Two questions concerning biogeochemical aspects of the climate system are addressed here. First, can biogeochemical changes lead to abrupt climate change? Second, can abrupt changes in the MOC further affect radiative forcing through biogeochemical feedbacks?
Abrupt changes in biogeochemical systems of relevance to our capacity to simulate the climate of the 21st century are not well understood (Friedlingstein et al., 2003). The potential for major abrupt change exists in the uptake and storage of carbon by terrestrial systems. While abrupt change within the climate system is beginning to be seriously considered (Rial et al., 2004; Schneider, 2004), the potential for abrupt change in terrestrial systems, such as loss of soil carbon (Cox et al., 2000) or die back of the Amazon forests (Cox et al., 2004) remains uncertain. In part this is due to lack of understanding of processes (see Friedlingstein et al., 2003; Chapter 7) and in part it results from the impact of differences in the projected climate sensitivities in the host climate models (Joos et al., 2001; Govindasamy et al., 2005; Chapter 10) where changes in the physical climate system affect the biological response.
There is some evidence of multiple equilibria within vegetation-soil-climate systems. These include North Africa and Central East Asia where Claussen (1998), using an EMIC with a land vegetation component, showed two stable equilibria for rainfall, dependent on initial land surface conditions. Kleidon et al. (2000), Wang and Eltahir (2000) and Renssen et al. (2003) also found evidence for multiple equilibria. These are preliminary assessments using relatively simple physical climate models that highlight the possibility of irreversible change in the Earth system but require extensive further research to assess the reliability of the phenomena found.
There have only been a few preliminary studies of the impact of abrupt climate changes such as the shutdown of the MOC on the carbon cycle. The findings of these studies indicate that the shutdown of the MOC would tend to increase the amount of greenhouse gases in the atmosphere (Joos et al., 1999; Plattner et al., 2001; Chapter 6). In both of these studies, only the effect of the oceanic component of the carbon cycle changes was considered.