Arctic Darkening and Pack–Ice Melting

Introduction

The Arctic is the area surrounding the North Pole. Unlike the South Pole, which is covered by land that is surrounded by water, the North is covered by water that is surrounded by land (Scandinavia, Greenland, Russia, and northern North America). This gives the Arctic very different properties from the Antarctic, including greater susceptibility to the effects of global climate change. Although Greenland—a large island fairly near the North Pole—is covered with an ice sheet several miles thick, this contains only about a tenth as much ice as the Antarctic ice sheet. The ice covering the pole itself is a skin of floating sea-ice only a few meters thick. During winter the area of floating ice widens and thickens, while during the summer it shrinks and thins. In recent years, the Arctic sea-ice sheet has shrunk in summer more than at any earlier time in recorded history. Scientists point to human-caused global climate change as the cause of this retreat and warn that Arctic ice-melting can in turn accelerate global warming. By 2040, perhaps sooner, the Arctic Ocean may be completely ice-free in summer. This may speed climate change in the Northern Hemisphere.

Historical Background and Scientific Foundations

The Arctic is kept cold partly by the whiteness of its ice cap and of the snow that covers much of the land in the northern parts of Asia, Europe, and North America during the winter. Light-colored surfaces reflect more light, so when more of the Arctic is covered with snow and ice, it is less warmed by the sun. However, small, dark particles of air pollution variously called black carbon or soot, which are emitted by diesel engines, coal-burning power plants, and the inefficient burning of wood, dung, and other biomass, absorb sunlight both while floating in the air and after settling to the surface. Ice-core records of Arctic soot pollution from 1788 to 2002 show higher soot concentrations from 1850 to 1951 and somewhat lower levels since that time (though not as low as pre-1850 levels). Most of this soot has been traced to sources in eastern North America, though in recent years an increasing amount is from Asia and Europe. Relatively lower soot levels since 1951 may reflect more efficient burning of fuels in North America.

Snowflakes efficiently gather soot particles from the air as they fall; studies in Japan and North America have found dozens to thousands of solid particles in each snowflake. Even in very small concentrations (a few parts per billion), these dark particles can significantly increase the rate at which snow and ice absorb solar energy. In 2004, researchers showed that soot was increasing the albedo or brightness of snow and ice in the Arctic sea ice by 1.5% and that of snow-covered land in the Northern Hemisphere by 3%. Moreover, each unit of solar energy absorbed at or near the surface by soot is twice as effective at raising surface air temperature (i.e., warming climate) as a unit of energy absorbed in Earth’s atmosphere by greenhouse gases such as carbon dioxide (CO2). Soot also increases climate warming while still floating in the air, absorbing sunlight. It does this most efficiently over snowy or ice regions such as the Arctic, where sooty haze layers in the atmosphere can absorb light reflected upward from snow and ice as well as light shining down from the sky.

Some climate scientists argue that soot particles may be the second-largest contributors to global warming, worldwide, after CO₂, causing accelerated melting not only of Arctic ice but of glaciers in the Himalayas. In the Arctic, soot may be causing as much global warming as greenhouse gases. Soot is also a direct threat to human health, killing over a million people each year worldwide by triggering lung cancer and other diseases.

The Arctic also produces strong feedbacks into climate warming. That is, there are several kinds of change that are pushed by climate warming and in turn cause the climate to warm. For example, trees absorb sunlight, decreasing albedo. As climate warms, snow melts sooner in spring and covers the ground later in the fall; this allows trees to grow in areas formerly covered by grassy tundra, darkening the land and increasing absorption, which enhances warming. Ocean waters exposed by melting sea-ice are much darker than ice and so absorb sunlight more efficiently, accelerating the climate warming that exposed them to begin with.

Impacts and Issues

In the Arctic, soot rivals greenhouse gases as a cause of climate warming. The Arctic has been experiencing global warming for the last several decades at about twice the world average. A recent effect of this warming has been the rapid retreat and thinning of Arctic summer sea ice, unprecedented in the period since historical observations began. A record for summer sea-pack shrinkage was set in 2005, then broken in 2007, as Arctic ice melted about twice as fast as climate scientists had predicted. By 2007, sea ice was retreating each summer about 40% more than the average retreat for 1979–2000. Even before the astonishing ice retreat of

WORDS TO KNOW

ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.

