Thorsten Markus sees Earth’s sea ice thinning

Thorsten Markus: We should not underestimate the importance of sea ice to the global climate system.

Thorsten Markus, of NASA’s Goddard Space Flight Center, has been using a NASA satellite – called ICESat – to track the thickness of sea ice around Earth’s poles. He said the thick, multi-year ice is getting thinner.

Thorsten Markus: We are able, for the first time really, to measure the sea ice thickness at large scales. Previously, the only means, really, was to drill holes through the ice. Results from ICESat have shown that just since 2003, when ICESat was launched, the sea ice thickness has decreased by over two feet. The thick, multi-year ice has decreased by over 40 percent during that time.

He said sea ice plays a key role in Earth’s climate system.

Thorsten Markus: Sea ice is white and it reflects most of the sunlight back into space, in drastic contrast to the dark ocean. Therefore, changes in the sea ice cover have a significant effect on the global energy balance.

In other words, less sea ice could cause more warming. The satellite Markus uses – ICESat – was retired in early 2010 after 7 years collecting data in Earth orbit. ICESat-II will launch in 2015. Until then, Dr. Markus will use what’s called Ice Bridge – aircraft in flight over the polar regions – to measure and track changes. In addition to the sunlight it reflects, Dr. Markus said that as salt water freezes to form sea ice, at a large scale it impacts weather and climate.

Thorsten Markus: When sea ice forms, it releases most of its salt into the ocean. And the increased salinity makes the top water masses of the ocean denser, heavier, which then sink down into the deeper parts of the ocean, starting an enormous heat pump, the oceanic thermohaline circulation, which strongly affects our weather and our climate.

The sea ice that rides the polar ocean water measures only about six feet, or two meters thick. But it blankets large swaths of water.

Thorsten Markus: On any given day, the area of the ocean covered by sea ice is as large as the continental United States. So it’s a huge area that we’re talking about.

Dr. Markus explained how ICESat measured sea ice.

Thorsten Markus: We have been able to monitor the extant of the sea ice from satellite passive microwave data for over 30 years. The part that has always been missing is the total sea ice volume. ICESat send laser pulses from 600 kilometer altitude, which is about 400 miles, to the Earth, and measures the time it takes for the pulse to get back to the satellite after reflection of the surface, at very high precision. The precision of ICESat measurements is about two centimeters, which is quite an accomplishment considering the altitude and effect that the satellite is going around the entire Earth in about an hour and a half. With those precise measurements of elevations, we are able to determine the sea ice freeboard, or sea ice thickness from space.

Earth’s sea ice is nothing like a still lake, but rather, said Markus, a huge and dynamic river.

Thorsten Markus: Winds and ocean currents cause the ice to move about. In addition to local melting, much of the sea ice that’s leaving the Arctic by sea ice drift, especially East of Greenland, sea ice is leaving the Arctic by vast amounts, like a huge river. With more and more thin, first-year ice present in contrast to the thick multi-year ice, which has been reduced by 40 percent since 2003, the ice becomes more movable and we may expect more events, like in 2007, when winds pushed much of the ice out of the Arctic ocean and caused a record minimum.

Our thanks today to NASA’s ICESat Mission, improving our understanding of the effects of Earth’s changing climate.

May 3, 2010

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