Arctic sea ice, the vast sheath of frozen seawater floating on the Arctic Ocean and its neighboring seas, has been hit with a double whammy over the past decades.
As its extent shrunk, the oldest and thickest ice has either thinned or melted away, leaving the sea ice cap more vulnerable to the warming ocean and atmosphere.
Researchers at NASA’s Goddard Space Flight Center suggest that this older, thicker ice is like the bulwark of sea ice: a warm summer will melt all the young, thin ice away but it can’t completely get rid of the older ice.
But this older ice is becoming weaker because there’s less of it and the remaining old ice is more broken up and thinner, so that bulwark is not as good as it used to be.
Direct measurements of sea ice thickness are incomplete across the Arctic, so scientists have developed estimates of sea ice age and tracked their evolution from 1984 to the present.
Now, a new NASA visualization of the age of Arctic sea ice shows how sea ice has been growing and shrinking, spinning, melting in place and drifting out of the Arctic for the past three decades.
In the early 2000s, scientists at the University of Colorado developed a way to monitor Arctic sea ice movement and the evolution of its age by using data from a variety of sources, but primarily satellite passive microwave instruments.
These instruments gauge brightness temperature: a measure of the microwave energy emitted by sea ice that is influenced by the ice’s temperature, salinity, surface texture and the layer of snow on top of the sea ice.
Each floe of sea ice has a characteristic brightness temperature, so the researchers developed an approach that would identify and track ice floes in successive passive microwave images as they moved across the Arctic.
The system also uses information from drifting buoys as well as weather data.
Ice in motion
Every year, sea ice forms in the winter and melts in the summer.
The sea ice that survives the melt season thickens with each passing year: newly formed ice grows to about 3 to 7 feet of thickness during its first year, while multi-year ice (sea ice that has survived several melt seasons) is about 10 to 13 feet thick.
The older and thicker ice is more resistant to melt and less likely to get pushed around by winds or broken up by waves or storms.
The motion of sea ice is not limited to its seasonal expansion and shrinkage. Except for coastal regions where sea ice is attached to the shore, the sea ice cap is in almost constant movement.
The primary driver of sea ice movement in the Arctic is wind, and there are two major features in the Arctic circulation.
One is the Beaufort Gyre, a clockwise ice circulation that makes ice spin like a wheel in the Beaufort Sea, north of Alaska. The other is the Transpolar Drift Stream, which transports ice from Siberia’s coast toward the Fram Strait east of Greenland, where the ice exits the Arctic basin and melts in the warmer waters of the Atlantic Ocean.
On a week-to-week basis, there are weather systems that come through, so the ice isn’t moving at a constant rate. Sometimes the Beaufort Gyre reverses or breaks down for a couple weeks or so. The Transpolar Drift Stream shifts in its direction, but the overall pattern is this one.
Then the spring melt starts and the ice shrinks back, disappearing from the peripheral seas.
The new animation shows two main bursts of thick ice loss.
The first one started in 1989 and lasted a few years due to a switch in the Arctic Oscillation, an atmospheric circulation pattern, which shrunk the Beaufort Gyre and enhanced the Transpolar Drift Stream, flushing more sea ice than usual out of the Arctic.
The second peak in ice loss started in the mid-2000s.
Unlike in the 1980s, it’s not so much as ice being flushed out -though that’s still going on too. What’s happening now more is that the old ice is melting within the Arctic Ocean during the summertime.
One of the reasons is that the multiyear ice used to be a pretty consolidated ice pack, and now relatively smaller chunks of old ice are interspersed with younger ice. These isolated floes of thicker ice are much easier to melt.
The Earth has lost most of the older ice. In the 1980s, multiyear ice made up 20% of the sea ice cover. Now it’s only about 3%.
The older ice was like the insurance policy of the Arctic sea ice pack. As we lose it, the likelihood for a largely ice-free summer in the Arctic increases.
News source: NASA.
Figure legend: This Knowridge.com image is credited to NASA’S Scientific Visualization Studio.