A sea captain in 1751 dropped a temperature gauge on a rope into the subtropical Atlantic and then picked it back up without knowing what he had done. There was no way to explain how shockingly cold the water was almost a mile below. The main thing he did with the discovery was chill his wine. It would be another fifty years before anyone realized what he had really found: proof of a huge, slow, planet-regulating machine that wound through all of Earth’s oceans.
Heat has been moving around the Earth for thousands of years thanks to the thermohaline circulation and its Atlantic branch, which is called AMOC (the Atlantic Meridional Overturning Circulation). It does this at a rate of about one petawatt of energy per second, which is fifty times the amount of energy that all of humanity uses together. When you look at these numbers on paper, they don’t feel real. But when you stand on the shore in Massachusetts or the English Channel and watch flood markers rise higher than ever before, you get the feeling that something huge and stable is no longer acting the way it should.
The thermohaline shift that is happening deep in the ocean is very small and easy to miss. This isn’t about a big underwater volcano erupting or a sudden geological event. Scientists are looking for changes in the delicate balance between temperature and salinity, which are the two things that move the water around in the deep ocean. Cold water goes down. Water that is salty goes down. It falls from the surface near Greenland and Iceland to depths of two to three thousand meters when the water is cold and salty enough. It then flows south like a slow river under the sea floor. The whole overturning system is driven by that sinking. There is less and less trust in that sinking.
Even though the effects are hard to fully understand, the mechanism is not hard to understand. As the Earth continues to warm, freshwater from melting glaciers and more rain in the Arctic will flow into the North Atlantic. Salty water is heavier than fresh water. It floats on top and fights the downward pull that keeps the blood moving. Everything slows down. When it slows down, not as much warm water from the tropics moves north to replace what sank, which means that the Northern Hemisphere doesn’t get as much heat. The idea that a warming world would make Europe colder, mess up the Gulf Stream, and cause sea levels to rise along the East Coast of the United States doesn’t make sense. However, that is exactly what the physics says.
Scientists are very worried about the thermohaline shift because it involves the idea of a “tipping point”—a level above which the slowdown starts to reinforce itself. Henry Stommel, an oceanographer, used math to find this possibility in 1961. What at the time seemed like just a theory has since been proven by dozens of models, including some of the most complex climate simulations ever made. It turns out that the AMOC can only be in two stable states: on or off. There’s no need for a catastrophe to go from on to off. You have to cross a line to do it.
Still, it’s not clear where that line is. The most honest thing that can be said about the current state of science is that the exact threshold is still unknown. This does not mean that the risk is too high. There are models that show the system might be stronger than people thought. Some people think that those models might not be fully capturing the problem because they are too focused on large-scale deep convection processes. The disagreement is real and important.
What is clear is what is already happening along the coasts. For years, the sea level has been rising faster along parts of the East Coast of the United States than the rest of the world. The reason for this is physical: when the AMOC is weaker, it pulls less water north from the coast, which lets it build up. Once, flooding would only happen during named storms. Now, it happens on clear days, caused by nothing more dramatic than a high tide and a weak deep current thousands of miles away.
There is something slightly disorienting about the thought that changes in temperature and salt levels, which are measured in fractions of a degree, can cause streets in Charleston or Boston to flood. There’s always been something big and cold about the ocean. It turns out that it is very big and doesn’t care what we put in the air above it.
