9.4 Understanding of Air Temperature Change During the Industrial Era

9.4.1 Global-Scale Surface Temperature Change

9.4.1.1 Observed Changes

Six additional years of observations since the TAR (Chapter 3) show that temperatures are continuing to warm near the surface of the planet. The annual global mean temperature for every year since the TAR has been among the 10 warmest years since the beginning of the instrumental record. The global mean temperature averaged over land and ocean surfaces warmed by 0.76°C ± 0.19°C between the first 50 years of the instrumental record (1850–1899) and the last 5 years (2001–2005) (Chapter 3; with a linear warming trend of 0.74°C ± 0.18°C over the last 100 years (1906–2005)). The rate of warming over the last 50 years is almost double that over the last 100 years (0.13°C ± 0.03°C vs 0.07°C ± 0.02°C per decade; Chapter 3). The larger number of proxy reconstructions from palaeodata than were available for the TAR indicate that it is very likely that average NH temperatures during the second half of the 20th century were warmer than any other 50-year period in the last 500 years and it is likely that this was the warmest period in the past 1.3 kyr (Chapter 6). Global mean temperature has not increased smoothly since 1900 as would be expected if it were influenced only by forcing from increasing greenhouse gas concentrations (i.e., if natural variability and other forcings did not have a role; see Section 9.2.1; Chapter 2). A rise in near-surface temperatures also occurred over several decades during the first half of the 20th century, followed by a period of more than three decades when temperatures showed no pronounced trend (Figure 3.6). Since the mid-1970s, land regions have warmed at a faster rate than oceans in both hemispheres (Figure 3.8) and warming over the SH was smaller than that over the NH during this period (Figure 3.6), while warming rates during the early 20th century were similar over land and ocean.