Schmidtko S, Heywood K J, Thompson A F, Aoki S (2014) Multidecadal warming of Antarctic waters. Science 346, 1227-1231. DOI: 10.1126/science.1256117
What would happen if all of Antarctic’s ice sheets were to suddenly melt? This is very unlikely, but if it did, we would experience some of the most dramatic changes in weather and an intrusion of seawater in almost every coastal community. Antarctica is the world’s largest reservoir of freshwater, storing nearly 70 percent and making up its own continent. If all the glaciers and ice sheets on Antarctica melted, global sea level would be 60 meters higher than it is today. Findings from a recent study reveal alerting trends that may make this unlikely scenario a brutal reality over the next century or two. Schmidtko and co-authors examine the spatial distribution of long-term and large-scale temperature trends over Antarctica’s continental shelf. They show that different water types around Antarctica have not been warming uniformly, but rather in distinct regional patterns.
Flowing clock-wise around Antarctica is a warm, salty deep-ocean current known as Circumpolar Deep Water (CDW). Water masses from the Atlantic, Pacific and Indian Oceans mix together to form CDW. Understanding how the temperature of this water is changing is critical, and if it warms, can undermine glaciers with melt water and reduce ice thickness. CDW has shown significant warming around Antarctica, which has raised temperatures beneath ice shelves at shallower depths through changes in easterly wind strength over the Antarctic shelf break.
Researchers examined data from oceanographic observations in and around Antarctica collected from the 1960s onward. Their research points to startling findings that West Antarctica shelf waters in the Bellingshausen Sea and Amundsen Sea have warmed considerably more over the past 5 decades than previously thought. Meanwhile, the Ross Sea and Weddell Sea are cooling slightly. However, the trends in shelf-water temperatures are highly sensitive to the time period of data being analyzed. This is one challenge the authors attribute to conflicting findings of warming patterns in past studies. Observations from the 1990s onward are more reliable than older measurements dating back to the 1970s due to poor sampling frequency.
The authors categorize CDW temperature properties into two flow regimes: (1) sloping upward or (2) sloping downwards. These two regimes are primarily caused by differences in wind patterns blowing east to west around the South Pole. The changes in the strength of these winds modify seawater temperatures across the continental shelf under the sea ice.
Shelf water temperatures are rising in regions where CDW slopes upward toward the shelf break. These changes in CDW are most pronounced in the warming Bellingshausen and Amundsen seas. In regions where CDW slopes downward towards the shelf break, offshore trends in CDW show no correlation with bottom water masses on continental shelves, and therefore have no correlation with warming.
Ice shelf melting and ice sheet collapse depends on delivery of warm water by ocean currents. Therefore, it is important to have accurate estimates of oceanic heat transport. Surfacing CDW has the potential to increase heat transport and must be represented in climate models for future ice shelf predictions.
Observed trends in water mass properties may influence Southern Ocean ecosystems through the redistribution of salps and krill due to their complex life cycles and spawning preferences. Salps and krill play a critical role in Antarctic ecosystems and a change in abundance could impact their predators.
The melting occurring under the West Antarctic ice shelves is considered irreversible, and there are no indications that this warming trend will stop anytime soon. The good news is that scientists have not yet seen other Antarctic regions beginning to melt as intensely as in the west, but they agree that a change may be looming with significant consequences for sea level rise.