The November 2007 sinking
of the cruise ship MS Explorer
after it hit an iceberg in Antarctic waters is a reminder that the Antarctic is a dangerous place to sail. Ever since British explorer Ernest Shackleton's
met its end when it become trapped and crushed in pack ice near Antarctica, the Antarctic waters have been a notoriously dangerous place for boats. For those of you unfamiliar with the story of Shackleton's ill-fated expedition, I highly recommend a reading of The Endurance: Shackleton's Legendary Antarctic Expedition
. The book details the most mind-blowing tale of survival and courage I have ever read. Shackleton's phenomenal leadership skills saved the lives of all of his men. Shackleton refused to sleep for over 30 consecutive days while leading his men in an arduous months-long trek over the treacherous Antarctic sea ice. His voyage to find help using an open boat in winter on the storm-tossed Scotia Sea may rank as the greatest navigation feat of all time.Figure 1.
Antarctic sea ice (purple colors) at the time the MS Explorer
hit an iceberg and sank. Summer is approaching in the Southern Hemisphere, leading to melting and break up of the sea ice and plenty of icebergs. Image credit: University of Illinois Cryosphere Today.Why talk about Antarctic sea ice?
You hear a lot of talk about Arctic sea ice,
but not about Antarctic sea ice. That's because Antarctic sea ice is relatively unimportant to the Earth's climate. Antarctica is a huge continent that rises thousands of feet above the ocean. It holds about 90% of the world's fresh water, locked up in its massive ice cap. The presence of such a titanic block of ice at the bottom of the world completely dominates the weather and climate of the region, and the year-to-year fluctuations of sea ice don't have a lot of impact on temperatures there.
The other reason to ignore Antarctic sea ice is that it hasn't changed much over the historical record. A look at the sea ice coverage since 1978 (Figure 2) shows very little change. Climate skeptics have pointed out that Antarctic sea ice has been near its maximum area the past few winters. However, this is not considered statistically significant, and there is no overall trend apparent in the data.
However, Antarctic sea ice may be important because of its ability to insulate and buttress glaciers and semi-permanent ice shelves along the coast. Recent melting of sea ice due to warming temperatures along the Antarctic Peninsula allowed warming ocean waters to penetrate close to shore, triggering the collapse of the Larsen B Ice Shelf
in 2002. This Rhode Island-sized chunk of ice had been around thousands of years, and disintegrated in just three days. Any decline of Antarctic sea ice in coming decades might cause a speedier retreat of the continent's glaciers and ice shelves.Figure 2.
Antarctic sea ice area as observed via satellite since 1978. The maximum area in winter has ranged between 14-16 million square kilometers, about the same amount of ocean that the Arctic ice covers in winter. However, the Antarctic sea ice almost entirely melts away in summer, something the Arctic sea ice does not do (yet). Image credit: University of Illinois Cryosphere Today.Antarctic cooling
What is significant is the fact that most of Antarctica cooled in recent decades (Figure 3). For example, the surface temperature at the South Pole cooled 0.05° C between 1980 and 1999 (Kwok and Comiso, 2002). However, the majority of Antarctica has shown no statistically significant warming over the past 50 years (Turner et al.
, 2005)--the cooling has just been over the past 25-30 years. In the period 2004-2007, much of the Antarctic warmed (Figure 4). Why did Antarctica cool between 1982 and 2004 if there was global warming going on?
Well, the globe, on average, has warmed about 1.1° F (0.65° C) in the 50 years ending in 2005 (IPCC, 2007). Given that there is a lot of natural variability in the climate, it should be expected that some areas of the globe would not see warming, given the relatively modest magnitude of global warming thus far.Figure 3.
Antarctic surface temperatures as observed via AHVRR satellite measurements between 1982 and 2004. Much of Antarctica cooled during this period. Image credit: IPCC The Physical Science Basis, Figure 3.32.Figure 4.
Antarctic surface temperatures as observed via AHVRR satellite measurements between 1981 and 2007. Note that the cooling trend observed from 1982-2004 has reversed, thanks to warming in the past few years. Image credit: NASA
In addition, the weather of the Antarctic is dominated by a strong band of westerly winds that blow around the pole. This circumpolar vortex extends from the surface to the stratosphere, and can attain very high wind speeds, thanks to the absence of large land masses to slow it down. This vortex tends to isolate Antarctica from the rest of the globe, keeping global warming from influencing Antarctica weather, and allowing the surface to cool. The Antarctic Peninsula, which sticks out from Antarctica towards South America, frequently lies outside the vortex. This has allowed the peninsula to warm significantly, compared to the rest of Antarctica (Figures 3 and 4). The Antarctic circumpolar vortex has strengthened in the past 25-30 years, forming an even stronger barrier than usual. Tree ring records (Jones and Widman, 2004) suggest that the circumpolar vortex has shown similar strengthening in the past, so the current cooling trend in Antarctica may be partly a natural cycle.
Another possibility, favored by climate modelers,
is that the strengthening of the circumpolar vortex and recent cooling in Antarctica are primarily due to a combination of the recent increase in greenhouse gases and the opening of the Antarctic ozone hole. The ozone hole opened up at about the same time as the recent cooling began. Ozone absorbs UV radiation which heats the atmosphere around it, so the absence of ozone has led to cooling in the stratosphere over Antarctica. This cooling has been about 10° C in October-November since 1985 (Thompson and Solomon, 2002), and has acted to intensify the circumpolar vortex, leading to surface cooling. If the climate modelers are right, the circumpolar vortex will weaken as the ozone hole diminishes in coming decades. This will allow the Antarctic to begin warming with the rest of the globe.References and resources
Intergovernmental Panel on Climate Change (IPCC), 2007, The Physical Science Basis
Jones, J.M., and M. Widman, "Atmospheric science: Early peak in Antarctic oscillation index,"
Nature 432, 290-291 (18 November 2004) | doi:10.1038/432290b; Published online 17 November 2004.
Kwok, R., and J.C. Comiso, "Spatial patterns of variability in Antarctic surface temperature: Connections to the Southern Hemisphere Annular Mode and the Southern Oscillation", GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 14, 10.1029/2002GL015415, 2002.
Thompson, D.W.J., and S. Solomon, "Interpretation of Recent Southern Hemisphere Climate Change", Science
3 May 2002: Vol. 296. no. 5569, pp. 895 - 899 DOI: 10.1126/science.1069270.
Turner, J. et al.
, 2005, "Antarctic climate change during the last 50 years", International Journal of Climatology
, Volume 25, Issue 3, pp 279-294. Arctic sea ice"Antarctic cooling, global warming?"
RealClimate.org post, 3 December 2004.