6.6.2 Southern Hemisphere Temperature Variability
There are markedly fewer well-dated proxy records for the SH compared to the NH (Figure 6.11), and consequently little evidence of how large-scale average surface temperatures have changed over the past few thousand years. Mann and Jones (2003) used only three series to represent annual mean SH temperature change over the last 1.5 kyr. A weighted combination of the individual standardised series was scaled to match (at decadal time scales) the mean and the standard deviation of SH annual mean land and marine temperatures over the period 1856 to 1980. The recent proxy-based temperature estimates, up to the end of the reconstruction in 1980, do not capture the full magnitude of the warming seen in the instrumental temperature record. Earlier periods, around AD 700 and 1000, are reconstructed as warmer than the estimated level in the 20th century, and may have been as warm as the measured values in the last 20 years. The paucity of SH proxy data also means that uncertainties associated with hemispheric temperature estimates are much greater than for the NH, and it is more appropriate at this time to consider the evidence in terms of limited regional indicators of temperature change (Figure 6.12).
The long-term oscillations in warm-season temperatures shown in a tree ring reconstruction for Tasmania (Cook et al., 2000) suggest that the last 30 years was the warmest multi-decadal period in the last 1 kyr, but only by a marginal degree. Conditions were generally warm over a longer period from 1300 to 1500 (Figure 6.12). Another tree ring reconstruction, of austral summer temperatures based on data from South Island, New Zealand, spans the past 1.1 kyr and is the longest yet produced for the region (Cook et al., 2002a). Disturbance at the site from which the trees were sampled restricts the calibration of this record to the 70 years up until 1950, but both tree rings and instrumental data indicate that the 20th century was not anomalously warm when compared to several warm periods reconstructed in the last 1 kyr (around the mid-12th and early 13th centuries and around 1500).
Tree-ring based temperature reconstructions across the Southern Andes (37°S to 55°S) of South America indicate that the annual temperatures during the 20th century have been anomalously high in the context of the past four centuries. The mean annual temperatures for northern and southern Patagonia during the interval 1900 to 1990 are 0.53°C and 0.86°C above the 1640 to 1899 means, respectively (Figure 6.12). In Northern Patagonia, the highest temperatures occurred in the 1940s. In Southern Patagonia, the year 1998 was the warmest of the past four centuries. The rate of temperature increase from 1850 to 1920 was the highest over the past 360 years (Villalba et al., 2003).
Figure 6.12. Temperature reconstructions for regions in the SH: two annual temperature series from South American tree ring data (Villalba et al., 2003); annual temperature estimates from borehole inversions for southern Africa and Australia (Huang et al, 2000); summer temperature series from Tasmania and New Zealand tree ring data (Cook et al., 2000, 2002a). The black curves show summer or annual instrumental temperatures for each region. All tree ring and instrumental series were smoothed with a 25-year filter and represent anomalies (°C) from the 1961 to 1990 mean (indicated by the horizontal lines).
Figure 6.12 also shows the evidence of GST changes over the last 500 years, provided by regionally aggregated borehole temperature inversions (Figure 6.11) from southern Africa (92 records) and Australia (57 records) described in Huang et al. (2000). The instrumental records for these areas show warmer conditions that postdate the time when the boreholes were logged; thus, the most recent warming is not registered in these borehole curves. A more detailed analysis of the Australian geothermal reconstruction (Pollack et al., 2006), indicates that the warming of Australia in the past five centuries was apparently only half that experienced over the continents of the NH during the same period and shows good correspondence with the tree-ring based reconstructions for Tasmania and New Zealand (Cook et al., 2000, 2002a). Contrasting evidence of past temperature variations at Law Dome, Antarctica has been derived from ice core isotope measurements and from the inversion of a subsurface temperature profile (Dahl-Jensen et al., 1999; Goosse et al., 2004; Jones and Mann, 2004). The borehole analysis indicates colder intervals at around 1250 and 1850, followed by a gradual warming of 0.7°C to the present. The isotope record indicates a relatively cold 20th century and warmer conditions throughout the period 1000 to 1750.
Taken together, the very sparse evidence for SH temperatures prior to the period of instrumental records indicates that unusual warming is occurring in some regions. However, more proxy data are required to verify the apparent warm trend.