Site: Palmer Antarctica LTER
Map of Antarctic sea ice duration trends, showing also yearly changes in PAL LTER ice season duration (inset).
Updated figures based on Stammerjohn et al (2011, DSRII, 58, pp. 999-1018) and Ducklow et al (Royal Society chapter.)

Changes in sea ice reflect changes in atmospheric and ocean circulation and properties, while the changing seasonality of sea ice plays a predominant role in controlling much of the polar marine ecosystem. Since the late 1970s satellites have allowed us to track sea ice changes from space.

For example, using long term satellite data we can determine the length of the ice season by tracking when sea ice first arrives in austral autumn and when it subsequently departs the following austral spring. We can then map the trends (Figure 1), which reveal remarkably large decreases in ice season duration in an area extending from the western Antarctic Peninsula (WAP) to the eastern Amundsen Sea. This area of decreasing sea ice stands in stark contrast to increasing sea ice trends observed most elsewhere around Antarctica, particularly in the western Ross Sea. When averaged over the PAL study region, we see an 86-day shorter ice season over the 1979-2010 period. The spatial pattern of decreasing ice season duration is conspicuous in that the largest decreases are over the continental shelf margin, in exactly the same region where the Antarctic Circumpolar Current delivers warm water to the shelf (see below). These decreases are also conspicuous in that they are adjacent to coastal areas experiencing the greatest ice mass losses from the West Antarctic Ice Sheet (WAIS). We now know that atmospheric warming extends from the Antarctic Peninsula over the entire WAIS, and that it cannot be explained without the influence of increasing greenhouse-gas concentrations that in turn appear to be impacting regional atmospheric circulation, sea surface temperature and sea ice (Steig et al., 2009).

Nutrients and heat are delivered to western Antarctica via the Antarctic Circumpolar Current, in the particular water mass: Upper Circumpolar Deep Water (UCDW). Nutrients provide food to the base of the foodchain, while heat in the ocean is particularly “potent” given that the ocean contains over 4000 times more thermal energy than the equivalent volume and temperature of air (e.g., seawater will melt over 4000 times more ice than the air). The UCDW supplies heat that warms the winter atmosphere and melts both sea ice and the undersides of coastal glaciers. PAL researchers have shown that the UCDW itself is warming (Fig. 2). These effects play major roles in the WAP climate that is showing the fastest atmospheric winter warming on Earth (Vaughan et al., 2003) and where 87% of the WAP glaciers are in retreat (Cook et al., 2005).

Glaciologists find that recent warming of the UCDW is responsible for accelerated melt of WAIS ice streams. In an attempt to determine the source of the UCDW warming, necessary to forecast future sea level contributions, we find that the WAP exponential warming mimics the rate that global deep waters of the world have warmed (Figure 2).This suggests that unless something changes, the accelerated rate of WAIS ice stream melt will continue for decades, regardless of whether global warming is stopped or not. We are now studying mechanisms for delivering the warmed water onto the continental shelf to evaluate the likelihood of changing the delivery in the future given other changes in the system (e.g., extreme glacial melt will release large quantities of fresh meltwater into the ocean that could possibly “cap” the heat coming in at depth, thus preventing further accelerated melt).

Graph for
Graph of 8 independent studies showing increase in global deep ocean heat content (State of the Planet, 2008). Red solid squares show increased heat content of UCDW an area extending from WAP to offshore of where WAIS ice stream enter sea.
Figure recently constructed for publication in Webb and Martinson, in Nature Geoscience on alarming ocean warming in the western Antarctic.