3.2.2.5 Consistency between Land and Ocean Surface Temperature Changes

The course of temperature change over the 20th century, revealed by the independent analysis of land air temperatures, SST and NMAT, is generally consistent (Figure 3.8). Warming occurred in two distinct phases (1915–1945 and since 1975), and it has been substantially stronger over land than over the oceans in the later phase, as shown also by the trends in Table 3.2. The land component has also been more variable from year to year (compare Figures 3.1 and 3.4a,c,d). Much of the recent difference between global SST (and NMAT) and global land air temperature trends has arisen from accentuated warming over the continents in the mid-latitude NH (Section 3.2.2.7, Figures 3.9 and 3.10). This is likely to be related to greater evaporation and heat storage in the ocean, and in particular to atmospheric circulation changes in the winter half-year due to the North Atlantic Oscillation (NAO)/NAM (see discussion in Section 3.6.4). Accordingly the differences between NH and SH temperatures follow a course similar to the plot of land air temperature minus SST shown in Figure 3.8.

Figure 3.8. Annual anomalies (°C) of global average SST (blue curve, begins 1850), NMAT (green curve, begins 1856) and land-surface air temperature (red curve, begins 1850) to 2005, relative to their 1961 to 1990 means (Brohan et al., 2006; Rayner et al., 2006). The smooth curves show decadal variations (see Appendix 3.A). Inset shows the smoothed differences between the land-surface air temperature and SST anomalies (i.e., red minus blue).