Polar Climate Trends Review:
The Antarctic
Antarctica’s geography is complex and how the South Polar region has responded to the last few decades of global
temperature rise is equally so. Putting this response into a longer-term context is inhibited by sparse long-term
weather and ice mass records. Even with today’s technology and equipment, data gathering both on site and
remotely is difficult and ice mass variation estimates remain broad. Understanding the context and mechanics of
recent trends related to ice mass variation is important, as past interglacial periods with temperatures only a few
degrees warmer than today had sea levels as much as 25 feet higher. Evidence suggests that melting Antarctic ice,
particularly ice from the West Antarctic ice sheet (WAIS), likely contributed significantly to the increased ocean
volume. The WAIS holds enough ice to raise global sea levels by 16 to 23 feet. The East Antarctic ice sheet holds
enough ice to raise sea levels by 200 feet, but is considered much more stable and resistant to warming tends.
Amundsen
Sea
Above: 1980 to 2007 temperature trends based on radiometer
measurements. Image: NASA Earth Observatory.
Temperatures: The 1957-58 International Geophysical
Year led to the establishment of a number of year-round
research stations and weather data records in Antarctica.
Most of these stations were located close to the coastline,
making estimates of trends in the continent’s interior
difficult. Automatic weather stations now dot Antarctica
and satellites have measured temperatures since 1980.
The sharpest temperature trend is on the Antarctic
Peninsula, which has warmed by five degrees Fahrenheit
over the past 50 years. Satellite data shows warming
along the coast of the Amundsen Sea, an area
experiencing ice shelf loss. The interior of the continent,
including some areas near the South Pole show no trend.
Warming near the coast and interior cooling are likely
related to two processes: a rise in the Southern Ocean’s
temperature slightly above the global rate and
simultaneous strengthening of the circumpolar vortex
winds that isolate the continental interior from marine air
intrusions. These winds have strengthened since the late
1950s, with the greatest seasonal increase (20 percent
strengthening) occurring during southern hemisphere
summer.
Predominant Winds and Pressure Centers: The circumpolar vortex wind strengthening is related to a positive
trend in the Southern Annular Mode (SAM), defined as the departure from the normal mean pressure difference
between the latitudes of 40 and 65 degrees South. When pressure differences are greater, the SAM is in a positive
phase. Positive phases mean stronger westerly circumpolar winds which are accompanied by a stronger
circumpolar current in the surrounding Southern Ocean. Variations in the SAM explain about 35 percent of
Antarctica’s interannual weather variability. Since 1957, the SAM has become more frequently and strongly
positive, which is due at least partially to the development of the ozone hole over the same period. The loss of
stratospheric ozone, which happens predominately in the spring, cools the stratosphere and strengthens
circumpolar winds. While stronger westerly winds work to keep Antarctica’s interior cool, the strengthened winds
also accentuate warming of the Antarctic Peninsula.
Influence from the tropical Pacific, specifically the El Niño-Southern Oscillation, is useful in understanding
temperature trends and variability: this is particularly true for temperatures on the Antarctic Peninsula and along the
Amundsen Sea. Fluctuations in tropical Pacific water temperatures remotely influence the strength of the
Amundsen Sea low pressure system, which is an important source of moisture transport for West Antarctica. The
increase in the frequency of central Pacific El Niños as opposed to eastern Pacific El Niños over the last 30 years
has strengthened the Amundsen Sea low, leading to increased warm air transport and concurrent warming trends.
A Program of
Ice Shelf Loss and West Antarctic Instability: The ice shelves on the Amundsen Sea are thinning by between
two and 17 feet per year. This thinning is driven in part by increased warm air inputs from the Amundsen Sea low
and in part by sub-glacial intrusions of circumpolar deep water (CDW), which is about half-a-degree Fahrenheit
warmer than it was 50 years ago. Ice shelves separate land ice from sea ice and serve as buttresses that help keep
inland glacier ice streams from sliding and discharging glacial ice into the ocean. Ice shelves are particularly
important to the WAIS. Unlike the East Antarctic Ice Sheet that sits on an elevated continental mass, the WAIS sits
on a series of troughs and basins as deep as 1.25 miles below sea level, making its existence precarious. A
combination of factors – the buttressing effect of ice shelves being one of the most important – has kept the WAIS
intact over the current (Holocene) interglacial period. Evidence for the importance of the buttressing effect came
after the collapse of the Larsen A (1995) and B (2002) ice shelves on the Antarctic Peninsula. Following these
events many ice streams accelerated towards the ocean. Those that did not accelerate were those that were still
buttressed by smaller ice shelves.
