 

If we want to understand variability of Arctic sea ice (and, for that matter, sea and air temperature) we should take our eyes off greenhouse gases, at least for a moment, and study multidecadal phenomena. We should also avoid the temptation of taking the last 20-30 years of data, computing a trend, and assuming that that trend will continue for 50-100 years. History tells us that long-term linear trends will not occur. In the words of Santayana, "Those who cannot remember the past are condemned to repeat it." Or make bad forecasts.

 

The author is a Certified Consulting Meteorologist, Corvallis, Oregon.

 

References

 

Chylek, P., Box, J.E. and Lesins, G. 2004. Global warming and the Greenland ice sheet. Climatic Change 63: 201-221.

 

Comiso, J.C., Wadhams, P., Pedersen, L.T. and Gersten, R.A. 2001. Seasonal and interannual variability of the Odden ice tongue and a study of environmental effects. Journal of Geophysical Research 106: 9093-9116.

 

Grumet, N.S., Wake, C.P., Mayewski, P.A., Zielinski, G.A., Whitlow, S.L., Koerner, R.M., Fisher, D.A. and Woollett, J.M. 2001. Variability of sea-ice extent in Baffin Bay over the last millennium. Climatic Change 49: 129-145.

 

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

 

Hurrell, J. North Atlantic Oscillation (NAO) Indices Information. Source: http://www.cgd.ucar.edu/cas/jhurrell/Data/naodjfmindex.1864-2004.xls

 

Jevrejeva, S. 2001. Severity of winter seasons in the northern Baltic Sea between 1529 and 1990: reconstruction and analysis. Climate Research 17: 55-62.

 

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

Maslanik, J.A., Serreze, M.C., and Barry, R.G. 1996. Recent Decreases in Arctic Summer Ice Cover and Linkages to Atmospheric Circulation Anomalies. Geophysical Research Letters 23: 1677-1680.

Omstedt, A. and Chen, D. 2001. Influence of atmospheric circulation on the maximum ice extent in the Baltic Sea. Journal of Geophysical Research 106: 4493-4500.

 

Naurzbaev, M.M., Vaganov, E.A., Sidorova, O.V. and Schweingruber, F.H. 2002. Summer temperatures in eastern Taimyr inferred from a 2427-year late-Holocene tree-ring chronology and earlier floating series. The Holocene 12: 727-736.

Parkinson, C.L., Cavalieri, D.J., Gloersen, P., Zwally, H.J., and Comiso, J.C. 1999. Arctic Sea Ice Extents, Areas, and Trends, 1978-1996. J. Geophys Res. 104: 20 837-20 856.

Parkinson, C.L. 2000. Variability of Arctic Sea Ice: the View From Space, an 18-Year Record. Arctic 53: 341-358.

Polyakov, I., et al., 2002. Trends and Variations in Arctic Climate Systems. EOS, Transactions, American Geophysical Union, 83, 547-548.

 

Polyakov, I., Walsh, D., Dmitrenko, I., Colony, R.L. and Timokhov, L.A. 2003a. Arctic Ocean variability derived from historical observations. Geophysical Research Letters 30: 10.1029/2002GL016441.

 

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. 2003b. Long-term ice variability in Arctic marginal seas. Journal of Climate 16: 2078-2085.

 

Polyakov, I., Alekseev, G.V., Timokhov, L.A., Bhatt, U.S., Colony, R.L., Simmons, H.L., Walsh, D., Walsh, J.E. and Zakharov, V.F., 2004. Variability of the Intermediate Atlantic Water of the Arctic Ocean over the Last 100 Years. Journal of Climate 17: 4485-4497.

 

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.

 

Schmith, T. and Hansen, C., 2003. Fram Strait Ice Export during the Nineteenth and Twentieth Centuries Reconstructed form a Multiyear Sea Ice Index from Southwestern Greenland. Journal of Climate 16: 2782-2791.

 

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