Drift Ice

In the world’s polar regions, ice that floats on the water’s surface is termed drift ice. Drift ice is named for its tendency to be carried by wind and currents. The Arctic and Antarctic ice packs (drift ice that is forced into a single mass) account for the majority of the Earth’s drift ice. Research has shown that increased water and air temperatures are causing this ice to melt. The extent of this ice depletion by climate change is unknown, and the long-term impact on ice-dependent species is not clear.

Drift ice plays a critical role in both climate and ecosystem habitats. The ice originates from the freezing of seawater and varies greatly in size. The extent of regions that are covered in drift ice (sea ice) during the past 30 years has significantly decreased. Current climate models predict a continual decrease in sea ice in the foreseeable future, which will likely have a significant impact on the Earth’s albedo, or its potential reflectivity of the sun’s energy. With decreasing ice and, more importantly, snow-covered ice, the albedo of the Earth is decreasing, whereby the amount of sun energy that is absorbed at the surface is increasing. Research has shown that decreased albedo will increase average water and air temperatures, similar to the greenhouse effect discussed in climate change models.

Annual and seasonal variations in any ice formations are well known. The annual fluctuation in Arctic ice ranges from 2 to 5 million sq. mi. (7 to 15 million sq. km) from the end of summer melt to its peak at the end of winter, respectively. The sea and drift ice area surrounding the Antarctic continent, similarly, ranges from 1 to 6 million sq. mi. (3 to 18 million sq. km) during the same period. Seasonal ice can range in thickness from 3 to 6 ft. (1 to 2 m) compared to the typically much thicker “permanent” ice that does not melt during the summer.

Impact on Species, Livelihoods, and Oceans 

Drift ice is a key contributor to local and regional food webs. Changes in ice densities impact microorganisms that are dependent on the ice for nutrients and shelter. Ice algae are the primary producers in ice-associated food webs. As primary producers, they are critical to the survival of all higher-level organisms. Ice habitats are also critical for the juvenile life stages of many microorganisms, such as zooplankton, that are dependent on these algal and bacterial populations as food. Fish seek shelter under the drift ice and feast on the abundant zooplankton. Some of the polar regions’ most sensitive macrofauna, including birds, seals, penguins, and polar bears, are dependent on this floating ice as platforms for rest, hunting, and a source of food.

Indigenous peoples of the Arctic are dependent on drift ice and sea ice for subsistence hunting of whales, seals, and fish. Decreases in and thinning of drift ice because of the climate-warming trend reduce the annual window that these groups have available for hunting. In addition, the populations of macrofauna are declining because of the limited sea ice. Those that survive spend less time in areas that are readily accessible to hunters, concomitantly decreasing the opportunity for a successful hunt that would feed indigenous families. These decreases in drift ice may prove to be beneficial to some indigenous peoples, however, with the opening of channels for new sea routes and future economic benefits with increased navigation and potential for trade. In either case, the lifestyles of indigenous peoples will be altered.

From a global climate perspective, melting of drift ice is likely to have significant impacts on ocean circulation patterns. Ocean circulation is driven by density differences of the water (thermohaline circulation). As ocean surface waters freeze, salt from that water is transferred to the surrounding waters, increasing its salinity (drift ice has very low salt content). The drift ice is then moved by winds and currents to far-reaching locales, contributing freshwater ice melt to these new areas.

These fluxes in ocean salinity will have far-reaching impacts on food webs at and well away from the polar regions, as nearly all organisms have a low salinity range tolerance. Salinity variation is not the only effect of increasing ice melt. If the oceans change from being primarily ice-covered to having more open waters, the productivity (phytoplankton) of the affected regions will increase dramatically, altering the oxygen and nutrients available for other organisms and shifting the food-web structure (i.e., ice algae would disappear). Regional macrofauna may be forced to move to new regions, as drift ice is no longer present, or their populations may simply become extinct.

Research has found that the rate at which drift ice is moving is also increasing. Some suggest significant increases in the movement of ice out of the Arctic regions, up to a 70 percent increase annually, likely because of the thinning of the ice (decreased overall ice mass). Recent data analyses suggest similar trends in the Antarctic, with a net ice export of about 11,500 sq. mi. (30,000 sq. km) per year from the Ross Sea alone. Tracking changes in drift ice has become an important focus of oceanic research in recent years, including new modeling techniques. Ice forecasting systems, such as those employed by the Naval Research Laboratory (NRL), are used to study changes in ice thickness, movement, and stability. New highresolution systems are = coupled with the forecast system already in place to better track changes in drift ice. These technologies will allow scientists to better predict the long-term effects that changes in drift ice will have, both regionally and globally.