Along the 1,200 kilometer western coastline of the Antarctic Peninsula, hundreds of glaciers stretch down to the sea. Glacier melt from this region is a major contributor to global sea-level rise. While scientists have looked to rising atmospheric temperatures to explain the rapid glacier melting in recent decades, a new study reveals that ocean temperatures may actually be the main cause of glacier retreat in the region.
The Antarctic Peninsula is in the northernmost part of the continent, and lies 1,000 kilometers from the tip of South America. Due to its latitude between 63 and 70 degrees South, the peninsula has the most moderate climate and — relatively speaking — warmest temperatures in Antarctica. As a result, glacier retreat in this area occurs at a faster rate than in most of the rest of the continent. However, melting has accelerated in recent years, raising concern in the scientific community. The atmospheric temperature record over the past several decades shows warming in the region. Rising atmospheric temperatures have, until now, been considered the largest contributing factor to glacier melting on the peninsula.
This study, published in Science on July 15, offers a new explanation in its surprising finding that ocean temperatures correlate more closely to glacier melt than air temperatures. The team, led by Alison Cook of Swansea University in the United Kingdom, investigated the relationship between ocean temperatures and glacier retreat in response to research, which showed that the air temperature record in the Antarctic Peninsula did not correctly predict the timing or location of glacier melt in the region.
Along the Antarctic Peninsula, there has been more ice loss in the colder southern end of the peninsula than in the warmer north. Air temperatures fail to explain this dramatic gradient along the peninsula, leading the team to seek another explanation. Using detailed data from the World Ocean Database, the researchers were able to track ocean temperatures along the Antarctic Peninsula between 1945 and 2009. When this data was compared to observed glacier retreat over time, a strong connection was revealed.
In the southern portion of the Antarctic Peninsula, mid-depth ocean temperatures were higher than in the north. By dividing the ocean near the peninsula into 6 study regions, the team of researchers from Swanea and British Antarctic Survey found that the ocean water composition was very different between the top and bottom half of the peninsula.
Along the southwestern coast of the peninsula, water from multiple oceans meets to form Circumpolar Deep Water (CDW). In this region, a mix of Antarctic, Pacific, and Atlantic water masses dominates the ocean composition. The temperature and salinity of the water along the southwestern coast is unique because of the mixing of different water sources — cold salty water sinks, while warmer water settles at mid-range depths. The CDW in the region has an average temperature of 4 degrees Celsius above the seawater freezing point.
However, Shelf Water and Bransfield Strait Water surround the northern portion of the peninsula. These waters are only 1 and 2 degrees above seawater freezing point, respectively. The warmer southern waters correspond to the areas of the peninsula that have had the most glacier melt, and explain why the southern peninsula has more ice loss than the northern area.
While it may seem that the surface temperature of the water would be the most important factor affecting glacier melt, the team found that it is actually the temperature of water 100 to 300 meters below the surface that correlates strongest with glacier melt — the bottom of the glaciers extending off the coastline fall within this range, and the warm water melts them from below the surface.
When fresh, cold water melts from the glaciers into the ocean, it causes upwelling — a process in which deep water rises to the surface. When warm Circumpolar Deep Water upwells onto the ice shelf, it accelerates the rate of glacier melt. In the north, where the deep water is still cold, this phenomenon does not occur.
The results show that warm ocean water is causing glacier retreat in a staggering 90 percent of the 674 glaciers that drain into the ocean. This important finding in the western Antarctic Peninsula means that conservation strategies need to be reconsidered and climate models readjusted, according to the authors. In order to accurately predict global environmental changes including sea-level rise, the temperature of coastal ocean water needs to be included as not only a factor, but the main factor in glacier melt.