D. Perovich1,2, W. Meier3, M. Tschudi4, S. Gerland5, J. Richter-Menge1
1ERDC - CRREL, 72 Lyme Road, Hanover, NH, USA
2Thayer School of Engineering, Dartmouth College, Hanover, NH, USA
3National Snow and Ice Data Center, University of Colorado, Boulder, CO, USA
4Aerospace Engineering Sciences, University of Colorado, Boulder, CO, USA
5Norwegian Polar Institute, Fram Centre,Tromsø Norway
December 3, 2012
- Record minimum Arctic sea ice extent occurred in September 2012; The lowest observed during the satellite record (1979-present) and 49% below the 1979-2000 average minimum.
- 2012 had the largest loss of ice between the March maximum and September minimum extents during the satellite record.
- The extent of multi-year ice continued to decrease.
- A severe storm in August accelerated ice loss in the Pacific Arctic.
Sea ice extent
Sea ice extent is used to describe the state of the Arctic sea ice cover. There is an accurate record of extent since 1979, determined from satellite-based passive microwave instruments. There are two months each year that are of particular interest: September, at the end of summer, when the ice reaches its annual minimum extent, and March, at the end of winter, when the ice is at its maximum extent. Ice extent in March 2012 and September 2012 are illustrated in Fig. 2.1.
Based on estimates produced by the National Snow and Ice Data Center, on September 16, 2012 the sea ice cover reached its minimum extent for the year of 3.41 million km2. This was the lowest in the satellite record; 18% lower than in 2007, when the previous record of 4.17 million km2 was recorded (Fig. 2.2). Overall, this year's minimum was 3.29 million km2 (49%) below the 1979-2000 average minimum of 6.71 million km2. The last six years, 2007-2012, have the six lowest minimum extents since satellite observations began in 1979.
In March 2012 ice extent reached a maximum value of 15.24 million km2 (Fig. 2.2), 4% below the 1979-2000 average. This was the highest maximum in 9 years, but 2004-2012 has the nine lowest maximum extents since 1979. The relatively high maximum extent in March 2012 was due to conditions in the Bering Sea, where ice extent was at or near record levels throughout the winter and spring.
After reaching maximum extent, the seasonal decline began slowly, particularly in the Bering Sea, and around mid-April, extent was close to the 1979-2000 average for the time of year. However, soon after that the decline accelerated and was faster than normal through much of the summer. August 2012 was a period of particularly rapid ice loss, in part due to a storm that passed through the region at the beginning of the month (see below, and the Air Temperature, Atmospheric Circulation and Clouds essay). Overall, 11.83 million km2 of ice was lost between the maximum and minimum extents. This is the largest seasonal decline in the record and 1 million km2 more than in any previous year.
Sea ice extent is decreasing in all months and virtually all regions (the exception being the Bering Sea during winter). The September monthly average trend is now -91,600 km2 per year, or -13.0 % per decade relative to the 1979-2000 average (Fig. 2.2). The magnitude of the trend has increased every year since 2001. Trends are smaller during March, but still decreasing and statistically significant. The March trend is -2.6% per decade (Fig. 2.2).
Average ice extents for each month are presented in Fig. 2.3. Three time periods are compared; the reference period 1979-2000, 2001 to 2006, and the last six years (2007-2012) beginning with the previous record minimum of 2007. The 1979-2000 period has the largest ice extent for every month, with the greatest difference between the time periods occurring in September. Comparing the two 21st Century periods shows that ice extent is similar in winter and spring, but summer values are significantly lower in 2007-2012.
Spatial distribution of sea ice
In 2007 persistent winds through the summer, created by an Arctic dipole circulation pattern (see Fig. A3 in Arctic Report Card 2007, and Fig. 1.5 in the Air Temperature, Atmospheric Circulation and Clouds essay), resulted in a compact ice cover and an ice edge far to the north on the Pacific side of the Arctic. However, the circulation also pushed ice onto the coast in the Laptev Sea, completely blocking the Northern Sea Route. In the years since 2007, the pattern of ice loss has varied, but a tongue of older ice in the East Siberian Sea has persisted through the summer. This tongue was particularly evident in 2010 and 2011. In 2012 that tongue of ice mostly melted away, aided by the August storm, and ice retreated significantly around the entire perimeter of the ice pack (Fig. 2.1, right panel). This includes the Atlantic side, north of Svalbard, where extents had been near normal in recent years. Overall, compared to 2007 there was more ice this year in the central Arctic north of the Bering Strait, but less ice nearly everywhere else.
Age of the ice
The age of the ice is another key descriptor of the state of the sea ice cover. Older ice tends to be thicker and thus more resilient to changes in atmospheric and oceanic forcing than younger ice. The age of the ice is determined using satellite observations and drifting buoy records to track ice parcels over several years (Tschudi et al., 2010). This method has been used to provide a record of ice age since the early 1980s (Fig. 2.4). The distribution of ice of different ages illustrates the extensive loss in recent years of the older ice types (Maslanik et al., 2011). Analysis of the time series of areal coverage by age category indicates the continued recent loss of the oldest ice types, which accelerated starting in 2005 (Maslanik et al., 2011). For the month of March, older ice (4 years and older) has decreased from 26% of the ice cover in 1988 to 19% in 2005 and to 7% in 2012. This represents a loss of 1.71 million km2 since 2005. In March 1988, 58% of the ice pack was composed of first-year ice (ice that has not survived a melt season). In March 2012, first-year ice dominated the pack (75%). Younger ice is typically thinner than older ice (e.g., Maslanik et al., 2007), so the current ice pack is likely thinner on average than it was in 1988. Note that, from March 2011 to March 2012, much of the three-year-old ice north of the Canadian Archipelago survived the melt season, resulting in an increase in four-year-old ice in March 2012 (5%, compared to 2% in March 2011). This increase was directly associated with a reduction in the fraction of three-year-old ice, which decreased from 9 to 7%.
Impact of an August Storm
A severe storm in the Chukchi and East Siberian seas in early August 2012 (Fig. 2.5) accelerated ice loss and helped to quickly remove ice from the region (also see the essay on Air Temperature, Atmospheric Circulation and Clouds). As Fig. 2.4 indicates, most of this region was covered by first year ice. The storm blew the ice southward into warmer water, where satellite observations indicated that it melted in a few weeks (Fig. 2.5). As the ice melted and diverged, ice concentration quickly fell below the detection limit of passive microwave sensors, though small amounts of ice were observed for a couple of weeks afterwards by operational ice analysts using other imagery.
Maslanik J. A., C. Fowler, J. Stroeve, S. Drobot, J. Zwally, D. Yi and W. Emery. 2007. A younger, thinner Arctic ice cover: Increased potential for rapid, extensive sea-ice loss. Geophys. Res. Lett., 34, L24501, doi:10.1029/2007GL032043.
Maslanik, J., J. Stroeve, C. Fowler and W. Emery. 2011. Distribution and trends in Arctic sea ice age through spring 2011. Geophys. Res. Lett., 38, L13502, doi:10.1029/2011GL047735.
Tschudi, M. A., Fowler, C., Maslanik and J. A., Stroeve. 2010. Tracking the movement and changing surface characteristics of Arctic sea ice. IEEE J. Selected Topics in Earth Obs. and Rem. Sens., 10.1109/JSTARS.2010.2048305.