July-August, Volume 87, No. 4

Figure 9. When the ocean-current conveyor belt is in a stable warm state (1), such as exists today, it can experience relatively large changes in freshwater inflow (2) to the North Atlantic without undergoing significant change. Ocean circulation is strong, and considerable heat is transported northward from the tropics, warming Europe. As freshwater inflow increases, circulation remains relatively strong and heat transport significant until a threshold is crossed (3), causing circulation and transport to plummet, and Europe to cool by at least 2 to 5 degrees Celsius (4). Likewise, a large drop in freshwater inflow (5) is required to cause a return to the warmer state. (Adapted from work by Stefan Rahmstorf, Potsdam Institute for Climate Impact Research, Germany.)