Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeBox 4.1. Figure 1. Arctic September sea ice extent (× 10 6 km 2 ) from observations (thick red line) and13 IPCC AR4 climate models, together with the multi-model ensemble mean (solid black line) and standarddeviation (dotted black line). Models with more than one ensemble member are indicated with an asterisk.From Stroeve et al., 2007 (updated to include 2008).once the tank was completely filled with coldwater. In addition there developed an extremelyshallow overturning circulation in the topmostfew centimeters, with warm water flowingtoward the cooling device directly at the surfaceand cooler waters flowing backwards directlyunderneath. This pattern persisted, but a deep,top-to-bottom overturning circulation did notexist in the equilibrium state.However, when Sandström (1908) put the heatsource at depth, then such a deep overturningcirculation developed and persisted. Sandströminferred that a heat source at depth is necessaryto drive a deep overturning circulation in anequilibrium state. Sources and sinks of heatapplied at the surface only can drive vigorousconvective overturning for a certain time, butnot a steady-state circulation. The tank experimentshave been debated and challenged eversince (recently reviewed by Kuhlbrodt et al.,2007), but what Sandström inferred for theoverturning circulation observed in the oceanremains true. Thus, if we want to understandthe AMOC in a thermodynamical way, we needto determine how heat reaches the deep ocean.One potential heat source at depth is geothermalheating through the ocean bottom. While itseems to have a stabilizing effect on the AMOC(Adcroft et al., 2001), its strength of 0.05 Terawatt(TW, 1 TW = 10 12 W) is too small to drivethe circulation as a whole. Having ruled thisout, the only other heat source comes from the123