If the WAIS ice sheet melted, seaways and islands would be uncovered. The largest of these is Pine Island – Pine
Island Glacier comprises about 10 percent of the WAIS mass. Several miles of grounding line retreat on Pine Island
occurred between 1995 and 2006, opening up a gap between the ice bottom and the ocean floor which allowed
ocean water in, causing further melting. Similar retreats and ice shelf weakening along the Amundsen Sea shore
have led to an acceleration of ice mass loss. The area now sends about 80 gigatons of ice into the ocean each
year, compared with 47 gigatons in the early-to-mid 2000s. Eighty (80) gigatons is about 0.22 millimeters of sea
level rise. For comparison, global sea level is rising at a rate of 3.1 millimeters per year. Most of Antarctica’s
contribution to sea level rise comes from the Amundsen area, which includes several other rapidly thinning glaciers.
Mass Balance Estimates: The 2007 Intergovernmental Panel on Climate Change put mass balance estimates for
Antarctica at between +50 and -250 Gigatons per year for the period 1992 to 2009. Mass balance is the difference
between input into an ice mass through precipitation and output through melting and ice flow. These numbers
reflect mass balance estimates from three main sources: the European Remote Sensing (ERS) satellites that make
radar altimetry measurements (measuring surface elevations with radar waves); the Input-minus-Output Method
(IOM), which uses a variety of tools and observations to determine how much precipitation is coming into Antarctica
and how much melt water and ice flow is going out; and NASA’s twin Gravity Recovery and Climate Experiment
(GRACE) satellites, which measure changes in Earth’s gravitational field to estimate changes in mass distribution.
The higher numbers (overall ice gain) come from the ERS measurements, while the lower numbers (overall ice
loss) come from the IOM and GRACE measurements.
More recent analysis suggests that the WAIS is losing ice while the East Antarctic Ice Sheet is stable or growing
slightly. The range of estimates is being narrowed by comparing data from other satellites and improving models of
how snow compaction, melting, snow drift and elevation changes influence ice density.
Map of Ice Stream Flow
Rignot, et al. (2011) used data from
Interferometric Syntheitc Aperture Radar
(InSAR) measurements to construct a map of
Antarctica’s ice streams – fast moving ice
channels flowing into the sea, flanked by
relatively stationary ice. Many satellites had
been used to take InSAR measurements over
time and coordination among international
space agencies to fill in data gaps made a
complete map possible.
This research shows there are several factors,
including bed and surface topography, basal
slipperiness, and ice shelf stability, which
control glacial movement. Lack of
measurements, especially for conditions
beneath the ice, make it difficult to model ice
flow and the results suggest that the dynamics
of Antarctica’s ice will be more unpredictable
than previously thought.
Image Right: The recent map of Antarctica’s ice streams. The
blue and purple colors indicate areas of rapid ice flow. Watch an
animation of the flow. Image: NASA/JPL-Caltech/UCI.
More Resources:
To see still images and animations depicting annual variations in Greenland and Antarctica’s ice masses from
GRACE data, visit: http://svs.gsfc.nasa.gov/vis/a000000/a003900/a003910/index.html
For a depiction of Antarctica’s annual sea ice growth and retreat, visit:
http://svs.gsfc.nasa.gov/vis/a000000/a003800/a003854/index.html
For an animation of the extent of Antarctica’s ozone hole and the circumpolar vortex from 1979 to 2004, visit:
http://svs.gsfc.nasa.gov/vis/a000000/a003200/a003264/index.html
A special thanks to Dr. Ian Joughin and Dr. Eric Steig at the University of Washington for his help with this paper.
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