Observations of Earth's climate over the past half-century reveal a global trend in warming temperatures of land surfaces, the lower atmosphere, and the oceans. The Arctic region is warming at a rate that is at least twice that of the global average. This is particularly evident in the central Arctic, two-thirds of which is the Arctic Ocean. In contrast to the Antarctic, which is dominated by a central continent and an enormous mass of ice with an average elevation of 7,500 feet, much of the Arctic is at sea level. The low elevation of the Arctic (and thus the greater sensitivity to surface warming), the effect of open water and bare land absorbing more solar-radiant energy than ice and snow, and the fact that water holds heat more effectively than land all contribute to the sharp difference in warming rates observed for Earth's two polar regions. Arguably no ocean is changing now or is likely to change in the near future as rapidly as the Arctic Ocean.
It is the extensive year-round coverage of sea ice that makes the Arctic Ocean distinct from the Atlantic and all other oceans. The Antarctic Ocean surface waters adjacent to Antarctica freeze and thaw annually, with very little sea ice surviving a summer melt season. The currents and high winds around Antarctica tend to facilitate dispersion and melting of Antarctic sea ice, while Arctic surface currents tend to retain ice from year to year in a central gyre.
The area of Arctic Ocean sea ice is shrinking at an alarming rate. As the Arctic cycles from a winter of total darkness to a summer with 24 hours of light, the extent of sea ice oscillates from a maximum in March to a minimum in September. From satellite sensor data, which were first available in 1979, we know that for nearly three decades, the summer minimum in sea-ice cover has varied annually, but on average it has shrunk about 1 percent per year. In 2007, however, the area of summer ice declined precipitously by an additional 20 percent. In 2008 a slight uptick in ice area occurred, but the 2008 September ice was thinner than the ice a year earlier, and total ice in the Arctic was the lowest yet recorded. The inventory of summer Arctic sea ice is now only about one-quarter of what it was in 1979. Some climate models project future ice-free summers in the central Arctic within a few decades.
What are the possible implications for the unique Arctic Ocean ecosystem? Without sea ice, the marine productivity of the Arctic would very likely increase but with different microalgae, different crustacea, and different fish. What about the well-known Arctic mammals that have evolved to depend on sea ice? Consider the consequences of a warmer Arctic for three ice-obligate marine mammals.
The ringed seal (Phoca hispida), by far the most abundant Arctic seal, feeds on fish and crustacea that live under the ice. This seal has evolved to forage deep within the central Arctic, long distances from open water. It is the only seal that thrives in completely ice-covered waters. Its stout front flippers and long claws are especially effective in creating and maintaining breathing holes, which can be seen all across the central Arctic. This seal creates a birthing den in the snow that drifts across one of its breathing holes, and in this lair, provides shelter for the pup until it is weaned. In warming regions of the Arctic there are increasing observations of prematurely collapsed dens and presumed higher pup mortality. Moreover, with ice-free summer conditions in the central Arctic, the area where this species can successfully forage will be dramatically reduced to parts of the Arctic archipelagos where winds and currents may still concentrate sea ice in summer.
The walrus (Odobenus rosmarus) feeds primarily on mollusks in bottom sediments at depths up to 260 feet. These gregarious mammals live in large herds, with clusters of dominant bulls and their harems. They spend two-thirds of their lives in water and have evolved to exploit the highly productive ice margin, where high rates of organic-matter deposition nourish the seabed community. As sea ice seasonally retreats from land surrounding the Arctic Ocean, the band of intense ice-edge productivity moves offshore as well, and walrus migrate with it as the ice retreats northward, consuming up to 5 percent of their body weight during spring and summer and using the ice to extend their coastal habitat by hauling-out onto ice floes for occasional rest. After feeding, walrus return to familiar coastal sites or haul-outs.
Walrus live up to 40 years. They have an unusually long, 15-month, gestation period, and calves nurse for up to six months. In a warming Arctic sea, ice retreats earlier in the season and soon is too far from continental-shelf feeding depths to be used effectively for rest after feeding, especially by young animals. During the summer of 2007, the Chukchi Sea, just north of the Bering Strait, was ice-free for 80 days. This resulted in unprecedented congestion in coastal haul-outs, with females and young coming ashore in summer, which is highly unusual. When humans or predators, such as polar bears, disturb walrus when they are on land, their natural response is to rush to safety in the sea. As many as 4,000 young walrus were trampled to death by stampeding adults during the summer of 2007. In recent years, walrus have also been observed to be in poor physical condition, and sightings of females separated from their young are increasingly common. For these and other reasons, in 2008 the Center for Biological Diversity (San Francisco) petitioned the Department of the Interior to list the walrus as a threatened or endangered species under the Endangered Species Act.
The polar bear (Ursus maritimus), more than any other species, has become the icon of a changing Arctic Ocean. It has evolved to have an absolute dependence upon sea ice for access to its preferred prey, the rotund ringed seal. This unusually specific diet is related to the female polar bear's need to acquire substantial lipid reserves prior to giving birth. After mating, a female will double or triple her weight before early autumn when she goes ashore to create a den in a snowdrift or snow bank. She will not feed again until spring, and her metabolism shifts to conservation mode. Absent food, placental nutrition is abbreviated, and two or three cubs weighing 2 pounds at birth are nourished on lipid-rich milk. The female does not eat, drink, defecate, or urinate but remarkably maintains her bone mass during the denning period of six or more months. She emerges from the den in spring with cubs weighing 20 pounds, and she begins her search for seal. Cubs will stay with their mother for two and a half years, but mortality is as high as 40 percent in the first year. Polar bears can live up to 30 years.
Already there are signs that pregnant female polar bears in some regions weigh less today than female polar bears did in 1980. These bears are excellent swimmers, covering distances of several miles per day. But if the ice retreats from shore before a female and her new cubs can make it from their den onto the ice, the female will be unable to catch seal to feed her young. In recent summers there have been more sightings of drowned bears far from land and ice. If by mid-century no continuous sea ice exists in the central Arctic Ocean, the habitat of the polar bear will probably be reduced like that of the ringed seal to areas in some of the Arctic archipelagos where winds and currents may still concentrate sea ice in summer.
These observations and others were used as evidence in support of the 2005 petition to the Department of the Interior by the Center for Biological Diversity (San Francisco) to list the polar bear as a threatened or endangered species under the Endangered Species Act. In May 2008, the U.S. Fish and Wildlife Service ruled, "upon the best available scientific and commercial information, that polar bear habitat -- principally sea ice -- is declining throughout the species' range, that this decline is expected to continue for the foreseeable future, and that this loss threatens the species throughout all of its range. Therefore, we find that the polar bear is likely to become an endangered species within the foreseeable future throughout all of its range."
All oceans are interconnected, and the oceans and the planetary atmosphere are tightly coupled components of Earth's climate. As climate changes due to natural and, increasingly, anthropogenic causes, ocean temperatures, ocean currents, the distributions of species and their productivity, even the organization of marine ecosystems can be expected to change. At the end of the last glacial maximum, 18,000 years ago, sea level was about 420 feet below today's level. The Laurentide ice sheet covered nearly half of North America, and Arctic sea ice extended well south of where it is today. The ranges of populations of terrestrial and marine organisms were correspondingly different from today's. Fossil evidence from the period of warming following the last glacial maximum tells us that nearly all of these species accommodated changes in sea level, extent of land and ocean ice, and general poleward extension of habitat ranges. After a stable period of several thousand years, habitats are again shifting with our warming climate. Now, however, the rate of change is faster by 10 to 100 times, and for Arctic marine species that absolutely require sea ice, critical habitat may disappear entirely.