Scientists say ocean currents are becoming slower and less stable, which could trigger extreme changes in weather patterns and climate.
The next time you gaze at seawater lapping at the shore, spare a thought for the bigger systems behind the movement of the world’s oceans. The combination of waves, tides, and currents is complex and a critical element of our climate — it’s also sensitive to the man-made changes affecting so many other aspects of our world today. And many experts are worried that climate change-induced slowdowns in one of the world’s most critical current systems could have serious consequences.
Oceans carry water of varying temperatures and salt content around the world in a series of currents known as the global conveyor belt. The Atlantic Meridional Overturning Circulation (AMOC) is a part of this conveyor belt and it transports warm water from the tropics to Europe, cooling as it goes, before turning around and heading for the Antarctic. This current, made possible by a specific interplay of wind, water temperature, and salinity, redistributes heat around the world and plays a role in the temperatures and weather patterns we experience.
But the current’s strength relies on the density of warmer and saltier water — so if colder and fresher water is introduced into the system, it could transition into what’s known as a weak mode. A weak mode would result in a slower northward transport of water by the AMOC, likely triggering significant changes in weather and climate, including more rapid sea-level rise, cooling of the Northern hemisphere, more extreme cycles of drought and rainfall, and severe temperature swings. Scientists believe we may be heading in that direction — and more quickly than expected — thanks in part to melting snow and sea ice from the Arctic due to climate change. Ocean circulation also plays an important role in carbon sequestration, so such a huge shift in the AMOC could impact how we plan to manage climate change itself.
The AMOC is bistable, which means it exists in one of two states at any given time. If the current reaches a weak mode, it would also mean a total collapse of its current strong mode. This loss of stability — distinct from the slowdown of these currents we’re seeing — is irreversible. If this occurs, it would create an ocean current system that functions in entirely different ways from how it does now, bringing about huge changes, like a cooler Northern hemisphere, and dramatically different weather patterns, like more frequent and intense storm systems and extreme temperatures.
So naturally, researchers are doing everything they can to learn more about what’s happening to the AMOC. While it’s impossible to predict exactly how quickly the system will change or observe shifts happening in real-time, some researchers have identified early warning signs that currents are slowing. Based on years of data on the Atlantic ocean’s salinity and temperature, researchers have also noticed indirect indications that the current system may be losing strength and stability.
“For many natural systems that can potentially exhibit abrupt regime shifts such as the AMOC, there are reasons to believe that such a shift would be preceded by characteristic changes in the dynamics of the system that are associated with a loss in stability and a weakening of the restoring forces that bring the system back to its current equilibrium state following some perturbation,” says Niklas Boers, a professor of Earth system modeling at the Potsdam Institute for Climate Impact Research who published a study in August finding “an almost complete loss of stability over the last century” in the AMOC.
Boers emphasizes that there’s still no way to know exactly how much closer we are to an unstable AMOC, but his research suggests that the vital ocean current system is moving closer to that critical tipping point. (An abrupt timeline would be a switch in the current system over a few decades.)
As with many of the other consequences of climate change, the main way to address the slowdown of the AMOC is to reduce greenhouse gas emissions as quickly and as much as possible, says Boers. As he told the Guardian, a collapse of the AMOC is “something you just can’t allow to happen.”
It’s hard to imagine a world in which some of our fundamental Earth systems function in a way that we’ve never known them to do. But it’s just another of many reasons to work toward a world in which we take real action to mitigate climate change.
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