air temperature occurred between 1920 and 1930; much of the warming of the twentieth century at this site took place during that decade (Figure 2.8, Durre, 2001).
Effects of those sudden changes on oceanic ecosystems were widespread (Intergovernmental Panel on Climate Change, 2001a, Ch. 13, 16; Dickson, 2001). Northward dislocations of plankton, fish, mammals, and birds were extreme, including the range of economically important species, such as herring (Figure 2.9) and cod. In the case of cod, the warm ocean greatly extended their range, as larvae were transported north by the Irminger Current from the waters around Iceland. The northern catch rose from nothing to 300,000 tons per year in the 1950s and 1960s, then declined abruptly when the surface cold water reappeared in the late 1960s. Earlier successful cod fisheries were seen near Greenland in the 1820s and 1840s, but none had occurred in the wide interval between then and 1920. Such effects were also widespread in the Nordic and Barents Seas.
Causes of the abrupt yet long-lasting northern warming are not clear. The Arctic region as a whole seems to amplify climate variability, as we are witnessing now. Relationship with the AO/NAO mode is not clear; the indexes of the mode were strongly negative in 1910-1920 and strongly positive in 1920-1930, indicating enhanced cyclone activity in the subpolar Atlantic through the 1920s with southerly winds at the entrance to the Nordic Seas. Climate-model simulations (Delworth and Mann, 2000) suggested that the warming could have occurred with strong natural variability, through an increase in oceanic meridional overturning circulation, which brought warm, saline surface waters into the sub-Arctic and Arctic. In that