Climate Change in Antarctica

Introduction

Antarctica is one of the most poorly understood parts of our planet. It’s larger, colder, and less accessible than even the Arctic. Direct observational data is scarce, especially in East Antarctica, and the satellite record for many metrics is short.

Ice Terminology:

Ice Sheet: thick layer of year-round ice sitting on continental bedrock that extends more than 20,000 square miles. Found in Antarctica and Greenland.

Ice Shelf: Part of an ice sheet that flows out from the bedrock and overhangs sea water. Stays in position year-round. Found mainly in the Antarctic, but smaller (and rapidly disappearing) shelves exist in Greenland and Northern Canada.

Sea Ice: Ice that forms in sea water, unattached to a continent. Melts, grows, and shifts position with seasons and ocean currents. Found in the Antarctic and Arctic.

Mass Balance: Whether an ice sheet grows or shrinks with time.

Many of Antarctica’s ice shelves are collapsing, and the continent’s land-based ice sheet is losing mass. Antarctica is one and a half times the size of the United States and differs greatly from east to west. West Antarctica is warming and losing ice, while East Antarctica is cooling, and may be gaining both land and sea ice.

Despite being the coldest place on Earth, Antarctica is vulnerable to global warming. It is melting, and this melt may accelerate if key ice shelves continue to be lost. The Antarctic ice sheet serves as a valuable climate indicator, as well as a potential source of dangerous sea level rise.

The strengthening of the jet stream in Antarctica is consistent with global warming, and may be part of the reason why East Antarctica has defied the global trend. Topographic factors such as the high elevation of East Antarctica also play a role. The details are still challenging scientists, but this backgrounder catalogues the state of our knowledge.


Polar Ice

Ice covers much of the Earth’s polar regions and it is a critical component of the Earth’s climate. The North Pole is covered with sea ice and surrounded

by the Arctic Ocean, which undergoes annual cycles of ice accumulation and melt. While the Arctic contains water surrounded by continents, the South Pole is in Antarctica, a continent surrounded by ocean. The majority of ice in Antarctica, known as an ice sheet, sits atop the continental bedrock. It is estimated that the Antarctic ice sheet contains around 90% of the Earth’s fresh water, which, if melted, would cause sea levels to rise by 58 meters. (Philander 2012)

Maps show the Arctic (above) and Antarctica (below), as well as the shift in sea ice (white) between winter and summer. Source: NSIDC


East and West Antarctica

The continent of Antarctica varies greatly from east to west. The differences mainly are due to topography. East Antarctica is larger, more mountainous, and at a higher elevation. The high ridges of the East Antarctic Plateau are home to the coldest places on Earth, where temperatures can dip below minus 133.6°F. The West is at a lower elevation, with some ice resting below sea level. More contact with ocean water means more opportunities for ice to melt (Hellmer et al. 2012), and over the last 15 years, scientists have increasingly observed that West Antarctic glaciers have been retreating and draining huge amounts of ice into the ocean. Warm Atlantic sea surface temperatures have been found to be a factor in this melt (Li et al. 2014)

The Pine Island Glacier, which is responsible for nearly 20 percent of the total ice loss in West Antarctica, is of particular concern. New research indicates that the glacier, because it is close to a current of warmer water, has crossed a tipping point and is melting irreversibly. This will lead to global sea level rises estimated at one centimeter over the next twenty years (Favier et al. 2014). The melting is also likely to cause broader instability in the West Antarctic ice sheet, with further implications for ice melt and sea level rise. 

The division between East and West Antarctica. Pine Island Glacier is shrinking especially fast. Source: BBC News | Colored regions represent major ice shelves, gray is the land-based ice sheet. Source: NSIDC

Ice-ocean interactions like those we are currently witnessing with the Pine Island Glacier may become even more influential in the future as sea levels rise, because floating ice shelves hold up much of the West Antarctic Ice Sheet. If floating ice shelves are threatened or eliminated due to sea level rise, the flow of non-floating land ice near the coast would accelerate, which could ultimately cause the interior of the ice sheet to melt. In this scenario, global sea level would rise by more than three meters (Joughin & Alley, 2011).

East and West Antarctica are also differentiated by ice-atmosphere interactions. The polar jet stream and westerly winds that encircle Antarctica trap cold air and block warm air. This effect is greater in the East, where the topography isolates the land from the surrounding environment; meanwhile the West is more open to intrusions of warm air (Stokes et al. 2013). 

Air circulation patterns are influenced by other climate variables (Mayewski et al. 2013). The ozone hole is one such variable, and many studies indicate reduced ozone enhances the jet stream and westerlies’ cooling effect (Jones 2013;Turner et al. 2009). This is because reduced ozone coverage alters air currents and increases winds that divert warm air. This effect further isolates East Antarctica from the full force of global warming. 

Ocean temperature is another variable that can influence Antarctic air circulation. Research suggests warming oceans increase the temperature gradient between the southern ocean and cold polar region, which in turn intensifies the effect of the polar jet stream and associated westerlies (Lamy et al. 2010).