ICE CORE: A cylindrical section of ice removed from a glacier or ice sheet in order to study climate patterns of the past.

PACK ICE: Floating sea ice that has been driven together into a single large mass.

SOOT: A black, powdery, carbonaceous substance produced by the incomplete combustion of coal, oil, wood, or other fuels.

2007, scientists with the U.S. National Center for Atmospheric Research had predicted that Arctic summers could be essentially ice-free (except on Greenland, where the ice sheet is far too thick to disappear so rapidly) by about 2020.

In April 2008, 35 researchers with the U.S. National Oceanic and Atmospheric Administration (NOAA) began a concerted research effort in the Arctic named the Aerosol, Radiation, and Cloud Processes Affecting Arctic Climate Change Project. The scientists sought to

IN CONTEXT: NORTHWESTPASSAGE OPENED BYRECORD MELTING

The area of ocean covered by ice around the North Pole varies seasonally, reaching a minimum every September. In August and September 2007, the north polar sea-ice cap shrank to the smallest size ever recorded. As of September 16, U.S. government scientists announced, the ice was 1.59 million square miles (4.14 million square kilometers) in size, about a fifth smaller than the previous record, set in September 2005. The Northwest Passage, which is the sea route from the Atlantic to the Pacific along the northern edge of North America, was ice-free for the first time in recorded history. Although surprised and concerned by the extent of the 2007 melting, climate scientists predict increased melting of the ice due to global climate change. Thus far, Arctic temperatures have warmed twice as fast as the rest of the world.

understand why the Arctic was melting so much faster than climate models had predicted. They planned to make observations of carbon-particle haze, carbon parties on snow, altered cloud patterns, low-altitude ozone, and other variables. Despite a colder-then-average winter in 2007–2008, as of the spring of 2008 only about 30% of the Arctic ice pack consisted of ice more than a year old (which is generally thicker and harder to melt than one-year-old ice), as opposed to historical coverage of 50-60%. Ice older than six years was down from about 20% to a mere 6%. This meant that despite the cold winter, much of the Arctic Ocean’s ice cover was unusually thin and therefore vulnerable to rapid summer melting.

The Arctic ecosystem is being affected at many levels by climate warming and pack-ice melting. For example, polar bear populations may be greatly reduced by the loss of sea ice, which they depend on when hunting seals during the winter. Polar bears live only in the Arctic. In 2007, scientists of the U.S. Geological Survey forecasted that the world polar-bear population might decline by two-thirds by about 2050 as a result of climate warming.

Melting of Arctic sea ice does not raise sea levels; when ice floating in a body of water melts, the level of the water does not change. However, ice on land, such as the Greenland ice sheet or mountain glaciers, does raise sea levels when it melts and flows into the ocean.

See Also Air Pollution; Climate Change; Global Warming

BIBLIOGRAPHY

Periodicals

Alley, Richard B. “‘C’ing Arctic Climate with Black Ice.” Science 317 (2007): 1333–1334.

Hansen, James, and Larissa Nazarenko. “Soot Climate Forcing Via Snow and Ice Albedos.” Proceedings of the National Academy of Sciences 101 (2004): 423-428.

Law, Kathy S., et al. “Arctic Air Pollution: Origins and Impacts.” Science 315 (2007): 1537–1540.

Revkin, Andrew C. “Analysts See ‘Simply Incredible’ Shrinking of Floating Ice in the Arctic.” New York Times (August 10, 2007).

Revkin, Andrew C. “Scientists Report Severe Retreat of Arctic Ice.” New York Times (September 21, 2007).

Web Sites

University Corporation for Atmospheric Research. “Abrupt Ice Retreat Could Produce Ice-Free Arctic Summers by 2040.” http://www.ucar.edu/news/releases/2006/arctic.shtml (accessed April 8, 2008).

Larry Gilman