FIGURE 2-1 Global, Northern Hemisphere, and Northern Hemisphere extratropical land area annual temperature anomalies in degrees Celsius from the HadCRUT2v surface temperature dataset. SOURCE: Jones et al. (2001).

latitude continental climatic zones generally exhibit larger temperature swings on virtually all timescales than other regions of the globe. The Northern Hemisphere and global estimates exhibit less variability because of the additional influence of SSTs, which have less variability than land air temperatures from year to year, mainly due to the higher heat capacity of the ocean mixed layer compared to the land surface. The evolving pattern of fluctuations is similar in these three large-scale averages because (a) the Northern Hemisphere extratropical land area stations form a major part of the larger-scale averages and (b) the larger variations in the Northern Hemisphere extratropical land area record tend to dominate the smaller variations in the remaining regions. Since 1978, instruments on satellites have monitored the temperature of the deep atmospheric layer above the surface and, though regional differences occur, global average trends agree with the surface warming of +0.16°C per decade within ±0.04°C per decade (CCSP and SGCR 2006).

In addition to substantial year-to-year variability, the global instrumental temperature record shows the following low-frequency features: a slight decline from 1856 to 1910, a rise of ~0.4°C between 1910 and 1945, a leveling or slight decline between about 1945 and 1975, and a rise of ~0.5°C from 1975 to the present. The overall rise during the 20th century was about 0.6°C, with an additional 0.1°C reported since then. If the 150 years of relatively reliable instrumental data are divided into three 50-year segments—1856–1905 (I), 1906–1955 (II), and 1956–2005 (III)—the average

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