Antarctica’s Mass Balance

Mass change of the Antarctic ice sheet over time, as measured by the GRACE satellite launched in 2002. Source: NSIDC

The Antarctic ice sheet has historically experienced very little melting from the surface due to the cooling effect of Antarctica’s winds. For similar reasons, the East Antarctic ice sheet is considered Earth’s most stable ice sheet. However, melting changes the total area of ice sheets regularly, primarily in West Antarctica where the ice comes into contact with water or warmer dry land at its base. 

Due to the complexity and difficulty measuring ice-atmosphere and ice-ocean interactions, scientists have struggled to get an accurate reading of Antarctica’s overall mass balance; however, as early as 1998 there has been mounting evidence that Antarctica is in fact melting. In the past several years, major, independent surveys have continued to support this view using a variety of techniques (Velicogna & Wahr, 2006Ramillien et al., 2006; Chen et al., 2007; Rignot et al. 2008Velicogna, 2009;Pritchard et al., 2011Rignot et al. 2011).

Using satellite measurements of mass changes, scientists are able to produce comprehensive and accurate assessments of ice sheet losses in Antarctica, and they have found a 5.5 ± 2 Gt/yr2 decrease in Antarctic surface mass balance since 1992 and an acceleration in mass loss of 14.5 ± 2 Gt/yr2 (Rignot et al. 2011). According to these numbers, the mass loss in Antarctica is comparable in magnitude to the mass loss in Greenland (Rignot et al. 2011).


Observational Challenges

Antarctica is one of the most challenging places on Earth to scientifically observe and measure. In a 2006 paper, Dr. Eric Rignot described the situation: “The observational record of Antarctic glaciers is short and limited because humans have seldom visited many Antarctic glaciers. In the last decade, the advent of satellite missions has revolutionized our knowledge of ice dynamics in Antarctica. These tools have provided observations of ice motion in most places for the first time.” 

The relatively limited span of the observational record and short timeframe of the satellite record make it difficult to draw strong conclusions about Antarctica. The East is especially poorly observed. Some commentators have suggested that the media’s focus on the clearly shrinking Arctic as opposed to the confusing Antarctic is part of an intentional cover-up. A much more likely explanation, however, is that the scarcity of data and the greater range of natural variability in the Antarctic provide less in the way of clear trends to talk about.

Direct Observation: The United States operates just three permanent research stations on the entire continent. The U.S. also participates in an interdisciplinary program called the West Antarctic Ice Sheet (WAIS) Initiative. There is no corresponding initiative for the East, though, thus revealing a significant knowledge gap. 

International coordination may help with such gaps. As of 2007, there were 37 total year-round stations in Antarctica operated by 20 nations, but these stations vary in size, funding, and research priorities. 

Money is an issue for Antarctic research, which is very expensive to do. The wealthy U.S. is not immune to money troubles in Antarctic research. For example, the government shutdown of 2013 caused the loss of research in the field for many teams, and budget debates have not been settled yet.

Direct observation in the Antarctic can also be dangerous. The Antarctic is even colder and less accessible than the Arctic, and during the nine-month winter researchers often cannot be removed even in an emergency. For example, in 1982 a fire destroyed the generators at Russia’s Vostok Station. The researchers survived by burning asbestos wicks dipped in diesel fuel. In the winter of 1961, the only doctor on the 12-person base came down with appendicitis and had to operate on himself (he survived). Today, key winter crewmembers must have their wisdom teeth and appendixpreemptively removed.

Satellite Observation: Data collection from space resolves many of the gaps associated with direct observation. The whole continent can be observed at once without interruption, and many different types of measurements can be taken. But to understand trends over time, it’s necessary to have a record with which to compare current observations, and the existing satellite record is relatively short.

Systematic observation of sea ice extent in Antarctica dates back to 1978, when NASA launched the SMMR satellite. The satellite and its successors used microwaves and could also measure surface wind speed and precipitation. Other satellites such as those in the LandSat program have been used to take visual photographs of Antarctica, but are less useful for scientific data, because they cannot “see” through clouds as microwave satellites can.

The GRACE satellite, launched in 2002, was the first satellite able to use changes in gravitational fields to measure the mass balance of Antarctica and can detect overall mass loss due to melting.

Reliable data on ice thickness, however, was not available until the launch of the CryoSat-2 in 2010. The CryoSat-2 uses radar to measure changes in ice thickness as small as 2cm. With only a few years of data so far from Cryosat-2, we don’t have a long record for reference, but as time goes by Cryosat-2 will help us learn a lot about trends in Antarctic ice volume.

Historical Reconstruction: A final way to try to extend our knowledge beyond the bounds of our data is through historical reconstruction. Researchers have used data from whaling records and ice cores to gain some insight into the state of Antarctic ice farther into the past.


