3.2.2.6 Temporal Variability of Global Temperatures and Recent Warming

The standard deviation of the HadCRUT3 annual average temperatures for the globe for 1850 to 2005 shown in Figure 3.6 is 0.24°C. The greatest difference between two consecutive years in the global average since 1901 is 0.29°C between 1976 and 1977, demonstrating the importance of the 0.75°C and 0.74°C temperature increases (the HadCRUT3 linear trend estimates for 1901 to 2005 and 1906 to 2005, respectively) in a centennial time-scale context. However, both trends are small compared with interannual variations at one location, and much smaller than day-to-day variations (Table 3.1).

The principal conclusion from the three global analyses is that the global average surface temperature trend has very likely been slightly more than 0.65°C ± 0.2°C over the period from 1901 to 2005 (Table 3.3), a warming greater than any since at least the 11th century (see Chapter 6). A HadCRUT3 linear trend over the 1906 to 2005 period yields a warming of 0.74°C ± 0.18°C, but this rate almost doubles for the last 50 years (0.64°C ± 0.13°C for 1956 to 2005; see FAQ 3.1). Clearly, the changes are not linear and can also be characterised as level prior to about 1915, a warming to about 1945, levelling out or even a slight decrease until the 1970s, and a fairly linear upward trend since then (Figure 3.6 and FAQ 3.1). Considered this way, the overall warming from the average of the first 50-year period (1850–1899) through 2001 to 2005 is 0.76°C ± 0.19°C. Clearly, the world’s surface temperature has continued to increase since the TAR and the trend when computed in the same way as in the TAR remains 0.6°C over the 20th century. In view of Section 3.2.2.2 and the dominance of the globe by ocean, the influence of urbanisation on these estimates is estimated to be very small. The last 12 complete years (1995–2006) now contain 11 of the 12 warmest years since 1850, the earliest year for which comparable records are available. Only 1996 is not in this list – replaced by 1990. 2002 to 2005 are the 3rd, 4th, 5th and 2nd warmest years in the series, with 1998 the warmest in HadCRUT3 but with 2005 and 1998 switching order in GISS and NCDC. The HadCRUT3 surface warming trend over 1979 to 2005 was more than 0.16°C per decade, that is, a total warming of 0.44°C ± 0.12°C (the error bars overlap those of NCDC and GISS). During 2001 to 2005, the global average temperature anomaly has been 0.44°C above the 1961–1990 average. The value for 2006 is close to the 2001 to 2005 average.