Islands in the Indian Ocean are not the first to come to mind when glacier retreat is mentioned. However, glaciers in the Kerguelen Islands, located at sub-polar latitudes in the southern hemisphere, have been experiencing widespread and rapid retreat in recent years.
While rising temperatures are generally assumed to be the main cause of glacial retreat, a recent study published in Nature revealed that a reduction in precipitation is the dominant factor controlling the retreat of glaciers in the Cook Ice Cap on the Kerguelen. Similar conclusions were made in a study of Kilimanjaro’s melting glaciers, and this study could offer further insight into the effect of circulation changes on glaciers.
The Kerguelen Islands are among the most isolated places on Earth. Located on the seaway between South Africa and Antarctica, they are only accessible by boat and serve as a nesting ground for many bird species, such as the Macaroni Penguin. Glaciers cover about 500 square kilometers of the islands, and the loss of ice from these glaciers was among the most serious in the world in the 2000s, according to the study mentioned above.
Uncertainty surrounding the effects of climatic changes on glaciers in the southern mid-latitudes is particularly high due to a lack of observational data on glaciers and ice caps. Along with a lack of full modeling studies, this has led to the general assumption that warming is the main driver of glacial loss, as is the case in the northern mid-latitudes.
A team of scientists led by Vincent Favier, a researcher at Université Grenoble Alpes, set out to test the hypothesis that glacial retreat on the Kerguelen was largely due to increasing temperatures. The Cook Ice Cap was a suitable site for study because it is mainly made up of glaciers, which links its mass variations more strongly to climate variations than other ice caps at similar latitudes. In addition, the availability of long term climate and glaciological observations in the region made it possible to produce accurate models of glacial mass balance from 1850-2011.
Using a combination of field data, satellite data, and climate and glacial models, the team was able to attribute 77% of ice loss since the 1960s to atmospheric drying, with temperature increases only amplifying the losses. The researchers used the decade between 1950 and 1960 as a reference period for glacial mass and modelled changes in glacial mass using different hypothetical temperature and precipitation values.
1000 different simulations were run, revealing that dryness is the dominant influence on glacier wastage despite the increase in temperatures in the Kerguelen since the 1960s. The dominant influence of precipitation is particularly evident in glacier mass balance trends between 1963-1975, when both temperatures and mass balance increased. This seemingly paradoxical observation was due to higher levels of precipitation experienced during this period.
Precipitation over the Kerguelen is influenced by the north-south movement of wind belt in the middle latitudes of the Southern Hemisphere – the Southern Annular Mode (SAM). It brings stormy weather to the Kerguelen when it is in a more northerly position, also known as its negative phase. Since 1975, the SAM has been in southerly positions more frequently, increasing atmospheric dryness over the Kerguelen. This is associated with ozone layer depletion and increases in greenhouse gas emissions, suggesting that the frequency of positive phases of SAM is likely to increase over the course of the century and worsen glacier retreat in the Kerguelen.
Darker surfaces exposed by this loss of glacial ice could exacerbate melting in what is known as the ice-albedo negative feedback mechanism. These surfaces absorb more heat than light colored surfaces like ice, amplifying the effects of temperature changes on glacier loss.
As the SAM is a hemispheric feature, other glaciers within similar latitudes may also have been affected. As Favier explained in an interview with GlacierHub, “We chose this location because we believe it is an example of what is occurring under the same latitude but at other longitudes, in particular in New Zealand… Indeed, this drying trend is suggested at a large scale in the mid latitudes.” However, the lack of long-term sets of observational data for other locations at similar latitudes makes it difficult to determine possible effects, he said.
The difficulty of determining the effects of these changes on other locations within similar latitudes is exacerbated by the poor simulation of temperature and precipitation patterns over the Kerguelen in climate models used by the Intergovermental Panel on Climate Change. 95% of the models used by Favier and his colleagues to model patterns of glacial mass loss in the Kerguelen underestimated glacial mass losses at Cook Ice Cap. As such, projections of ice losses in the southern mid-latitudes based on model simulations should be used with great caution, particularly in areas where circulation changes are expected.
While glacier retreat is usually associated with increasing temperatures, it seems that circulation changes are important too. Remote locations like the Kerguelen Islands can offer clues about some of the impacts of climate change.