What We Know: Category Breakdown

Overall Trends

Antarctica is melting. Ice-ocean interactions play a central role, where oceans deliver more heat to ice shelves from underneath, causing instability of inland ice and faster rates of ice discharge to the sea. While this chain of events is widely accepted as the cause for ongoing ice melt in West Antarctica, the interior regions do not exhibit a long-term trend (Rignot 2011).

West Antarctica, Ice Sheet

Trend: Warming and shrinking

What’s Physically Happening: Ice sheets are flowing off the continent into the ocean faster, likely influenced by warmer ocean water and ice sheet collapse.

In West Antarctica, the flow of the Pine Island Glacier accelerated exponentially over the last 30 years: 0.8% in the 1980s, 2.4% in the 1990s, 6% in 2006 and 16% in 2007–2008 (Rignot 2008) and quadrupled its thinning rate in 1992–2008 (Wingham et al. 2009). Neighboring glaciers are widening their flow (Rignot 2006). Simple model projections predict a tripling in glacier speed once the grounding line retreats to a deeper and smoother bed (Thomas et al. 2004). Dynamic losses are therefore likely to persist and spread farther inland in this critical sector.

Melting influences are also evident in the collapse of major Western ice shelves, including the Larsen ice shelf, which disintegrated in 2002. The ice shelf was 220 meters thick and is believed to have existed for 12,000 years. Warming over time caused gradual stresses and fractures, which culminated in a sudden breakup.

Why it Matters: Land ice melt contributes to sea level rise. It’s also susceptible to “threshold” effects from increased velocity, thinning and retreat and could suffer a collapse once enough mass is lost (Joughin & Alley, 2011). Ice shelves are a key part of this process in that they stem the flow of glaciers into the sea, and their collapse can accelerate glacier flow (Rignot 2009).

West Antarctica, Sea Ice

Trend: Shrinking

What’s Physically Happening: According to the IPCC, “In the winter, negative trends are evident at the tip of the Antarctic Peninsula and the western part of the Weddell Sea” (AR5, WGI, chapter 4, pg. 18). In the summer and autumn, sea ice loss trends are confined to the Bellingshausen/Amundsen Seas. Warm ocean currents are being driven towards West Antarctica and melting sea ice from below (Jacobs et al., 2011).

Why it Matters: Melting sea ice doesn’t contribute directly to sea level rise, but it is a useful indicator of ocean warming. It also means that land ice will flow into the ocean faster without the sea ice blocking it (Pritchard et al., 2011).

East Antarctica, Ice Sheet

Trend: Slight growth

What’s Physically Happening: Analysis of satellite data from the Gravity Recovery and Climate Experiment (GRACE) indicates that East Antarctica has gained mass (King et al., 2012). This could be due to reduced levels of ozone in the upper atmosphere (stratosphere) that are strengthening the polar wind vortex and preventing warm air from reaching the region (Miles et al. 2013). Briny water, due to its relatively high density and lower freezing temperature, is also slowing the process of ice melt (Kahazender et al. 2013). Finally, warmer temperatures and increased air humidity in the surrounding area have contributed to increased snowfall in East Antarctica (Winkler et al. 2012).

The observed rate of growth is small compared to natural variation, meaning it may be a negligible natural variation (Boening et al. 2012). Furthermore, new research suggests that the East Antarctic ice sheet has melted in the past during the Pliocene era, meaning it may be more vulnerable to melting than previously thought (Cook et al. 2013; Miles et al. 2013). 

Thinning has been reported along the East Antarctic ice sheet, but it is still unclear whether glaciers are advancing or retreating (Miles et al. 2013).

Why it Matters: Although GRACE satellite estimates indicate slight growth in the mass of the East Antarctic Ice Sheet, it is not substantial enough to counter or offset the larger trend of ice loss in the Antarctic continent overall. Trends in East Antarctica are an outlier in that nearly every other part of the globe is consistently warming and melting. Furthermore, the data record is short enough that the trend may simply be natural variation. Overall, East Antarctica is the coldest, most sparsely observed and poorly understood part of the continent.

East Antarctica, Sea Ice

Trend: Slowly growing (maybe)

What’s Physically Happening: Compared to West Antarctica, seasonal and annual trends in East Antarctic sea ice have not been the subject of extensive research and are less well understood. A recent analysis notes that sea ice trends off East Antarctica yield  “mixed signals on regional to local scales” with regions of ice growth in some cases located right next to regions of ice loss (Massom et al. 2013).

In general, East Antarctic sea ice appears to be protected from melt by the same changing air currents that protect the Eastern ice sheet (Turner et al. 2009). However, a new study suggests that the apparent growth in sea ice may be an illusion caused by a change in the way satellite sea ice observations are processed (Eisenman et al. 2014). More research will be needed to confirm this.

Why it Matters: It doesn’t tell us much about the bigger picture. The same issues with understanding the data regarding the East Antarctic ice sheet apply to sea ice as well.