19.3.5.3 Possible changes in the North Atlantic meridional overturning circulation (MOC)
The sensitivity of the North Atlantic meridional overturning circulation (MOC) (cf., WGI AR4 Glossary; Bindoff et al., 2007 Box 5.1) to anthropogenic forcing is regarded as a key vulnerability due to the potential for sizeable and abrupt impacts (Tol, 1998; Keller et al., 2000; Mastrandrea and Schneider, 2001; Alley et al., 2003; Rahmstorf et al., 2003; Link and Tol, 2004, 2006; Higgins and Schneider, 2005; Sathaye et al., 2007).
Palaeo-analogues and model simulations show that the MOC can react abruptly and with a hysteresis response, once a certain forcing threshold is crossed (Randall et al., 2007; Meehl et al., 2007). Estimates of the forcing threshold that would trigger large-scale and persistent MOC changes rely on three main lines of evidence. The first, based on the analysis of coupled Atmosphere-Ocean General Circulation Models (AOGCMs), do not show MOC collapse in the 21st century (Meehl et al., 2007 Box 10.1). Assessing the confidence in this is, however, difficult, as these model runs sample only a subset of potentially relevant uncertainties (e.g., Challenor et al., 2006) and do not cross the forcing thresholds suggested by the second line of evidence: simulations using Earth system models of intermediate complexity (EMICs) (Randall et al., 2007 Section 8.8.3; Meehl et al., 2007 10.3.4). EMIC simulations, which use simplified representations of processes to explore a wider range of uncertainties, suggest that the probability that forcing would trigger an MOC threshold response during the 21st century could exceed estimates derived from AOGCM runs alone (e.g., Challenor et al., 2006). The third line of evidence, not assessed by Working Group I, relies on expert elicitations (sometimes combined with the analysis of simple climate models). These MOC projections show a large spread, with some suggesting a substantial likelihood of triggering a MOC threshold response within this century (Arnell et al., 2005; Rahmstorf and Zickfeld, 2005; McInerney and Keller, 2006; Schlesinger et al., 2006; Yohe et al., 2006).
Potential impacts associated with MOC changes include reduced warming or (in the case of abrupt change) absolute cooling of northern high-latitude areas near Greenland and north-western Europe, an increased warming of Southern Hemisphere high latitudes, tropical drying (Vellinga and Wood, 2002, 2006; Wood et al., 2003, 2006), as well as changes in marine ecosystem productivity (Schmittner, 2005), terrestrial vegetation (Higgins and Vellinga, 2004), oceanic CO2 uptake (Sarmiento and Le Quéré, 1996), oceanic oxygen concentrations (Matear and Hirst, 2003) and shifts in fisheries (Keller et al., 2000; Link and Tol, 2004). Adaptation to MOC-related impacts is very likely to be difficult if the impacts occur abruptly (e.g., on a decadal time-scale). Overall, there is high confidence in predictions of a MOC slowdown during the 21st century, but low confidence in the scale of climate change that would cause an abrupt transition or the associated impacts (Meehl et al., 2007 Section 10.3.4). However, there is high confidence that the likelihood of large-scale and persistent MOC responses increases with the extent and rate of anthropogenic forcing (e.g., Stocker and Schmittner, 1997; Stouffer and Manabe, 2003).
19.3.5.4 Changes in the modes of climate variability
Change in the modes of climate variability in response to anthropogenic forcing can lead to key impacts because these modes dominate annual-to-decadal variability, and adaptation to variability remains challenging in many regions. For example, some studies suggest that anthropogenic forcings would affect El Niño-Southern Oscillation (ENSO) variability (Timmermann et al., 1999; Fedorov and Philander, 2000; Fedorov et al., 2006; Hegerl et al., 2007 Section 9.5.3.1; Meehl et al., 2007 Section 10.3.5.3-5). Current ENSO projections are marked by many uncertainties, including
- the potential for an abrupt and/or hysteresis response,
- the direction of the shift,
- the level of warming when triggered.
ENSO shifts would affect agriculture (Cane et al., 1994; Legler et al., 1999), infectious diseases (Rodo et al., 2002), water supply, flooding, droughts (Kuhnel and Coates, 2000; Cole et al., 2002), wildfires (Swetnam and Betancourt, 1990), tropical cyclones (Pielke and Landsea, 1999; Emanuel, 2005), fisheries (Lehodey et al., 1997), carbon sinks (Bacastow et al., 1980) and the North Atlantic MOC (Latif et al., 2000).
The North Atlantic Oscillation (NAO) and the Annular Mode in both the Northern and Southern Hemispheres (also known as the Arctic Oscillation, AO, and the Antarctic Oscillation, AAO; Meehl et al., 2007 Section 10.3.5.6; Hartmann et al., 2000; Thompson and Wallace, 2000; Fyfe et al., 1999; Kushner et al., 2001; Cai et al., 2003; Gillett et al., 2003; Kuzmina et al., 2005) are likely to be affected by greenhouse forcing and ozone depletion. For example, the average of the IPCC WGI AR4 simulations from thirteen models shows a positive trend for the Northern Annular Mode that becomes statistically significant early in the 21st century (Meehl et al., 2007 Section 10.3.5.6). Such changes would affect surface pressure patterns, storm tracks and rainfall distributions in the mid and high latitudes of both hemispheres, with potentially serious impacts on regional water supplies, agriculture, wind speeds and extreme events. Implications are potentially severe for water resources and storminess in Australia, New Zealand, southern Africa, Argentina and Chile, southern Europe, and possibly parts of the USA where Mediterranean-type climates prevail.
Current forcing may have caused changes in these modes but observed changes are also similar to those simulated in AOGCMs in the absence of forcing (Cai et al., 2003). There is some evidence for a weakening of major tropical monsoon circulations (AR4 WGI 3.7.1, 9.5.3.5). Projections of monsoon precipitation show a complex pattern of increases (e.g., Australia in the southern summer and Asia), and decreases (e.g., the Sahel in the northern summer) (Meehl et al., 2007 Section 10.3.5.2). Confidence in projections of specific monsoonal changes is low to medium.