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% in the 1980s. The thinner, younger ice is more vulnerable to completely melting in the summer, resulting in lower minimum ice extents.
Sea Ice Thickness
Satellite remote sensing and regular airborne survey programs continued to record changes in Arctic sea-ice thickness and volume. In 2017, the ESA CryoSat-2 radar altimeter mission completed its 7th year of operation since its launch in 2010, providing sea-ice thickness between October and April (Laxon et al., 2013) from freeboard observations. The CryoSat-2 freeboard measurements expand the data record of satellite and submarine based observations that document the decline in sea-ice thickness in the last decades (Kwok and Rothrock, 2009; Lindsay and Schweiger, 2015).
In spring 2017, CryoSat-2 products from the Alfred Wegener Institute (AWI) indicated a pattern of ice thickness change that was more variable (Fig. 4a) compared to previous years (2011-2016) in the observational period (Fig. 4b). The April 2017 thickness anomaly (Fig. 4c) shows below average thicknesses in the multiyear ice region north of the Canadian Archipelago, the Chukchi Sea and the shelf regions of the East Siberian Sea. Above-average thicknesses were observed in the Beaufort Sea and the eastern part of the central Arctic Ocean. The magnitude of these anomalies was generally below 1 meter.
(a) Sea-ice thickness from CryoSat-2 radar altimeter data for April 2017, (b) mean April data record, for the period 2011-2016, and (c) the April 2017 sea-ice thickness anomaly. (Data source: AWI CryoSat sea ice product, www.seaiceportal.de
The CryoSat-2 sea-ice volume estimates (Fig. 5) for the central Arctic Ocean showed a continued decline from 2014 through 2017. The April 2017 sea-ice volume of 13.19 ± 1.15 thousand cubic kilometers ranks as the third lowest spring volume after April 2012 (13.14 ±1.27) and 2013 (12.56 ± 1.21) in the AWI CryoSat-2 data record. The difference between the three lowest volume estimates lies within the observational uncertainties.
Fig. 5. (a) Monthly time series of sea ice