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June 2012 Issue

Researchers Find Decline in Antarctic Bottom Water Volume
Scientists estimate that the volume of Antarctic bottom water, the cold dense water that drives global ocean currents, has declined as much as a 60 percent in the past four decades, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) announced in May.

Teams of U.S. and Australian researchers compared temperature and salinity samples, collected across 77 sites between Antarctica and Fremantle, Australia, during the Australian Antarctic Program’s 2012 Southern Ocean marine science voyage, to historical data dating back to 1970.

The new measurements, which have not yet been published, suggest the densest waters in the world’s oceans are gradually disappearing and being replaced by less dense waters. The ocean profiles also show that the dense water formed around Antarctica has become less saline since 1970.

“The amount of dense Antarctic bottom water has contracted each time we’ve measured it since the 1970s,” said Steve Rintoul of CSIRO and the Antarctic Climate and Ecosystems Cooperative Research Centre. “There is now only about 40 percent as much dense water present as observed in 1970.”

Changes in winds, sea ice, precipitation or melt of floating glacial ice around the edge of Antarctica may be responsible. The new data could show how the Southern Ocean and the Antarctic Continent affect global climate processes. The scientists also deployed nine ice-capable Argo floats, which will transmit temperature and salinity profiles every 10 days for five years.


UK Expedition to Focus on Risks Of Ocean Acidification to Corals
A team of scientists set off in May on a month-long research voyage to study how Scotland’s cold-water coral ecosystems function and how they could be affected by changes in sea temperature and ocean chemistry.

The “Changing Oceans” expedition will visit sites in U.K., Irish and international waters, including the shallow reefs off Mingulay, and the deeper reefs on Rockall Bank and the Logachev Mound. Researchers will use the Holland I Deepwater ROV, built by SMD Ltd. (Wallsend, England), to film ecosystems like cold-water coral reefs and deep-sea sponge grounds and examine if corals will continue to grow or adapt to temperature and pH changes due to increased carbon dioxide levels in the atmosphere.

The scientists will also conduct seabed experiments and collect samples, which will be transported to a lab for further study. The team will collect cores using gravity corers, an NIOZ box corer and an SMBA box corer.


Subglacial Basin Site of Potential Instability in Antarctic Ice Sheet
Using ice-penetrating radar instruments flown on aircraft, a team of scientists from the U.S. and U.K. have uncovered a 20,000-square-kilometer subglacial basin nearly the size of New Jersey beneath the West Antarctic Ice Sheet (WAIS) near the Weddell Sea. The location, shape and texture of the mile-deep basin suggest the region of the ice sheet is at a greater risk of collapse than previously thought, according to the team’s findings published in Nature Geoscience in May.

Team members at the University of Texas at Austin compared data about the newly discovered basin to data they previously collected from other parts of the WAIS that also appear highly vulnerable, including Pine Island Glacier and Thwaites Glacier.

The scientists found two features of the basin particularly worrisome. First, its edges slope down steeply. If the grounding line begins to retreat upstream, seawater will replace it and more ice will begin to float. The study authors predict that this positive feedback mechanism would sustain retreat of the ice sheet until eventually all of the ice filling the basin goes afloat. Additionally, the bed of the basin on which the ice rests is smooth. There are few big bumps, or “pinning points,” to hold back sliding ice.

“The only saving grace is that losing the ice over this new basin would only raise sea level by a small percentage of the several meters that would result if the entire WAIS destabilized,” said co-author Don Blankenship.

In a related paper published simultaneously in the journal Nature, computer models reveal that the Weddell Sea region may experience warmer ocean conditions at the end of the 21st century, which could provide the trigger for ice sheet change.


Groundwater Pumping Leads to Sea Rise, Cancels Dams’ Effect A new study calculates that by 2050, groundwater pumping will cause a global sea level rise of about 0.8 millimeters per year. In the coming decades, groundwater contributions to sea level rise are expected to become as significant as those of melting glaciers and ice caps outside of Greenland and the Antarctic, said authors of the study, which was published in Geophysical Research Letters in May.

Between 1970 and 1990, sea level rise caused by groundwater pumping was canceled out as people built dams, trapping water in reservoirs so the water wouldn’t empty into the sea, Yoshihide Wada, lead author of the study, said. His research shows that starting in the 1990s, this changed as populations started pumping more groundwater and building fewer dams.

The study authors calculate that by mid-century, the net effect of these additional factors is an additional 0.05 millimeters per year of annual sea level rise, on top of the contribution from groundwater pumping alone.

Assuming that where there is groundwater, people will find a way to extract it, the researchers estimated the impact of groundwater depletion since 1900 using data from individual countries on groundwater pumping, simulations of groundwater recharge and reconstructions of how water demand has changed.

They also compared and corrected those estimates with observations from sources like the GRACE (Gravity Recovery and Climate Experiment) satellite, which uses gravity measurements to determine variations in groundwater storage.



2013:  JAN | FEB | MARCH | APRIL | MAY | JUNE | JULY | AUG | SEPT | OCT | NOV | DEC
2012:  JAN | FEB | MARCH | APRIL | MAY | JUNE | JULY | AUG | SEPT | OCT | NOV | DEC

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