Australia: The Land Where Time Began

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West Antarctica Warming Rapidly

It has been believed that the Antarctic Peninsula was the only part of Antarctica to have warmed in recent decades. This has now been shown to be incorrect, West Antarctica is actually warming rapidly.

For a long time it was believed that Antarctica was not taking part in the warming that has been occurring in the rest of the world, the Antarctic Peninsula, a relatively small area of land that juts from Antarctica towards South America, being thought to be an anomaly in that it has been rapidly warming for almost 100 years. It has now been shown that central West Antarctica, about 1,500 km inland from the Peninsula, has been warming over the past 5 decades as fast as any region on Earth (Bromwich et al., 2013).

According to the authors1 there may be some regions of Antarctica that are not sensitive to changing climate that is taking place around the rest of the Earth. Persistent westerly winds encircle Antarctica limiting the advection of heat from lower latitudes to the north of Antarctica and in the past decades these westerlies have strengthened, partly as the result of decreased amounts of ozone in the stratosphere and consequent vertical temperature structure changes in the atmosphere (Thompson et al., 2011). In East Antarctica the Polar Plateau comprises 2/3 of the Antarctic continent and an altitude of an average of 3,000 m. Atmospheric eddies, that are the only significant source of heat in the polar winter, are prevented from moving far into the East Antarctica interior by the steep continental slope between the plateau and the ocean. West Antarctica is much lower than East Antarctica and, unlike the climate of East Antarctica, the presence of the adjacent Amundsen and Bellingshausen Seas moderate its climate. The interior of West Antarctica is often penetrated by marine storms that are relatively warm and moist, on occasion reaching the South Pole across the entire ice sheet of West Antarctica. A significant source of local heat and storm activity has been provided as the sea ice cover has declined almost monotonically in the region for at least the past 30 years. The authors1 suggest there is reason to believe the ice sheet in West Antarctica has been warming along with the Antarctic peninsula, though proving this has been difficult because of a critical lack of instrumental weather data in central Antarctica.

The first direct evidence of warming in West Antarctica has been collected from satellite observations, in winter and spring in particular, but the data has been available only since 1982. Evidence that West Antarctica has been warming for at least 20 years prior to 1982 has been provided, primarily by the interpolation of the temperature data from Weather stations situated a considerable distance from West Antarctica (Steig et al., 2009), though according the authors1 the results of interpolation cannot rival direct local observations.

Byrd Station, that is close to the centre of the ice sheet, 80oS, 120oW, is the only location on the West Antarctic ice sheet that has collected direct temperature observations over the period of the last 50 years. According to the authors1 there are some gaps in the record, as well as a switch to automated systems following the hiatus from 1975-1980, which may have introduced errors resulting from the calibration of the temperature sensors.

Bromwich et al. salvaged the long-term temperature record for West Antarctica. Analysis of the salvaged data has shown that the austral winter and spring are the times when the greatest warming occurred at the Byrd Station, which agrees with earlier findings (Steig et al., 2009; Schneider, Deser & Okumura, 2011; Küttel et al., 2012). Their results have shown that there was a greater degree of warming than had been reported previously (Steig et al., 2009; Schneider, Deser & Okumura, 2011; Küttel et al., 2012), leading them to conclude that austral summer has also warmed significantly. In the updated Byrd Record the annual warming indicated that the overall warming rate averaged almost 0.5oC per decade from 1958 to 2010, comparable to that observed in the Antarctic Peninsula.

The warming that has been documented by Bromwich et al. has been supported by borehole temperature measurements. The surface temperatures affected only by smoothing of the signal by slow heat diffusion, a record has been preserved by polar snow and ice. Given the changes of surface temperatures that have been documented by Bromwich et al., the expected subsurface temperature profile, compares well with the subsurface temperature profile collected from a borehole in the ice sheet, 160 km east of Byrd Station in West Antarctica, provides independent support for the validity of the updated record from Byrd Station.

According to the authors1 the rapid warming of central West Antarctica that has been demonstrated is at odds with the cooling in summer that is expected because of the ozone depletion in the stratosphere above Antarctica. It does not necessarily imply that the understanding of the effects of ozone depletion is wrong, as the summer warming might be greater if there was no cooling resulting from ozone depletion, though it does imply that other influences on the Antarctic climate in summer are more important. Warming trends in West Antarctica in winter and spring have previously been attributed to changes in the circulation above the Amundsen Sea that are driven by anomalous convection in the tropical Pacific (Schneider, Deser & Okumura, 2011; Ding et al., 2011). The summer sea surface temperatures that have been observed near West Antarctica during the prominent El Nino event of 2009-2010 (Lee et al., 2010) indicate that similar mechanisms can also be important in summer. This earlier work is supported by the findings of Bromwich et al. which adds to a growing body of literature that suggests a significant role is played by the low latitudes in driving the change of climate in Antarctica.

The authors1 suggest there are implications for the ice sheet from the warming of West Antarctica, with atmospheric circulation changes bringing increased temperatures to West Antarctica have also driven ocean circulation changes (Steig, Battisti & Jenkins, 2012). The increased delivery, that result from these changes, of warm ocean waters to the West Antarctic ice sheet margin has resulted in the thinning of the floating ice shelves that has been observed (Jacobs et al., 2011). The flow of outlet glaciers draining the ice sheet have accelerated as a result of this thinning, which contributes to the rise in global sea level (Rignot, 2008). It has been suggested by Bromwich et al. that significant melting of the surface of ice shelves, that are already vulnerable, could possibly occur if the current trends continue (Bromwich et al., 2013). The collapse of ice shelves on the Antarctic Peninsula have resulted primarily from the mechanism of surface melting in summer.

According to the authors1 it is presently unclear if the trends that have been observed in the temperature of West Antarctica are likely to continue. The warming that has been confirmed by Bromwich et al. is now being captured by climate model simulations (Connolly & Bracegirdle, 2007) that capture accurately the trends in circulation and sea ice over the past few decades. The authors1 suggest that it is implied by the results from unrestrained models that yield trends that are more widely divergent, that the climate change of the future over West Antarctica remains considerably uncertain.

Central West Antarctica has been shown by the Byrd temperature record reconstruction of Bromwich et al. (Bromwich et al., 2013) that West Antarctica is one of the most rapidly warming regions on Earth. The authors1 suggest that these results should end speculation about the possibility that the region is warming, and the updated Byrd Station temperature record should be routinely incorporated into compilations of global temperature change.

Sources & Further reading

  1. Steig, Eric J., and Anais J. Orsi. "Climate Science: The Heat Is on in Antarctica." Nature Geosci 6, no. 2 (02//print 2013): 87-88.
Author: M. H. Monroe
Last updated 20/12/2012 

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