Is 'Missing Heat' in the Greenland Sea?

By Christa Marshall, E&E reporter
10/8/2013 10:05:33 AM

The deep waters of the Greenland Sea warmed much faster than the rest of the ocean in recent decades, a trend that raises new questions about sea-level rise and the underwater storage of heat, according to a new study.

The research found that the Greenland Sea's water temperature at deep depths -- between 6,561 feet and the seafloor -- increased 0.3 degree Celsius in the last 30 years.

That level of warming is about 10 times greater than the rest of the ocean and unprecedented in the Greenland Sea in the last century, the researchers said. Bordering Iceland and Greenland, the sea connects to the Arctic Ocean via the Fram Strait.

"Although the warming trends in the deep ocean sound quite small, they represent a large fraction of the heat accumulated in the Earth's climate system, and we should not assume that this increasing heat will stay there forever," said study lead author Raquel Somavilla Cabrillo, an oceanographer at the German-based Alfred Wegener Institute for Polar and Marine Research. The work was published in late September in Geophysical Research Letters.

The findings underscore the need for more research in polar regions, as there is a lack of knowledge about the extent and full effects of deep ocean changes there, Cabrillo said. It's not fully understood, for example, how the Greenland Sea's deep waters -- which now hold enough heat to warm Europe's atmosphere by 4 C -- will move and distribute their warmth, she added.

On March 18, 2011, the P-3 flew the IceBridge campaign's third sea ice flight over northwest Greenland at low altitude (1,500 feet) under conditions that varied from hazy ice fog to clear sunny skies. Open and refrozen leads in the sea ice are visible. Credit: NASA/ATM automatic Cambot system/

An underexamined heat sink

Deep Arctic waters "have been sparsely represented in global ocean heat content change estimations so far," the paper states.

Additionally, the heat is causing the area to play a disproportionate role in rising sea levels, largely through thermal expansion of water. The deep Greenland Sea covers roughly 0.05 percent of the global ocean total, but its heat is responsible for 0.1 percent of global sea-level rise between 1982 and 2009, according to the study.

The researchers assessed temperature and water salinity data in the Greenland Sea between 1950 and 2010, partially through annual measurements taken from 1993 onward via treks of the icebreaker Polarstern.

The scientists concluded that the temperature spike likely resulted from a unique combination of two warming effects. With the first, relatively warm water funneled into the Greenland Sea from the interior Arctic Ocean. There was a simultaneous slowdown of the transfer of cold, dense surface water at the surface to the Greenland Sea's bottom.

The slowdown occurred because of a decrease in winter heat losses from the ocean to the atmosphere, a phenomenon related to higher air temperatures in the Greenland Sea, Cabrillo explained.

That reduced cooling created a pool of lighter waters at the surface.

"The lighter (but still cold) waters at the surface were not dense enough to reach the bottom of the Greenland Sea," Cabrillo said.

At the same time, a conveyer belt of relatively warm water moved through the Arctic Ocean into the Greenland Sea. The warm water resulted from a multistep process, starting with the formation of sea ice at the edge of the Arctic Ocean. When that occurred, the ice lost salt, forming a saline layer underneath the ice.

Continuing puzzles of ocean storage

The salty water then sank because of its density, Cabrillo said. As it moved down the Arctic shelf, it eventually intermingled with warmer waters from the north Atlantic, picking up heat much in the way a rolling snowball picks up snow, she added. The warm water then moved on a loopy current into the interior Arctic and the Greenland Sea.

The paper notes an ongoing controversy about "missing heat" -- a term referring to the fact that temperature increases in the past decade were not as steep as expected, considering rises in greenhouse gas emissions.

The Greenland Sea typically has been overlooked as a storage spot for this missing heat and should get more attention with modeling, according to the study. A paper in April in Nature Climate Change pointed to ocean storage of heat as a possible explanation for why air temperatures did not seem to match rising greenhouse gas levels (ClimateWire, April 8).

Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research, said the study was a good paper but added that the data set was "sparse." Additional research verifying that the heat came from the loss of deep convection of the waters, as opposed to other factors, would solidify the findings, he said.

"One would like a bigger picture to make sure that the waters were not just slopping around," he added.

The study could help fill a significant knowledge gap with the missing heat over the last decade, considering that there still is not a full explanation or accounting for all of it, Trenberth said.

"The deep ocean ultimately has to warm to achieve a new equilibrium, and this may perhaps be regarded as part of that," he said.

Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500.

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