Scientific Symposium Presentation Abstracts
September 19
Dr. Claire L. Parkinson: Changes in Polar Sea Ice Coverage
Sea ice is a critical element in the climate of the polar regions, reflecting back toward space much of the solar radiation incident on it, providing insulation between the ocean and the atmosphere, and having multiple additional impacts on polar climate and ecosystems. Its distribution is well determined from satellite observations, which have provided high quality monitoring of the polar sea ice covers since the late 1970s. These records have revealed considerable interannual variability in both polar regions, but they also reveal a strong overall decline in Arctic sea ice coverage that has been countered in part by a much weaker increase in Antarctic sea ice coverage.
In the Arctic, sea ice expands each year from a minimum extent of approximately 6 million square kilometers (about 2.3 million square miles) in September to a maximum extent of approximately 15 million square kilometers (about 5.8 million square miles) in March. However, overall, these numbers have been decreasing in the past several decades. The satellite record indicates annually averaged decreases at a rate of approximately 3% per decade since the late 1970s, with the decreases being particularly strong, at a rate of approximately 8% per decade, in September, the month of minimum ice coverage. As the ice extent lessens, more ice-free ocean is exposed to the atmosphere, leading to increased absorption of solar radiation and hence providing a positive feedback to the warming that likely precipitated the ice reductions. In addition to climate impacts, the ice decreases are affecting polar ecosystems, with the impacts on polar bears in particular raising widespread interest and concern. Polar bears depend on the platform provided by the ice as they hunt for seals and other marine life. In fact, the ice is so important to the bears that questions have been raised regarding whether the polar bear species can survive if the Arctic sea ice decreases continue to the point of having an ice-free late-summer Arctic Ocean, a scenario projected by some as plausible by the end of the twenty-first century.
In the Antarctic, the sea ice trends since the late 1970s have been quite different from those in the Arctic. Antarctic sea ice has a considerably larger seasonal cycle, expanding from a minimum of approximately 3.5 million square kilometers (about 1.35 million square miles) in February to a maximum of approximately 18 million square kilometers (about 7 million square miles) in September. On an annually averaged basis, the Antarctic ice extents have, overall, increased rather than decreased since the late 1970s, at an average rate of approximately 1% per decade. In the Antarctic, the trends have a strong regional dependence, with sea ice increases in the Ross Sea and around much of East Antarctica, but sea ice decreases for a wide expanse to the west of the Antarctic Peninsula and a smaller expanse to the east of the Peninsula. This pattern is consistent with temperature records, as the Antarctic Peninsula, like the Arctic, shows temperature increases exceeding the global average, whereas much of the rest of the Southern Ocean region does not yet show a strong warming signal.
In the case of sea ice, there is no known “tipping point” beyond which the changes will necessarily continue in the same direction. Paleoclimate data over millions of years indicate that the sea ice covers in both polar regions can decrease to amounts far less than today’s or increase to amounts far greater than today’s, and can rebound from either of the more extreme conditions. However, on time scales of years to decades, as sea ice decreases, the replacement of a highly reflective ice surface by a strongly absorbing ocean surface does encourage absorption of solar radiation and thereby heating of the ocean and further reduction of sea ice coverage. In the Arctic, the sea ice reductions so far have already disrupted some animal life and are bound to be more disruptive if they continue. Although less sea ice has some benefits, such as greater ease of shipping in the polar regions, even the perceived benefits could prove more harmful than advantageous, as they could lead directly to increased pollution, increased disruption of native wildlife, and increased political tensions.
