The Rapidly Changing Arctic and Its Impact on Global Climate
Arctic sea ice has significant seasonal variability. Prior to the 2000s, it retreated about 15% in summer and fully recovered in winter. However, by the year 2007, Arctic sea ice extent experienced a catastrophic decline to about 4.28×106 km2, which was 50% lower than conditions in the 1950s to the 1970s (Serreze et al., 2008). That was a record low over the course of the modern satellite record, since 1979 (note that the year 2012 became the new record low). This astonishing event drew wide-ranging attention in 2007-2009 during the 4th International Polar Year. The dramatic decline of sea ice attracts many scientists' interest and has become the focus of intense research since then. Currently, sea ice retreat is not only appearing around the marginal ice zone, but also in the pack ice inside the central Arctic (Zhao et al., 2018). In fact, premonitory signs had already been seen through other evidence. Before the disintegration of the Soviet Union, US naval submarines had been conducting an extensive survey under the sea ice and taking measurements of sea ice thickness. Their measurements revealed a gradual decrease of ice thickness to 1.8 m during winter by the end of the 20th century, in contrast to the climatological mean of 3.1m (Rothrock et al., 1999). However, this alarming result did not draw much attention since the Arctic was still severely cold at that time.