Monday, April 1, 2013

Antarctic Paradox: ocean warming melting ice shelves causing sea ice expansion

Antarctic sea ice extent is a complex and puzzling beast, not following a simple process of rapid Arctic sea ice diminishment. A new paper in Nature Geoscience argues that warm deep ocean currents are contributing to ice shelf melt and causing Antarctic sea ice expansion. While ice mass loss from Antarctica is presently at 250 Gigatonnes per year and accelerating, sea ice is also expanding slowly but to record levels. What a paradox!

The trend for a gradual increase in Antarctic Sea ice has puzzled scientists for some time. The expansion is at odds with general climate model results. I reported on the puzzle of Record Antarctic Sea Ice Growth in November 2012. In that article I explained that (Holland et al 2012) argued that atmospheric variability in the form of local winds was pushing the ice northwards creating space for new sea ice to form. While climate deniers claimed record Antarctic sea-ice was a refutation of climate change, in reality it showed they could not fathom that the cryosphere and the planet's climate are complex processes which we are still trying to understand, with some surprising processees not readily open to be modelled.

The authors of this new paper (Bintanja et al 2013) from the Royal Netherlands Meteorological Institute in Utrecht argue that the freshly melted cool and fresh water forms a surface layer protecting sea ice and helping it's expansion insulating it from the deeper warm salty currents.

"The paradox is that global warming leads to more cooling and more sea ice around Antarctica," explained Richard Bintanja, a climate researcher at the Royal Netherlands Meteorological Institute in Utrecht in a Nature article accompanying the publication of the research study.

The study authors analysed satellite and buoy observations of ocean temperature and salinity for the period 1985-2010 around Antarctica. They compared this observational data with output from a global climate model which simulated the loss of 250 giggatonnes of ice mass each year and how it would affect surrounding ocean conditions. The model showed that meltwater would form a cool freshwater layer over the deeper warm water, effectively working as an insulation layer allowing sea ice to more rapidly form.

The researchers argue this is the primary trend influencing the growth of Antarctic sea ice. Wind variability and influence through the Southern Annular Mode (SAM) they argue, can only explain about one quarter of the observed Antarctic sea ice trend.

Paul Holland, an ocean modeller at the British Antarctic Survey and author of a previous paper on Antarctic sea ice formation commented in the Nature article, "The mechanism could be completely true, but this study does not demonstrate that increased melting has made a significant contribution to the increase in sea-ice cover,"

Bintanja et al also raise for discussion whether a similar process may exist around Greenland where the "proposed mechanism may be sufficiently strong to locally and seasonally offset the strong and accelerating Arctic warming and sea ice retreat, even though the area of influence is restricted to the waters directly adjacent to Greenland."

The authors assess that this process may impact estimates of sea level rise contribution from Antarctica. In my opinion I suspect any slow down in sea level rise contribution from this process will be relatively minor if at all.

More than likely both processes - wind currents and cold meltwater forming an insulating layer - may both play a part in Antarctic sea ice formation. It is an interesting puzzle which still isn't settled.

While the reasons for the slow growth of Antarctic sea-ice is interesting, it is not the primary process in play in Antarctica determining sea level rise. The authors readily accede to the findings of previous studies "indicating that the Antarctic ice sheet is losing mass by subsurface melt of its buttressing ice shelves (Yin et al., 2011; Joughlin and Alley, 2011; Pritchard et al., 2012), and that the mass loss is accelerating."

The study abstract for - Important role for ocean warming and increased ice-shelf melt in Antarctic sea-ice expansion:

"Changes in sea ice significantly modulate climate change because of its high reflective and strong insulating nature. In contrast to Arctic sea ice, sea ice surrounding Antarctica has expanded, with record extent in 2010. This ice expansion has previously been attributed to dynamical atmospheric changes that induce atmospheric cooling. Here we show that accelerated basal melting of Antarctic ice shelves is likely to have contributed significantly to sea-ice expansion. Specifically, we present observations indicating that melt water from Antarctica's ice shelves accumulates in a cool and fresh surface layer that shields the surface ocean from the warmer deeper waters that are melting the ice shelves. Simulating these processes in a coupled climate model we find that cool and fresh surface water from ice-shelf melt indeed leads to expanding sea ice in austral autumn and winter. This powerful negative feedback counteracts Southern Hemispheric atmospheric warming. Although changes in atmospheric dynamics most likely govern regional sea-ice trends, our analyses indicate that the overall sea-ice trend is dominated by increased ice-shelf melt. We suggest that cool sea surface temperatures around Antarctica could offset projected snowfall increases in Antarctica, with implications for estimates of future sea-level rise."

I have written a number of articles on ice mass loss from Antarctica and the threats posed from basal melting of ice sheets by warm ocean currents and the threat of destabilisation of the West Antarctic Ice Sheet (WAIS). See some of my related articles on Climate change and Antarctica: