Usually, when we think of going up in latitude, we think of climates getting colder. But across the globe we see anomalies: in the winter, Toronto is a frigid hellscape while Barcelona, which shares the same latitude, still boasts nude beaches. The same goes for the strangely balmy coast of Ireland, which shares its latitude with Siberia, and even for Iceland (which, despite its name, experiences very mild winters), which is situated at the same latitude as Alaska. It turns out the reason behind all of these anomalies has to do with the ocean, which uses a system that transports heat, nutrients, and oxygen across the entire planet. This system, often referred to as the Great Ocean Conveyor Belt, is called thermohaline circulation. What would our world look like without this massive regulator, you ask? We may know soon.
Thermohaline circulation was named after the two factors that contribute to this planet-wide conveyor system: thermo- for temperature, and -haline for saltiness. Thermohaline circulation is driven by water density, and works on the principle that colder, saltier water will sink below warmer, less salty water because it is more dense. When oceanic currents, which are constantly moving all over the globe, carry cold, deep water from cold regions (like the poles) to warm regions (near the equator), the water gets warmer and rises, and the large amounts of rainfall in the tropics help dilute it so it’s less salty.
Eventually this water travels back towards the poles, where it releases its heat and becomes colder, and where high evaporation and low rainfall make it saltier. This now high-density water sinks down to the seafloor, where it begins the process again. In this way, water circulates around the globe, bringing the heat from warm water to cold regions and the cold, nutrient rich water from the poles to ecosystems on the equator. This phenomenon is why we see native populations of penguins on the Galapagos Islands, which are located as far from the north or south poles as they possibly could be, and why Europe is warmer than Canada.
One of the most impressive examples of how thermohaline circulation affects our world can be found in the Gulf Stream, a massive, warm-water current off the east coast of the United States that transports more water than all of the world’s rivers combined. It acts like a river, too, carrying warm, tropical waters along the surface of the ocean from the Caribbean all the way to the cold, sub-polar regions of the North Atlantic ocean.
As it travels north, the heat dissipates from its waters and warms western Europe, ensuring a temperate climate for Great Britain, Spain, and even as far east as Norway. When it arrives in the North Atlantic it is cold and salty, ready to sink to the bottom of the ocean and begin its journey south as a current now called the North Atlantic Deep Water, which travels underneath the warm waters of the Gulf Stream.
We call this system of currents Atlantic Meridional Overturning Circulation (AMOC), which is a critical part of global circulation that determines the climates of eastern North America and Europe through the transport of heat.
Here’s where we get to the problem: a group of scientists led by Michael Mann and Stefan Rahmsdorf recently found that AMOC is shutting down. It’s happening slowly, and it’s still under debate how much it will affect our lives, but we are confident of one thing: we are the ones causing it.
The transport of heat and water by AMOC and the currents that contribute to it rely on the conditions of the ocean in certain regions to push them along: namely, a warm equatorial region to drive North Atlantic Deep Water to the surface, and a cold, salty North Atlantic to drive Gulf Stream waters down to the ocean floor. These temperature differences between the two regions keep these currents moving like conveyor belts, driving heat and nutrients through the Atlantic Ocean.
However, as global warming causes more and more ice to melt in the Arctic, the waters near the poles are becoming less salty, and therefore less dense. In the summer of 2016, the Arctic lost 23,600 square miles of ice in one day, and the Greenland ice sheet, which is the second largest body of ice on earth after Antarctica, reached a record low. This massive input of fresh water makes the cold, salty water coming up from the Gulf Stream less likely to sink, and has contributed to a slowdown of AMOC of 15-20% in the past century. As more sea ice melts and fresh water is added to the system, AMOC will slow even further, and could shut down entirely.
To conduct this study, a team of scientists led by Stephen Rahmsdorf analyzed multiple factors across the Atlantic Ocean. The shrinking of the Greenland ice sheet and the input of fresh water was an obvious driving force behind the slowing of AMOC. But they also noticed an acceleration of sea level rise on the east coast of the United States, 2 to 3 times faster than the global average.
This is because as the Gulf Stream moves water up towards the North Atlantic, it pulls water east away from the Atlantic coast of North America, resulting in a 3-5 foot lower sea level on this coast than on the other side of the Gulf Stream. As these currents slow, this sea level difference is offset, and we see increases in sea level across the eastern seaboard. As AMOC continues to slow, cities like New York and Boston may face rising sea levels of up to 5 feet, in addition to the global sea level rise expected due to global warming. For coastal cities, this may require massive changes in architecture to accommodate rising seas.
Another indication that AMOC has been slowing came in the form of a temperature anomaly on a global warming survey conducted by NOAA. Although temperature averages have continued to increase across the entire globe, a small oceanic region south of Greenland seemed to have actually gotten colder, reaching record cold temperatures in 2015.
At first, the scientists thought their measurements in that region had been inaccurate. But Rahmsdorf and his team were able to explain this phenomenon: this region is now colder because it was previously warmed by a powerful Gulf Stream, which has since weakened, resulting in less warm water transport into the North Atlantic. This suggests the warm water that drives this circulation is getting less efficient, and that we may be in for large changes to the oceanic ecosystem in the coming years.
Rahmsdorf believes that a weakening of global currents this severe has not occurred in a thousand years or more, and that it could result in drastic consequences to the environment in the next century.
What could some of these consequences look like? The last time thermohaline circulation shut down, it caused an ice age that lasted almost 100,000 years, one which ended only about 11,000 years ago. Humans evolved and spread across the earth during this period, but the ice age caused extinction on the massive scale, resulting in the die-offs of more than half of all large mammals that have ever lived on earth.
The impacts of THC shutdown now, taken with the effects of climate change, would likely look different than they did last time. Michael Mann, one of the lead researchers on this project, believes a significant slowdown of AMOC would “dramatically change ocean patterns, influence the food chain, and negatively impact fisheries.” While in theory, the shutdown of AMOC should result in a much colder climate in Europe, the authors actually believe this cooling of Europe will be offset by rising average temperatures caused by global warming.
However, the extra sea level rise on the east coast, the destruction of fisheries, and the alteration of nutrient cycling in the oceans poses real threats for us and for the ecosystems that support us. Regions like the Galapagos are so rich with life because they are fed by nutrient-rich, oxygenated deep water currents that originate in the poles, and a shutdown of this system could remove much of the nutrients sustaining these hotspots of life. Furthermore, any change in major planetary heat transport systems tends to have dramatic implications for our weather patterns. This could lead to stronger storms in the North Atlantic, reduced crop productivity in Europe, and changes in tropical precipitation patterns.
The reality, however, is that we are not entirely sure how the slowing of AMOC is going to affect our world just yet. This is an extremely difficult system to predict: we still cannot accurately track fresh water as it incorporates into thermohaline circulation, or account for what could be caused by seasonal changes or by our own activities. But AMOC is slowing, and combined with the other factors changing our climate, we may have to adapt to some major shifts in the coming years.
The changes happening in our world are real, and just because we cannot fully predict them does not mean we should not be wary of their consequences. We would be wise to be diligent in our efforts to reduce our carbon footprint, to study these systems in order to better understand them, and to treat our planet like the fragile, interconnected network that it is.