Against the odds Antarctic sea ice is growing– here’s why
Laura Nielsen for Frontier Scientists –
"On any given day, sea ice cover in the oceans of the polar regions is about the size of the U.S.," Thorsten Markus reminds us. He's a research scientist at NASA’s Goddard Space Flight Center. "Far-flung locations like the Arctic and Antarctic actually impact our temperature and climate where we live and work on a daily basis."
According to the National Snow and Ice Data Center, September Arctic sea ice extent (area) has declined by 13.7 percent per decade since 1979, while Antarctic sea ice extent has been increasing at 1.1 percent per decade. It seems like it doesn't add up. Even the frigid Antarctic South Pole is impacted by rising global atmospheric and oceanic temperatures. In fact, the South Ocean that surrounds Antarctica is warming faster than any other ocean.
Sea Ice versus Ice Sheets
To unravel this mystery, first remember that sea ice regularly forms at the poles on the ocean's surface, gaining area during the winter months and losing it during summer. We are not talking about the ice sheets that crouch atop the continent's stone and linger in Antarctica and Greenland year-round. Sea ice incorporates mostly ocean water and always displaces its own mass, so when it melts it does not raise global sea levels; in contrast, when ice sheets melt they release water that had been frozen for many hundreds of thousands of years into the ocean, and raise sea levels.
In the 1990s when our satellite technology could reliably measure ice sheet volume, the ice sheets as a whole were at a balance... not losing or gaining substantial amounts. Since then, global temperatures have warmed, and ice sheets started losing mass. The melt is accelerating in both Antarctica and Greenland ice sheets.
Below, we're focusing not on the continental mass of ice in Antarctica, but on the sea ice that grows on the ocean around it. A complex set of factors govern the growth of Antarctic sea ice.
Cyclone winds and the Ozone Hole
The ozone hole lets more ultraviolet light reach the surface of the planet. It also leaves our atmosphere less protected. Near the south pole, ozone depletion and the ozone hole cause the atmospheric layer called the stratosphere (6-37 miles [10 to 60 kilometers] above the surface) to cool. The more radical temperatures and related pressure mismatches between the atmospheric layers encourage stronger winds. Those colder, faster winds join the polar vortex, a region of powerful stormy cyclone winds that dominate Antarctica and the Southern Ocean during the winter. According to NASA: "Since 1980, the strength of the polar vortex has intensified by about 15 percent due to ozone depletion." The related storms and sustained freezing temperatures play havoc with Antarctic ice.
Jinlun Zhang, University of Washington oceanographer, used super-computer generated simulations to model the stronger winds. He found they could explain up to 80 percent of the increase in Antarctic sea ice volume in the last three decades. We have observed that the strong converging storm winds can drive sea ice. Some ice crashes together violently, causing deformation and ridging, making ice ridges that shelter new snowfall in their valleys and ultimately creating a thicker ice pack. Other winds at the edge of the storm push ice away from the South Pole, spreading out the ice pack's area. And whenever ice is moved it leaves behind patches of open water called polynyas that are vulnerable to the freezing winter temperatures that create yet more ice.
Ocean circulation and salinity
Ice behavior at the poles affects far-away climate because it impacts ocean circulation. Ocean water circles endlessly, a balance maintained by differing temperatures and salinity (salt content) variations in the ocean. It's called thermohaline circulation. Normally at the South Pole, convection causes deeper warmer water to rise to meet a layer of very cold surface water. The two layers mix.
However, warming global temperatures can cause the layers to become more stratified and less likely to mix. Warmer temperatures encourage precipitation– lots of snowfall. Melted snowfall and melting ice sheets inject lots of fresh water into the ocean. Fresh water is less dense than salt water, which means it doesn't easily mix with the deeper saltier and warmer ocean layer. Fresher colder water lingers on top, protecting the ice from melting due to circulating ocean heat.
More snowfall has other implications: deep snow cover accumulating on sea ice can get so heavy that it pushes the ice raft lower in the water. Waves creep over the side and inundate the snow, then the slush re-freezes. This snow-to-ice conversion forms ice with different isotope levels, and scientists have found it in sea ice all around Antarctica in every season.
Since satellite records began in 1979, we've seen Antarctic sea ice increasing at 1.1 percent per decade despite warming air and ocean temperatures. It's a good reminder that Earth's climate is complex, and we're still exploring how best to understand and model its variations.
The resilience of Antarctic sea ice is probably slowing some affects of anthropogenic (human-impacted) climate change. First, lingering sea ice is a good reflective material that bounces sunlight back into space. In contrast, the dark ocean waters exposed by lessened sea ice extent in the Arctic has low albedo (reflectiveness) and absorbs heat from the sun. Second, sea ice that crowds around Antarctica's ice sheets puts the "brakes" on glaciers that would otherwise be calving ice into the sea, raising sea levels.
Heightened sea levels are a risk to all of our fellow humans who live in low-lying areas. Please consider making a donation to the Red Cross to support relief efforts in areas devastated by Super Typhoon Haiyan.
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
- 'Arctic sea ice avoids last year’s record low; Antarctic sea ice edges out last year’s high' National Snow and Ice Data Center (2013)
- 'Is Antarctica losing or gaining ice?' Skeptical Science (2010)
- 'Modeling the impact of wind intensification on Antarctic sea ice volume' Jinlun Zhang, Journal of Climate (2013)
- 'Sea Ice May Be on Increase in the Antarctic: A Phenomenon Due to a Lot of 'Hot Air'?'
Gretchen Cook-Anderson, NASA Goddard Space Flight Center (2005)
- 'Stronger winds explain puzzling growth of sea ice in Antarctica' Hannah Hickey, University of Washington (2013)
- 'What's Holding Antarctic Sea Ice Back From Melting?' Adam Voiland, NASA Earth Science News Team (2009)