Climate Change and the Oceans

Our Changing Climate


Ice Response


As the atmosphere and oceans warm, the rising temperatures bring about other changes in the climate system as well. Two key components in climate are sea ice and the ice sheets of Antarctica and Greenland. Sea ice acts as an insulating blanket between the ocean and atmosphere, limiting exchanges of heat and gases between them. Sea ice is also an excellent reflector: the more sea ice, the more solar radiation is reflected back into space and the less is absorbed by the ocean. The major ice sheets are important for a different reason. Ice sheets lock away fresh water that would otherwise be included in the oceans. During the last glacial maximum, about 20,000 years ago, sea level was about 130 m (430 feet) lower than today. But if the Greenland ice sheet all melted (a possible scenario in the next 1-3 centuries), sea level would rise an extra 20 feet.

There are signs that significant changes in sea ice are already underway, and that ice sheet thinning from Greenland is occurring, while fairly small changes in air temperature might trigger significant loss from the Antarctic ice sheet in the same time frame. We discuss some of these prospects below.


Decrease in Sea Ice

from Walsh and Chapman, 2000
 This figure shows the decrease in the Arctic Sea ice extent over the past century. Data for the first part of the record were taken from ship and aircraft observations. Satellites are the primary source for measurements from about 1970 on. The ice extent in September 2000 (not shown in this plot) is only about 5 x 106 km2, the smallest ever observed in this century (Dr. Norbert Untersteiner).

 

Satellite images show decrease in sea ice extent

NASA

2 August 2001 (summer), ice is retreating from both the Northern Sea Route and Northwest Passage. Both routes have historically been usable only with heavy icebreaker support.

NASA

Sea ice extent during early February 2001(winter) was less than at any time previously recorded. Green line shows typical winter ice extent. North Slope native accounts contain no similar instances, and native subsistence hunting and fishing suffered as a result of the loss of ice cover.



Bering Strait was effectively ice-free in February 2001.

Anonymous photo

This was the first time in recorded history that Bering Strait was ice-free, with open water continuous southward into the northern Bering Sea. Impacts on the local marine ecosystem, where larger predators depend upon the ice cover as a platform from which to hunt, were significant.

 

Top Global Climate News Story of 2002:
Major hunk of Antarctic ice shelf shatters and drifts away.

An entire ice shelf - the Larsen B Ice Shelf - on the eastern side of the Antarctic Peninsula, disintegrated over a one-month period in 2002. Reasons for the breakup are uncertain but are believed to include stresses due to wind waves generated offshore in the open waters of the western Weddell Sea, which is normally ice-covered throughout the Antarctic summer (NSIDC).

Ice shelves are the floating portion of the ice sheet, formed by thousands of years of snow-fall and subsequent compaction over the continent. When floating ice melts, it does not add to sea level: try melting an ice cube in a cup and see that the level does not change. However, ice shelves do have the potential to slow down or stop the flow of the ice sheet off the continent and into the ocean. Recent studies following the collapse of Larsen-B indicate that the glaciers previously behind the ice shelf have accelerated by a factor of 2-4 compared with their pre-collapse speeds. Larsen-B and the glaciers behind it are small, and so will not have much affect on global sea level. But if larger ice shelves disappear, sea level might rise dramatically, and also very rapidly.