Antarctics

Climate Change Traps Deep Ocean Heat, Eliminating Ice-Free Antarctic Enclaves

Think Progress,

3 March, 2014

Climate change is not all just devastating droughts and scorching heat waves. Rather it’s a myriad of impacts playing out in the incalculably complex biosphere we know as Earth. For example, this year two polar vortices usurped most of the eastern U.S. during what was globally one of the warmest Januaries on record. In Antarctica, a new study has revealed another pocket of counterintuitive outcomes — in which climate change is causing the disappearance of ice-free regions, known as polynyas.

Researchers from the McGill University and University of Pennsylvania have discovered evidence that heat from the depths of the ocean is being trapped under the Antarctic ice shelf due to climate change. This in turn has effectively closed off a New Zealand-sized large, open body of water within the Antarctic’s Weddell Sea for the last four decades, according to the study.

“The fact that we can still have a surprise like this after studying the climate system for decades shows just how complex and dangerous climate change is,” study co-author Eric Galbraith told CTVNews.ca. “The deep ocean is like the basement of the climate system and without polynya there’s a trapped door to the basement.”

Published on March 2nd in the journal Nature Climate Change, the researchers found that deep ocean heat is trapped under a freshwater lid — making it unable to melt the winter Antarctic ice pack as it used to. Satellite images from the mid-1970s, the first taken of the Antarctic during the polar winter, showed the huge ice-free region enclosed within the Weddell Sea ice pack. The open area, or polynya, remained open for three full winters before closing, a lasting phenomenon that was due to warm water rising up from miles below the ocean’s surface. The polynya did not reappear in nearly 40 years, and came to be considered a rare event by scientists.

However, in this new study, analysis of measurements made across the region over the last six decades show that the ocean surface has been getting less salty since the mid-20th century, thus preventing the freshwater from mixing with warm waters beneath and instead freezing over.

“Deep ocean waters only mix directly to the surface in a few small regions of the global ocean, so this has effectively shut one of the main conduits for deep ocean heat to escape,” Casimir de Lavergne, a recent graduate of McGill’s Master’s program in Atmospheric and Oceanic Sciences and lead author of the paper, said in a statement from the university.

Jaime Palter, a professor in McGill’s Department of Atmospheric and Oceanic Sciences and co-author of the study, said the observations are in line with the principle that climate change will cause dryer regions to become dryer and wetter regions to get wetter. “True to form, the polar Southern Ocean — as a wet place — has indeed become wetter. And in response to the surface ocean freshening, the polynyas simulated by the models also disappeared.”

Another recent study on Antarctic sea ice also found drastic changes in distribution of ice and open water over the last 50 years, documenting a decrease of sea ice in the Bellingshausen-Amundsen sector, but an increase of sea ice in the Ross Sea sector of the continent. Over the last few years Antarctic sea ice has reached record levels, which one study found to be driven by stronger winds that drive ice faster and expose water to frigid winds causing more ice growth.

“The duration of ice-free days on the Ross Sea continental shelf has decreased by over two months over the past three decades,” write the study’s authors. However, “future projections of regional air temperature change suggest that substantial warming will occur in the next century in the Ross Sea sector. These changes are expected to reverse the sea-ice trends in the future; however the projected changes in heat content on the continental shelf and ecosystems dynamics that will occur as a result of such changes remain far from certain.”

According to their model, summer sea ice in the Ross Sea could decrease by more than half by 2050 and more than 75 percent by 2100. Funded by the National Science Foundation (NSF), the team of four researchers found that these ice-free periods affect the life cycles of both predators and prey. For instance, more humpback whales entering the Ross Sea in the summer could lead to over-predation of krill. “Regardless of the exact nature of the alterations,” the researchers write, “substantial portions of the food web that depend on ice in their life cycles will be negatively impacted, leading to severe ecological disruptions.”

In January, an international team of researchers found that Antarctica’s Pine Island Glacier, the single largest Antarctic contributor to sea-level rise, could add as much as one centimeter to ocean levels within the next 20 years.

“Ultimately, it’s apparent the relationship between ozone depletion, climate warming from greenhouse gases, natural variability, and how Antarctic ice responds is all very complicated,” wrote Jason Samenow, Chief Meteorologist at the Washington Post, about recent Antarctic conditions. “In sharp contrast, in the Arctic, there seems to be a relatively straight forward relationship between temperature and ice extent.”