Is Antarctica losing or gaining ice?
What the science says...
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|While the interior of East Antarctica is gaining land ice, overall Antarctica is losing land ice at an accelerating rate. Antarctic sea ice is growing despite a strongly warming Southern Ocean.|
It's important to distinguish between Antarctic land ice and sea ice which are two separate phenomena. Reporting on Antarctic ice often fails to recognise the difference between sea ice and land ice. To summarize the situation with Antarctic ice trends:
- Antarctic land ice is decreasing at an accelerating rate
- Antarctic sea ice is increasing despite the warming Southern Ocean
Antarctic Land Ice is decreasing
Measuring changes in Antarctic land ice mass has been a difficult process due to the ice sheet's massive size and complexity. However, since 2002 the Gravity Recovery and Climate Experiment (GRACE) satellites have been able to comprehensively survey the entire ice sheet. The satellites measure changes in gravity to determine mass variations of the entire Antarctic ice sheet. Initial observations found that that most of Antarctic mass loss comes from Western Antarctica (Velicogna 2007). Meanwhile, from 2002 to 2005, East Antarctica was in approximate mass balance. The ice gained in the interior is roughly balanced by the ice loss at the edges. This is illustrated in Figure 1 which contrasts the ice mass changes in West Antarctica (red) compared to East Antarctica (green):
Figure 1: Ice mass changes (solid lines with circles) and their best-fitting linear trends (dashed line) for the West Antarctica Ice Sheet (red) and East Antarctica Ice Sheet (green) for April 2002 to August 2005 (Velicogna 2007).
As more GRACE data came in, a clearer understanding of the Antarctic ice sheet emerges. Figure 2 shows the ice mass changes in Antarctica for the period April 2002 to February 2009 (Velicogna 2009) . The blue line/crosses show the unfiltered, monthly values. The red crosses have seasonal variability removed. The green line is the best fitting trend.
Figure 2: Ice mass changes for the Antarctic ice sheet from April 2002 to February 2009. Unfiltered data are blue crosses. Data filtered for the seasonal dependence are red crosses. The best-fitting quadratic trend is shown as the green line (Velicogna 2009).
With the longer time series, a statistically significant trend now emerges. Not only is Antarctica losing land ice, the ice loss is accelerating at a rate of 26 Gigatonnes/yr2 (in other words, every year, the rate of ice loss is increasing by 26 Gigatonnes per year) It turns out that since 2006, East Antarctica has no longer been in mass balance but is in fact, losing ice mass (Chen 2009). This is a surprising result as East Antarctica has been considered stable because the region is so cold. This indicates the East Antarctic ice sheet is more dynamic than previously thought.
This is significant because East Antarctica contains much more ice than West Antarctica. East Antarctica contains enough ice to raise global sea levels by 50 to 60 metres while West Antarctica would contribute around 6 to 7 metres. The Antarctic ice sheet plays an important role in the total contribution to sea level. That contribution is continuously and rapidly growing.
Antarctic Sea Ice is increasing
Antarctic sea ice has shown long term growth since satellites began measurements in 1979. This is an observation that has been often cited as proof against global warming. However, rarely is the question raised: why is Antarctic sea ice increasing? The implicit assumption is it must be cooling around Antarctica. This is decidedly not the case. In fact, the Southern Ocean has been warming faster than the rest of the world's oceans. Globally from 1955 to 1995, oceans have been warming at 0.1°C per decade. In contrast, the Southern Ocean has been warming at 0.17°C per decade. Not only is the Southern Ocean warming, it is warming faster than the global trend.
Figure 3: Surface air temperature over the ice-covered areas of the Southern Ocean (top). Sea ice extent, observed by satellite (bottom). (Zhang 2007)
If the Southern Ocean is warming, why is Antarctic sea ice increasing? There are several contributing factors. One is the drop in ozone levels over Antarctica. The hole in the ozone layer above the South Pole has caused cooling in the stratosphere (Gillet 2003). This strengthens the cyclonic winds that circle the Antarctic continent (Thompson 2002). The wind pushes sea ice around, creating areas of open water known as polynyas. More polynyas lead to increased sea ice production (Turner 2009).
Another contributor is changes in ocean circulation. The Southern Ocean consists of a layer of cold water near the surface and a layer of warmer water below. Water from the warmer layer rises up to the surface, melting sea ice. However, as air temperatures warm, the amount of rain and snowfall also increases. This freshens the surface waters, leading to a surface layer less dense than the saltier, warmer water below. The layers become more stratified and mix less. Less heat is transported upwards from the deeper, warmer layer. Hence less sea ice is melted (Zhang 2007).
In summary, Antarctic sea ice is a complex and unique phenomenon. The simplistic interpretation that it must be cooling around Antarctica is decidedly not the case. Warming is happening - how it affects specific regions is complicated.
Last updated on 20 July 2010 by John Cook.