3.2.2.4 Land and Sea Combined Temperature: Global, Northern Hemisphere, Southern Hemisphere and Zonal Means

Gridded data sets combining land-surface air temperature and SST anomalies have been developed and maintained by three groups: CRU with the UKMO Hadley Centre in the UK (HadCRUT3; Brohan et al., 2006) and NCDC (Smith and Reynolds, 2005) and GISS (Hansen et al., 2001) in the USA. Although the component data sets differ slightly (see Sections 3.2.2.1 and 3.2.2.3) and the combination methods also differ, trends are similar. Table 3.3 provides comparative estimates of linear trends. Overall warming since 1901 has been a little less in the NCDC and GISS analysis than in the HadCRUT3 analysis. All series indicate that the warmest five years have occurred after 1997, although there is slight disagreement about the ordering. The HadCRUT3 data set shows 1998 as warmest, while 2005 is warmest in NCDC and GISS data. Thus the year 2005, with no El Niño, was about as warm globally as 1998 with its major El Niño effects. The GISS analysis of 2005 interpolated the exceptionally warm conditions in the extreme north of Eurasia and North America over the Arctic Ocean (see Figure 3.5). If the GISS data for 2005 are averaged only south of 75°N, then 2005 is cooler than 1998. In addition, there were relatively cool anomalies in 2005 in HadCRUT3 in parts of Antarctica and the Southern Ocean, where sea ice coverage (see Chapter 4) has not declined.

Table 3.3. Linear trends (°C per decade) in hemispheric and global combined land-surface air temperatures and SST. Annual averages, along with estimates of uncertainties for CRU/UKMO (HadCRUT3), were used to estimate trends. For CRU/UKMO, global annual averages are the simple average of the two hemispheres. For NCDC and GISS the hemispheres are weighted as in Section 3.2.2.1. Trends are estimated and presented as in Table 3.2. R2 is the squared trend correlation (%). The Durbin Watson D-statistic (not shown) for the residuals, after allowing for first-order serial correlation, never indicated significant positive serial correlation, and plots of the residuals showed virtually no long-range persistence.

Hemispheric and global series based on Brohan et al. (2006) are shown in Figure 3.6 and tropical and polar series in Figure 3.7. Owing to the sparsity of SST data, the polar series are for land only. The recent warming is strongest in the NH extratropics, while El Niño events are clearly evident in the tropics, particularly the 1997–1998 event that makes 1998 the warmest year in HadCRUT3. The warming over land in the Arctic north of 65°N (Figure 3.7) is more than double the warming in the global mean from the 19th century to the 21st century and also from about the late 1960s to the present. In the arctic series, 2005 is the warmest year. A slightly longer warm period, almost as warm as the present, was observed from the late 1920s to the early 1950s. Although data coverage was limited in the first half of the 20th century, the spatial pattern of the earlier warm period appears to have been different from that of the current warmth. In particular, the current warmth is partly linked to the Northern Annular Mode (NAM; see Section 3.6.4) and affects a broader region (Polyakov et al., 2003). Temperatures over mainland Antarctica (south of 65°S) have not warmed in recent decades (Turner et al., 2005), but it is virtually certain that there has been strong warming over the last 50 years in the Antarctic Peninsula region (Turner et al., 2005; see the discussion of changes in the Southern Annular Mode (SAM) and Figure 3.32 in Section 3.6.5).

Figure 3.6. Global and hemispheric annual combined land-surface air temperature and SST anomalies (°C) (red) for 1850 to 2006 relative to the 1961 to 1990 mean, along with 5 to 95% error bar ranges, from HadCRUT3 (adapted from Brohan et al., 2006). The smooth blue curves show decadal variations (see Appendix 3.A).

Figure 3.7. Annual temperature anomalies (°C) up to 2005, relative to the 1961 to 1990 mean (red) with 5 to 95% error bars. The tropical series (middle) is combined land-surface air temperature and SST from HADCRUT3 (adapted from Brohan et al., 2006). The polar series (top and bottom) are land-only from CRUTEM3, because SST data are sparse and unreliable in sea ice zones. The smooth blue curves show decadal variations (see Appendix 3.A).