Figure 27. Thermohaline current. A. A schematic of the ocean circulation system, often called the Great Ocean Conveyor, that transports heat throughout the world oceans. Red arrows indicate warm surface currents. Blue arrows indicate deep cold currents (from Gagosian, 2002). B. New data shows that North Atlantic waters at depths between 1000 and 4000 meters are becoming dramatically less salty, especially in the last decade. Red indicates saltier-than-normal waters. Blue indicates fresher waters. Oceanographers say we may be approaching a threshold that would shut down the Great Ocean Conveyor and cause abrupt climate changes (from Gagosian, 2002). C. Conceptualized climate system, representing the temperature in and around the North Atlantic Ocean as a function of fresh- water input to the northern North Atlantic. The upper branch (red) features strong deep circulation. Along the lower branch (blue), the circulation is collapsed. The modern climate, with its freshwater flux F, is on the upper branch, as shown in green. When the fresh water reaches a threshold (F + DF), the system flips rapidly to the lower, cold regime. Going back to the warm mode would require a greatly reduced freshwater input (F - DF'). This diagram is highly schematic; the exact position of the modern climate with respect to bifurcation points is largely unknown. Moreover, the shape (particularly the width (DF + DF') of the hysteresis loop depends on parameters that are external to the ocean-atmosphere system. (reprinted with permission from Edouard Bord, 2002, Climate shock: Abrupt changes over millennial time scales: Physics Today, December, Copyright 2002, American Institute of Physics).