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What if the Ocean Conveyor Belt Stalled?

Imagine yourself drifting in the North Atlantic, attached to a bucket of water. You could be drifting in the middle of the ocean for months. Chances are that you will slowly drift towards the Caribbean, and enter one of the strongest currents on Earth, the Gulf Stream. Within a few weeks you will have been swept northward, past the coasts of Florida, Georgia, South Carolina and North Carolina. After passing Cape Hatteras, the current leaves the continent, and before you know it you will be in the middle of the Atlantic again. Now you have two choices. You can take the southern branch of the current, which will bring you straight across the Atlantic towards Africa. You might catch a glimpse of the Azores before you slowly drift south again towards the tropics. Or you can take the northerly branch. This will take you to Great Britain; from there you will drift north to Norway. Weather starts to deteriorate. The air gets colder, it starts to rain. Your call.

Suppose you took the northern branch. You might find yourself floating around in the Greenland Sea. Suddenly, your bucket of water will have cooled off so much that it becomes heavier than the water beneath you. It feels like the bottom drops out from under you, and you start to sink. You experienced a convection event, that will take you down to a depth of a few kilometers. Here, in the darkness of the abyss, you feel yourself slowly moving south again. You drift along the U.S. east coast again, but now in the opposite direction and several kilometers beneath the surface. You continue to drift, passing South America until suddenly you are swept eastward—you have entered the strong Antarctic Circumpolar Current. This majestic current flows around Antarctica, swept forward by the horrendous storms of the Southern Ocean. It might take you decades—and many trips around Antarctica—before you finally reach the surface again. If you happen to end up in the Atlantic Ocean, you will start to drift north again, first taking the Benguela Current to cross the South Atlantic from Africa to Brazil, then the North Brazil Current, across the Equator, until finally, hey! you are back in the Gulf Stream!

If you're still with me: congratulations! You just completed a loop of the global conveyor belt circulation! This ocean drift is very important for the climate system. When your water bucket cooled off in the Greenland Sea, it gave up its heat to the atmosphere, thereby heating the high northern latitudes, and in particular western Europe. There were times in the past when the water didn't get this far. During the ice ages, you would have sunk south of Iceland. Large parts of Europe, Asia and North America were covered by huge ice sheets, in part because the ocean couldn't get its heat as far north as it does today.

Climate scientists fear that there might be a hidden threshold in the conveyor belt. If the atmosphere heats up over the Greenland Sea, your bucket of water might not cool off enough to sink. In addition, chances are that the precipitation will increase as well. The extra freshwater in your bucket makes it even lighter in comparison to the cold and salty water underneath you. Without your bucket sinking, and drifting south, there is no place for new import of warm water. Global warming might thus slow down—and finally halt— the conveyor belt. If this happens, the impact will be felt mostly in the North Atlantic region; it will reduce or halt the trend of global warming. However, many consequences are hard to predict. What will happen to the sea ice in the Arctic? The ice sheets of Greenland? The North Atlantic fisheries that depend so strongly on the mild ocean temperatures? European agriculture? On the global scale, the impacts will be more subtle. The conveyor extracts a lot of heat and carbon dioxide (CO2) from the atmosphere. Without this sink, will CO2 levels and global temperatures go through the roof? As the deep ocean warms and expands, will the resulting raising sea levels mean the end of New Orleans, parts of Florida, the Netherlands?

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