The Arctic has seen extensive ice loss in the past decade or two.
While the effects of climate change on glaciers, ice sheets and ice shelves of Arctic, and Western Antarctica have been extensively studied, there has been little gathering of data on East Antarctica which has the world’s largest ice sheet.
A recent paper in Nature by Dr.Chris Stokes and his team at Durham University, U.K., has presented the results of studies on outlet glaciers on the Pacific coast of East Antarctica. Data on 175 ocean-terminating glaciers along 5,400 kilometres of the East Antarctic ice sheet revealed three epochal patterns — sixty-three per cent of the glaciers retreated from 1974 to 1990, seventy-two per cent advanced from 1990 to 2000 and fifty-eight per cent advanced from 2000 to 2010.
These figures indicate that forces other than global warming are at play here unlike in Western Antarctica and the Arctic which have seen extensive ice loss in the past decade or two.
The authors point to the Southern Annular Mode (SAM) as one of the reasons for less glacier retreat in recent decades and link a positive phase of SAM to greenhouse gas concentrations and ozone depletion resulting in cooler temperatures over East Antarctica.
SAM describes the north–south movement of the westerly wind belt that circles Antarctica, dominating the middle to higher latitudes of the southern hemisphere.
In a positive SAM event, the belt of strong westerly winds contracts towards Antarctica. When the SAM is in a positive mode, it strengthens the westerly winds that circulate around Antarctica during the austral winter.
These strong circumpolar winds (known as the polar vortex) effectively lock in cold air and low pressure to the south of them and prevent warmer, more maritime air, influencing the ice sheet from the north.
Dr. Stokes notes in an email to this correspondent: “Put simply, there is less heat exchange between the tropics and the pole. This explains how positive phases of the SAM are linked to cooler temperatures over our study area in East Antarctica – the polar vortex is strong and locks in the cool air.”
In recent decades the polar vortex has strengthened, leading to cooler temperatures over East Antarctica. The cause for this strengthening is unclear and may be due to natural variability, ozone depletion or the green house gases factor. But many studies have shown that a combination of factors must be at play, the most important being ozone depletion. Loss of ozone (creation of the ozone-hole) leads to what is known as photochemical-induced cooling and that is a dominant process.
Regarding greenhouse gases, most climate models indicate that increases in carbon dioxide will change sea level pressures around the globe, with a decrease in high latitudes and an increase in mid-latitudes, which will reinforce the polar vortex, hence increasing the likelihood of a positive SAM index.
A positive phase of SAM has also resulted in coastal upwelling of different bodies of water. This upwelling led to basal melting resulting in glacier retreat explaining why the advance of glaciers was only 58 per cent in 2000 to 2010 compared to 72 per cent in 1990-2000.
The mechanism behind the upwelling and basal melting involves a strengthening of the westerly winds during the SAM’s positive phase.
These westerly winds drag surface currents and actually take them a little further north. Surface water is replaced by warmer water dragged up from the deep.
Dr. Stokes notes: “The water only has to be a few degrees above zero to impact on any of the glaciers that flow into the ocean. Even if the air temperature is several degrees below zero, relatively warm ocean currents can circulate underneath the terminus of the glacier (which is floating) and cause melting.”