Change is happening most rapidly, and can be seen most vividly, in the Arctic. The Arctic Ocean is the least understood of all the world’s oceans, but we know it is warming at approximately twice the rate of the rest of the oceans. That is causing the rapid retreat of Arctic sea ice. In September 2007, the minimum ice extent at the end of summer was 23 percent lower than what it had been in 2005, the previous record low, and 50 per- cent lower than was typical in the 1950s through the 1970s.26 Scientists from the National Snow and Ice Data Center and the National Center for Atmospheric Research have found that Arctic sea ice is melting even faster than models have projected, giving rise to predictions that the Arctic might be seasonally ice free as soon as 2013, and possibly earlier. Such rapid change will lead to the local loss—or, in some cases, complete extinction—of certain Arctic species. Ice-associated marine algae and amphipods provide the base of the unique food web that includes a rich variety of invertebrates, fish, and birds. Ice-dependent ocean mammals, such as bowhead whales, narwhals, polar bears, ringed seals, and walruses, will also be directly affected by loss of habitat. The changes in the extent of Arctic sea ice will also have profound consequences for the world’s climate, increasing the retention of solar heat and reducing the vital temperature gradient between the warmer tropics and colder polar regions, thus altering ocean currents and weather patterns throughout the Northern Hemisphere.