IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability

12.6.2 Thermohaline circulation changes in the North Atlantic: possible impacts for Europe

Earlier studies of the possible impacts of rapid change in Meridional Overturning Circulation (MOC), also known as the thermohaline circulation (THC), in the North Atlantic are now being updated (Vellinga and Wood 2002, 2006; Alley et al., 2003; Jacob et al., 2005; Rahmstorf and Ziekfeld, 2005; Stouffer et al., 2006; Schlesinger et al., 2007). Model simulations of an abrupt shut-down of the Atlantic MOC indicate that this is unlikely to occur before 2100 and that the impacts on European temperatures of any slowing in circulation before then are likely to be offset by the immediate effects of positive radiative forcings under increasing greenhouse gases (Arnell et al., 2005; Gregory et al., 2005; Vellinga and Wood, 2006; Meehl et al., 2007). Under slowing or full Atlantic MOC shut-down, temperatures on Europe’s western margin would be most affected, together with further rises in relative sea level on European coasts (Vellinga and Wood, 2002, 2006; Jacob et al., 2005; Levermann et al., 2005; Wood et al., 2006; Meehl et al., 2007). Although there are no indications of an imminent change in the North Atlantic THC (Dickson et al., 2003; Curry and Mauritzen, 2005) it is recognised that MOC shut-down, should it occur, is likely to have potential socio-economic impacts for Europe and more widely (Table 12.3). Hence, it would be valuable to consider these impacts in developing climate policy (Defra, 2004c; Keller et al., 2004; Arnell et al., 2005; Schneider et al., 2007). Such policies are currently difficult to quantify (Manning et al., 2004; Parry, 2004). Assessment of the likely impacts of an abrupt Atlantic MOC shut-down on different economic and social sectors in Europe has been made using integrated assessment models, e.g., FUND (Tol, 2002, 2006; Link and Tol, 2004). Results suggest that the repercussions for socio-economic factors are likely to be less severe than was previously thought.

Table 12.3. Main types of impact for Europe following a rapid shut-down of the Meridional Overturning Circulation relative to the ‘pre-industrial’ climate (after: Arnell et al., 2005; Levermann et al., 2005; Vellinga and Wood, 2006).

  • Reductions in runoff and water availability in southern Europe; major increase in snowmelt flooding in western Europe.
  • Increased sea-level rise on western European and Mediterranean coasts.
  • Reductions in crop production with consequent impacts on food prices.
  • Changes in temperature affecting ecosystems in western Europe and the Mediterranean (e.g., affecting biodiversity, forest products and food production).
  • Disruption to winter travel opportunities and increased icing of northern ports and seas.
  • Changes in regional patterns of increases versus decreases in cold- and heat-related deaths and ill-health.
  • Movement of populations to southern Europe and a shift in the centre of economic gravity.
  • Requirement to refurbish infrastructure towards Scandinavian standards.