When we think of climate change, often we think about how the atmosphere is changing – how it’s getting hotter. But there is also a story of change unfolding in the ocean, which absorbs some 90 percent of the excess heat from greenhouse gas emissions. A system of currents in the Atlantic Ocean called AMOC (Atlantic Meridional Overturning Circulation) – in which the Gulf Steam is included – are particularly sensitive to climate change. The AMOC is driven by two ocean properties: temperature and salt. In the North Atlantic, water that flows northward off the US east coastline is warm and salty, carrying heat from the equator. Once the water reaches the mid-latitudes, it starts to cool. Around Greenland, the cooling of the water – and the saltiness – creates a density difference enough that this water sinks deep beneath the surface. The deep water then makes it way to the Southern Hemisphere, all the way to Antarctica as part of a “global ocean conveyor belt”, which is responsible for redistributing heat worldwide.
The transport of heat northward with the AMOC is what moderates Europe’s climate. With climate change in the picture, Greenland is warming – the consequence of that being a lot of melting of the ice, and subsequent freshwater pouring into the ocean. This reduces the salinity and thus the density of the water, inhibiting sinking and thus weakening the flow of the AMOC. If you’ve seen the movie, “The Day After Tomorrow,” you’ll remember how it depicts what happens on the planet due to a sudden shutdown of the AMOC. In reality, a steady weakening of the current is what we’re in store for, if we don’t do something about our greenhouse gas emissions.
So, what are the consequences of a weakening AMOC? It’s an area of active research, but can include more extreme weather in Europe (more winter storms; more summer heat waves), more intense hurricanes, and even enhanced sea level rise on the US east coast. The explanation for this is linked to the Coriolis effect: northward flow of the AMOC leads to a deflection of water masses to the right, away from the US east coast due to Earth’s rotation that diverts moving objects such as currents to the right in the northern hemisphere and to the left in the southern hemisphere. As the AMOC slows down, this effect weakens and more water can pile up at the US east coast, leading to an enhanced sea level rise. Fortunately, we don’t have a “day after tomorrow” event unfolding in our future. But we do need to act – globally – to curb greenhouse gas emissions and start fixing this planet – for the benefit of all species.