19.3.5 Geophysical systems

A number of Earth-system changes may be classified as key impacts resulting in key vulnerabilities.

19.3.5.1. Global biogeochemical cycles

The sensitivity of the carbon cycle to increased CO₂ concentrations and climate change is a key vulnerability due to its magnitude, persistence, rate of change, low adaptive capacity and the level of confidence in resulting impacts. Models suggest that the overall effect of carbon–climate interactions is a positive feedback (Denman et al., 2007 Section 7.1.5). As CO₂ concentrations increase and climate changes, feedbacks from terrestrial stores of carbon in forests and grasslands, soils, wetlands, peatlands and permafrost, as well as from the ocean, would reduce net uptake of CO₂ (Denman et al., 2007 Sections 7.3.3 and 7.3.4). Hence the predicted atmospheric CO₂ concentration in 2100 is higher (and consequently the climate is warmer) than in models that do not include these couplings (Denman et al., 2007 Section 7.1.5). An intercomparison of ten climate models with a representation of the land and ocean carbon cycle forced by the SRES A2 emissions scenario (Denman et al., 2007 Section 7.3.5; Meehl et al., 2007 Section 10.4.1) shows that, by the end of the 21st century, additional CO₂ varies between 20 and 200 ppm for the two extreme models, with most of the models projecting additional CO₂ between 50 and 100 ppm (Friedlingstein et al., 2003), leading to an additional warming ranging between 0.1 and 1.5°C. A similar range results from estimating the effect including forcing from aerosols and non-CO₂ greenhouse gases (GHGs). Such additional warming would increase the number and severity of impacts associated with many key vulnerabilities identified in this chapter. In addition, these feedbacks reduce the emissions (Meehl et al., 2007 Section 10.4.1) compatible with a given atmospheric CO₂ stabilisation pathway (**)

At the regional level (see Chapters 4, 10, 11, 12 and 14), important aspects of the carbon–climate interaction include the role of fire (Denman et al., 2007 Section 7.3.3.1.4) in transient response and possible abrupt land-cover transitions from forest to grassland or grassland to semi-arid conditions (Claussen et al., 1999; Eastman et al., 2001; Cowling et al., 2004; Rial et al., 2004).

Warming destabilises permafrost and marine sediments of methane gas hydrates in some regions according to some model simulations (Denman et al., 2007 Section 7.4.1.2), as has been proposed as an explanation for the rapid warming that occurred during the Palaeocene/Eocene thermal maximum (Dickens, 2001; Archer and Buffett, 2005). A rising eustatic (global) contribution to sea level is estimated to stabilise hydrates to some degree. One study (Harvey and Huang, 1995) reports that methane releases may increase very long-term future temperature by 10-25% over a range of scenarios. Most studies also point to increased methane emissions from wetlands in a warmer, wetter climate (Denman et al., 2007 Section 7.4.1.2).

Increasing ocean acidity due to increasing atmospheric concentrations of CO₂ (Denman et al., 2007 Section 7.3.4.1; Sabine et al., 2004; Royal Society, 2005) is very likely to reduce biocalcification of marine organisms such as corals (Hughes et al., 2003; Feely et al., 2004). Though the limited number of studies available makes it difficult to assess confidence levels, potentially severe ecological changes would result from ocean acidification, especially for corals in tropical stably stratified waters, but also for cold water corals, and may influence the marine food chain from carbonate-based phytoplankton up to higher trophic levels (Denman et al., 2007 Section 7.3.4.1; Turley et al., 2006).