Global Warming - A Chilling Possibility

Retreating Arctic ice, 1979-2003, based on data collected by the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSMI)

Some scientists worry that melting Arctic sea ice will dump enough freshwater into the North Atlantic to interfere with sea currents. Some freshwater would come from the ice-melt itself, but the main contributor would be increased rain and snow in the region. Retreating ice cover exposes more of the ocean surface, allowing more moisture to evaporate into the atmosphere and leading to more precipitation.

Because saltwater is denser and heavier than freshwater, this "freshening" of the North Atlantic would make the surface layers more buoyant. That's a problem because the surface water needs to sink to drive a primary ocean circulation pattern known as the "Great Ocean Conveyor." Sunken water flows south along the ocean floor toward the equator, while warm surface waters from tropical latitudes flow north to replace the water that sank, thus keeping the Conveyor slowly chugging along. An increase in freshwater could prevent this sinking of North Atlantic surface waters, slowing or stopping this circulation.

AMSR-E is collecting new data that will help scientists evaluate this possibility. For one thing, it provides greatly improved ground resolution over previous all-weather sensors. AMSR-E images reveal smaller cracks and fissures in the ice as it breaks up in the spring. This detail allows scientists to better understand the dynamics of ice break-up, says Cavalieri, a member of the AMSR-E team.

"Other important pieces of the puzzle, like rainfall, sea-surface temperatures, and oceanic winds, are also detected by AMSR-E. Looking at those variables together should help scientists assess the likelihood of a change in the Atlantic currents," adds Spencer.

Image courtesy Argonne National Laboratory A global ocean circulation between deep, colder water and warmer, surface water strongly influences regional climates around the world.

Once considered incredible, the notion that climate can change rapidly is becoming respectable. In a 2003 report, Robert Gagosian cites "rapidly advancing evidence [from, e.g., tree rings and ice cores] that Earth's climate has shifted abruptly and dramatically in the past." For example, as the world warmed at the end of the last ice age about 13,000 years ago, melting ice sheets appear to have triggered a sudden halt in the Conveyor, throwing the world back into a 1,300 year period of ice-age-like conditions called the "Younger Dryas."