For sea ice, age matters

Most people picture the Arctic Ocean as miles upon miles of thick sea ice. This icy expanse has become threatened as Arctic sea ice shifts from mainly old ice to much younger, thinner ice. How does this shift impact the Arctic environment? And what is the connection between the average age of ice found in the Arctic and the overall sea ice decline?

What is the difference?

Photograph of pancake ice in the Arctic Ocean

Pancake ice indeed looks like pancakes floating on the chilly sea. While typically very thin, often no more than 4 inches thick, these ice floes can grow to a diameter of ten feet. When ice floes form on choppy water, they bump into one another, eroding them into a rounded pancake shape. These constant collisions also form raised rims and ridges at their edges. (Courtesy Mark Brandon)

Each autumn, large parts of the Arctic Ocean, and the far northern Atlantic Ocean and Bering Sea, freeze over with a thin sheet of ice, which glaciologists call first-year ice. Scientists base the names of some types of first-year ice on their appearance, which makes spotting and identifying ice much easier. For example, grease ice, as its name implies, looks like a thin layer of grease on top of the ocean, formed by a continuous sheet of wet, slushy ice crystals. It represents one of the first stages of ice formation.  Grease ice may transform into pancake ice, which has a round shape that ranges from 1 to 9 feet in diameter and up to 4 inches thick. Throughout winter, the ice thickens and may eventually become 1 to 3 feet thick. Once spring and summer hit, however, much of the first-year ice quickly melts.

If the first-year ice survives that first summer, it is promoted to multiyear ice. New ice can then grow on the bottom of the floes through the following winter. Multiyear ice is hence usually thicker than first-year ice, sometimes measuring 6 feet to as much as 25 feet thick. Sea ice often still has droplets of ocean water, or brine, encased in the frozen mass. Multiyear ice contains less brine than first-year ice, because the brine trapped within the first-year ice has escaped through tiny channels. Particularly thick multiyear ice occurs through collisions with other ice blocks, which cause ridging and rafting. The greater thickness of multiyear ice makes it more difficult for icebreakers to churn through. This ice also provides a source of reliable fresh water for hunters or other visitors to the Arctic Ocean. Scientists often melt multiyear ice for fresh drinking water when they are conducting research in the field.

The Arctic Ocean is a better environment than the Antarctic to create and maintain multiyear ice because it is relatively landlocked. In the Antarctic, ocean currents tend to sweep the ice into warmer waters over the course of a year, leaving few areas where ice can persist for the longer term.

Why sea ice age matters

Photograph of a researcher sipping water from a sea ice melt pond

Zachary Brown of Stanford University sips freshwater from a melt pond on sea ice in the Arctic Ocean. Multiyear ice grows much thicker than first-year ice, and meltponds spot its surface in the summer months. After a continual process of melting, brine draining, and reforming, multiyear ice provides freshwater to scientists, Arctic peoples, and animal species. (Courtesy NASA/Kathryn Hansen)

During the 1980s, multiyear ice comprised 50 to 60 percent of the ice in the Arctic Ocean. However, by the end of summer in 2010, only 15 percent of the remaining sea ice was more than two years old. Because first-year ice is thinner and more prone to melting than multiyear ice, the increase in the amount of first-year ice drastically alters the dynamics of sea ice in the Arctic. The thinner ice is more easily pushed around by winds, and fractured by waves. It moves with currents more rapidly and easily, and so the ice is flushed out of the Arctic faster than it was before.

Previously, when storms raged through the Arctic, multiyear ice could resist the turbulent ocean. First-year ice is more likely to be broken up and consumed by winds and waves. In August 2012, a very strong storm formed over the coast of northeastern Eurasia and quickly surged to the central Arctic Ocean. After several days of high winds, the storm broke up the ice pack, pushing new ice out into warmer ocean waters and contributing to the record low sea ice extent observed in September of that year.

In September 2012, the Arctic sea ice extent hit an all-time low at 1.32 million square miles. While the media popularly discusses the importance of the sea ice extent, scientists continue to study how the link between the extent and age of ice impacts this icy region. In the last decade, the decline in multiyear sea ice has accelerated, putting the amount of multiyear ice at 55 percent less than the average during the 1970s. Higher temperatures and storms easily destroy first-year ice, weaken the multiyear ice that remains, and perpetuate the cycle of decreasing sea ice extent. If this cycle continues and resilient multiyear ice disappears, scientists estimate that the world may see ice-free summers in the Arctic well within this century.

References

Comiso, J. C. 2012. Large decadal decline of the Arctic multiyear ice cover. Journal of Climate 25(4): 1,176-1,193.

Polyakov, I.V., J.E. Walsh, and R. Kwok. 2012. Recent changes of Arctic multiyear sea ice coverage and the likely causes. Bulletin of the American Meteorological Society 93: 145-151.

NSIDC’s All About Sea Ice site (http://nsidc.org/cryosphere/seaice/)

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