Like the orderly flow of blood through the body, ocean and air currents distribute energy around the globe, regulating planetary temperature and climate. Two primary factors control the process: wind and water density. Winds direct surface water, while water density, which is controlled by temperature and salinity (the colder and saltier, the denser), dictates when and where currents descend to flow deep beneath the surface. The entire exchange operates like a giant conveyor belt. In the Atlantic Ocean, wind pushes warm, salty water north, where it cools -- releasing heat to the atmosphere -- and begins to sink and flow in the opposite direction, sucking more warm water northward. But as global warming melts the polar ice caps, the influx of freshwater may reduce the salinity of the North Atlantic, lowering its density enough to stop it from sinking. If that happens, the conveyor belt could shut down.
The idea of the oceans and atmosphere as a single, interconnected planetary thermostat is several decades old, and scientists have developed tools that allow them to watch the thermostat in action. Satellites detect infrared radiation from the oceans, allowing scientists to measure surface temperatures over large areas. Buoys tethered to the ocean floor have sensors that record the temperature, flow direction, and speed of deep water and surface currents. And free-floating devices rigged with a GPS system let scientists map the movement of a single ocean current; some even have salinity and temperature sensors. Together, these technologies help scientists understand exactly how earth regulates its body temperature, and will allow them to model more precisely what will happen as the earth warms.
Disturbingly, the oceans store four times more energy than the air, and satellite measurements over the past decade reveal that our seas are absorbing more heat than ever before. Scientists don't yet know for certain how long it will be before the oceans inevitably release this excess heat back into the atmosphere, but current models predict that we may have as few as 10 years before it happens. When it does, atmospheric temperatures are expected to jump by 1.1 degrees Fahrenheit, adding to the already quickening pace of global warming.