Global-average surface temperature records show two principal periods of warming during the twentieth century. The warming following 1970 has occurred widely, yet with regions of concentration in northern Asia and northwestern North America. Anthropogenic forcing is widely suspected to be a contributing cause (Intergovernmental Panel on Climate Change, 2001b). The warming earlier in the century is less likely to be of anthropogenic origin, although this is an unsettled issue; solar and volcanic forcing may have played a role (Delworth and Knutson, 2000; Stott et al., 2000; Hegerl et al., 2000).
Global temperature has evolved very differently, in space, during the two warmings (Figure 2.7). The earlier twentieth-century episode was concentrated in the far north. The warming seems to have appeared first in the Barents Sea; for example, in the records from Turuhansk, a city on the Yenisey River in northern Siberia. Sparse temperature records are supplemented by observations of a strong increase in ocean salinity and decrease in sea ice cover in the Nordic Seas (Kelley et al., 1982, 1987). Good records from Danish stations on the west Greenland coast show the strongest and most abrupt arrival of warming at Upernavik, where a 4°C rise in surface