Image courtesy Argonne National Laboratory
Seawater moves through the Atlantic as part of the Global Ocean Conveyor, the regular pattern by which seawater travels the world’s oceans. The water in the Global Ocean Conveyor circulates because of differences in water density. In the North Atlantic, the differences in water density are mainly caused by differences in temperature. Colder water is denser than warmer water. Water heated near the Equator travels at the surface of the ocean north into cold high latitudes where becomes cooler. As it cools, it becomes denser and sinks to the deep ocean. More warm surface water flows in to take its place, cools, sinks, and the pattern continues.
Melting Arctic sea ice could change this pattern, or stop it altogether.
Recent research shows that Arctic sea ice is melting faster than expected. As the Earth continues to warm and Arctic sea ice melts, the influx of freshwater from the melting ice is making seawater at high latitudes less dense. In fact, data shows that the North Atlantic has become fresher over the past several decades. The less dense water will not be able to sink and circulate through the deep ocean as it does currently. This will disrupt or stop the Global Ocean Conveyor. Scientists estimate that, given the current rate of change, the Global Ocean Conveyor may slow or stop within the next few decades.
Even through the disruption to ocean circulation is because of global warming, it could cause cooling in Western Europe and North America. The ocean currents carry warmth from the tropics up to these places, so they are a bit milder. If the Global Ocean Conveyor were to stop completely, the average temperature of Europe would cool 5 to 10 degrees Celsius.
This would not be the first time that the Global Ocean Conveyor was halted. There is evidence from sedimentary rocks and ice cores that it has shut down several times in the past and those shut downs have caused changes in climate. One of the most well-known, the Younger Dryas Event, happened about 12,700 years ago and caused temperatures to cool about 5 C in the region.
source: windows to the universe