1.4.2 Joint attribution

Joint attribution involves attribution of significant changes in a natural or managed system to regional temperature changes, and attribution of a significant fraction of the regional temperature change to human activities. This has been performed using studies with climate models to assess observed changes in several different physical and biological systems. An assessment of the relationship between significant observed changes from Section 1.3 and significant regional temperature changes is presented in Section 1.4.2.3.

1.4.2.1 Attributing regional temperature change

It is likely that there has been a substantial anthropogenic contribution to surface temperature increases averaged over each continent except Antarctica since the middle of the 20th century (Hegerl et al., 2007, Section 9.4.2). Statistically significant regional warming trends over the last 50 and 30 years are found in many regions of the globe (Spagnoli et al., 2002; Karoly and Wu, 2005; Karoly and Stott, 2006; Knutson et al., 2006; Zhang et al., 2006; Trenberth et al., 2007, Figure 3.9). These warming trends are consistent with the response to increasing greenhouse gases and sulphate aerosols and likely cannot be explained by natural internal climate variations or the response to changes in natural external forcing (solar irradiance and volcanoes).

Attributing temperature changes on smaller than continental scales and over time-scales of less than 20 years is difficult due to low signal-to-noise ratios at those scales. Attribution of the observed warming to anthropogenic forcing is easier at larger scales because averaging over larger regions reduces the natural variability more, making it easier to distinguish between changes expected from different external forcings, or between external forcing and climate variability.

The influence of anthropogenic forcing has also been detected in various physical systems over the last 50 years, including increases in global oceanic heat content, increases in sea level, shrinking of alpine glaciers, reductions in Arctic sea ice extent, and reductions in spring snow cover (Hegerl et al., 2007).