Seawater moves through the Atlantic as part of the thermohaline circulation (also called 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, which are caused by differences in temperature and salinity. Colder water is denser than warmer water, while saltier water is denser than less salty water or fresh water. Water heated near the Equator travels at the surface of the ocean north into cold high latitudes where it 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. But melting Arctic sea ice could change this pattern.
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 salty and hence less dense. In fact, data shows that the North Atlantic has already become fresher over the past several decades. Unless the temperature of the water decreases enough to compensate for this freshening of the surface water, the surface water will become less dense, and this less dense water will not be able to sink and circulate through the deep ocean as it does currently. The melting of the Arctic sea ice therefore has the potential to disrupt or slow down the Global Ocean Conveyor.
Paradoxically, this ocean circulation interference caused by global warming could lead to a cooling in Western Europe. Currently the ocean currents carry warmth from the tropics up to the high latitudes. That warmth is lost to the atmosphere keeping the temperatures of places like England, Norway, and many other counties in northern Europe a bit milder than other places at the same latitude. If the Global Ocean Conveyor were to stop completely, the average temperature of Northern Europe would cool 5° to 10° Celsius, but even a slow down could lead to a measurable cooling.
Changes in the Ocean Conveyor due to a freshening of the water in the North Atlantic have happened before. There is evidence from sedimentary rocks and ice cores that it the Ocean Conveyor has shut down or weakened several times in the past and those shut downs have caused changes in climate. One of the most pronounced of these, the Younger Dryas Event, happened about 12,700 years ago, when temperatures cooled about 5°C in the North Atlantic Region. One theory is that this cooling was caused by a Heinrich event, during which the ocean circulation was disrupted when large numbers of icebergs broke off of glaciers in Greenland and melted in the North Atlantic, decreasing the density of the water. Another theory is that the ocean circulation may have been disrupted during the Younger Dryas as a large lake of glacial meltwater emptied into the North Atlantic, decreasing salinity, and hence density, rapidly.