Why Arctic sea ice is shrinking

Second of three vacation blogs from Dr. Masters. Enjoy.

Since 1979, coverage of Arctic sea ice has shrunk by about 10% in winter and 20% in summer. The vertical thickness of the ice has also shrunk. According to the "official" Intergovernmental Panel on Climate Change (IPCC) report in 2001, Arctic sea ice decreased in thickness by about 40% between 1958 and 1997. This figure came from submarine sonar measurements taken over five cruises in the autumns of 1958, 1960, 1962, 1970, and 1976 when compared with three cruises in the autumns of 1993, 1996, and 1997 (Rothrock et. al, 1999). However, according to modeling studies by Holloway and Sou (2001), these results are highly uncertain, since not all years or locations in the Arctic were sampled. They found that if the first five cruises had been done just one year earlier (September 1957, 1959, 1961, 1969, 1975) and the three latter cruises had been done one year later (September 1994, 1997, 1998), the sonar measurements would have shown only an 11%-15% decrease in thickness. Arctic sea ice varies greatly in thickness, and currents and winds are always pushing the ice around, making it difficult to measure how the average thickness has been declining.

Warmer air and water temperatures have contributed to the sea ice decline

Annual average surface temperature has increased about 1 degree C since 1980 over the Arctic, which accounts for much of the sea ice melt. In addition, some melting has occurred from beneath the ice, due to warmer ocean waters. Global warming has heated up both the North Pacific and North Atlantic waters significantly over the past 30 years. Warmer waters have been brought into the Arctic Ocean from the Pacific via an ocean current flowing through the Bering Strait between Alaska and Russia, and from the Atlantic via an ocean current flowing northwards along the European coast.



Figure 1. The Arctic Oscillation (AO) index from 1899 - 2006. The AO is a measure of the difference in surface pressure between the north pole and about 45 degrees north latitude. Image credit: Dave Thompson of Colorado State University.

Wind patterns are a major cause of sea ice loss


The Arctic Oscillation is an observed natural pattern of surface pressure variations in the Northern Hemisphere. The "positive index" of the AO is defined when the surface pressure is below normal at the north pole and above normal at about 45 degrees north latitude. Positive Arctic Oscillation conditions steer storms farther north, bringing stronger surface westerly winds in the North Atlantic and warmer and wetter than normal conditions to the Arctic and northern Europe. The winds and ocean currents during the positive Arctic Oscillation mode tend to drive sea ice from west to east along the north shore of Canada, then out of the Arctic Ocean through the channel of water to the east of Greenland (Fram Strait).

When one looks at the wintertime pattern of the Arctic Oscillation (AO) over the past 100 years, a mostly random pattern of positive and negative AO modes is apparent (Figure 1). However, one anomalous period is very striking: a string of seven consecutive years with a positive AO, including two years (1989 and 1990) with the highest AO index ever observed. During this period, strong westerly winds rapidly flushed more than 80% of the oldest, thickest sea ice out of the Arctic Ocean, leaving most of the Arctic covered with ice less than three years old (Figure 2). Younger ice is much thinner, and melts much more readily. Rigor and Wallace (2004) estimate that at least half of the loss of sea ice in the Arctic since 1979 is due to these six years of strange weather with very low surface pressure over the Arctic. Did climate change cause this unusual pattern between 1989 and 1995? It is possible, but no one has published any papers showing how this might have occurred. For now, the assumption is that this major loss of Arctic sea ice due to wind patterns between 1989-1995 is natural.

The big concern is that since the strange positive Arctic Oscillation years of 1989-1995, a number of years with negative AO have occurred. Normally, during negative AO years, ice extent and thickness increase in the Arctic. But instead, ice extent and thickness during 2002-2006 have shown an unprecedented series of record minima, giving rise to fears that we are on our way to an ice-free Arctic later this century.



Figure 2. The change in age and thickness of sea ice between 1987 and 2005. In 1987, most of the Arctic sea ice was old and thick, generally more than ten years old. A period of strong positive Arctic Oscillation conditions between 1989 and 1995 created winds and currents that flushed most of this old ice out of the Arctic Ocean, through Fram Strait to the east of Greenland. The new ice that replaced the old ice is much thinner. Image credit: Rigor, I. G., and J. M. Wallace (2004), "Variations in the age of Arctic sea-ice and summer sea-ice extent," Geophys. Res. Lett., 31, L09401, doi:10.1029/2004GL019492.


This is the third in a series of five blogs on climate change in the Arctic that will appear every Monday and Thursday over the next two weeks. Next blog: Future abrupt loss of Arctic sea ice.

Also, be sure to visit our new Climate Change blog, written by Dr. Ricky Rood of the University of Michigan.

Jeff Masters

References
Holloway, G. and T. Sou, 2001, "Has Arctic Sea Ice Rapidly Thinned?", Journal of Climate 15, p1691-1701, 2001.

Rigor, I. G., and J. M. Wallace (2004), "Variations in the age of Arctic sea-ice and summer sea-ice extent," Geophys. Res. Lett., 31, L09401, doi:10.1029/2004GL019492.

Rothrock D.A., Y. Yu, and G.A. Maykut, 1999: "Thinning of the Arctic sea ice cover." Geophys. Res. Lett., 26, 3469-3472.