t for biological activity, and (4) limiting human access to the Arctic Ocean. Sea ice extent (designated by a 15% sea ice concentration threshold) has been monitored using passive microwave instruments on satellite platforms since 1979. The months of September and March are of particular interest because they are the months when the Arctic sea ice typically reaches its minimum and maximum extents, respectively. Figure 1 shows maps of monthly average ice extents in March 2017 and September 2017.
Average monthly sea ice extent in March 2017 (left) and September 2017 (right) illustrate the respective winter maximum and summer minimum extents. The magenta line indicates the median ice extents in March and September, respectively, during the period 1981-2010. Maps are from NSIDC at nsidc.org/data/seaice_index
(Fetterer et al., 2002).
Based on estimates produced by the National Snow and Ice Data Center (NSIDC) Sea Ice Index (Fetterer et al., 2002), the sea ice cover reached a maximum extent of 14.42 million km2 on March 7, 2017. This was 8% below the 1981-2010 average. For the third straight year, the Arctic sea ice has experienced a new record lowest maximum value in the satellite record. The maximum extent occurred 5 days earlier than the 1981-2010 average (12 March).
On September 13, 2017, the sea ice cover reached a summer minimum value of 4.64 million km2. This was the eighth lowest extent of the satellite record. While it represents a modest increase from the 2016 minimum, it was 25% less than the 1981-2010 median minimum ice extent. The ten lowest September extents have occurred in the last 11 years (Parkinson and DiGirolamo, 2016). The September ice extent has not returned to pre-2007 levels.
Sea ice extent has decreasing trends in all months and virtually all regions except for the Bering Sea during winter (Meier et al., 2014). The September monthly average trend for the entire Arctic Ocean is now -13.2% per decade relative to the 1981-2010 average (Fig. 2). Trends are smaller during March (-2.7% per decade), but the decrease is statistically significant.
Fig. 2. Time series of ice extent anomalies in March (the month of maximum ice extent) and September (the month of minimum ice extent). The anomaly value for each year is the difference (in %) in ice extent relative to the mean values for the period 1981-2010. The black and red dashed lines are least squares linear regression lines. The slopes of these lines indicate ice losses of -2.7% and -13.2% per decade in March and September, respectively. Both trends are significant at the 99% confidence level.
Age of the Ice
The age of sea ice is another key descriptor of the state of the sea ice cover. It serves as an indicator for ice physical properties, including thickness, surface roughness, and melt pond coverage (Tschudi et al., 2016). Compared to younger ice, older ice tends to be thicker, stronger and more resilient to changes in atmospheric and oceanic forcing. 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; Maslanik et al., 2011). This method has been used to provide a record of the age of the ice from 1985 to the present (Tschudi et al., 2015).
Very old ice (>4 years old) continues to make up a small fraction of the Arctic ice pack in March, when the sea ice extent is at its maximum (Fig. 3). In 1985, 16% of the ice pack was very old ice, but by March 2017, this ice category only constituted 0.9% of the ice pack. The extent of the oldest ice declined from 2.54 million km2 in March 1985 to 0.13 million km2 in March 2017. The distribution of ice age in March 2017 was similar to that in March of the previous year, although there was a decrease in the oldest ice fractional coverage, from 1.2% in March 2016 to 0.9% in March 2017.
Fig. 3. (a) Sea ice age coverage by year, 1985-2017. Sea ice age coverage maps for (b) March 1985 and (c) March 2017.
First-year ice now dominates the winter sea ice cover, comprising ~79% of the ice cover in March 2017, compared to about 55%