When people think about the risks of climate change, the idea of abrupt changes is quite frightening. Films like “The Day After Tomorrow” fuel that fear, with visions of unimaginable storms and populations fleeing to escape rapidly changing temperatures.
While Hollywood clearly takes liberties with the speed and scale of disasters, several recent real-world studies have raised alarm bells that a crucial ocean current that circulates heat to northern lands could stall this century, with potentially disastrous consequences.
That scenario has happened in the past, most recently more than 16,000 years ago. However, it depends on Greenland throwing a lot of ice into the ocean.
Our new research, published in the journal Science, suggests that while Greenland is indeed losing huge and worrying amounts of ice at the moment, this may not last long enough to cut off the flow on its own. A closer look at past evidence shows why.
Blood and water
The Atlantic Current system distributes heat and nutrients on a global scale, just as the human circulatory system distributes heat and nutrients throughout the body.
Warm water from the tropics circulates northward along the U.S. Atlantic coast before crossing the Atlantic Ocean. As some of the warm water evaporates and the surface water cools, it becomes saltier and denser. Denser water sinks, and this colder, denser water circulates back south at depth. The variations in heat and salinity fuel the pumping heart of the system.
If the Atlantic circulation system weakens, it could lead to a world of climate chaos.
Ice caps are made of fresh water, so the rapid release of icebergs into the Atlantic Ocean can lower the ocean’s salinity and slow the pumping heart. If surface waters can no longer sink deeply and the circulation collapses, dramatic cooling is likely to occur across Europe and North America. Both the Amazon rainforest and Africa’s Sahel region would become drier, and the warming and melting of Antarctica would accelerate, all within a few years to decades.
Today, the Greenland ice sheet is melting rapidly, and some scientists worry that the Atlantic Current system is heading toward a climate tipping point this century. But is that concern justified?
To answer that we have to look back in time.
A radioactive discovery
In the 1980s, a young scientist named Hartmut Heinrich and his colleagues extracted a series of deep-sea sediment cores from the ocean floor to investigate whether nuclear waste could be safely buried in the deep North Atlantic Ocean.
Sediment cores contain a history of everything that has accumulated on that part of the ocean floor over hundreds of thousands of years. Heinrich found several layers containing many mineral grains and rock fragments of the land.
The sediment grains were too large to have been transported to the middle of the ocean by wind or ocean currents alone. Heinrich realized that they must have been brought there by icebergs, which had picked up the rock and mineral when the icebergs were still part of glaciers on land.
The layers with the most rock and mineral debris, from a time when the icebergs must have been coming out with force, coincided with a serious weakening of the Atlantic current system. Those periods are now known as Heinrich events.
As paleoclimate scientists, we use natural data such as sediment cores to understand the past. By measuring uranium isotopes in the sediments, we were able to determine the deposition rate of sediments that fell through icebergs. The amount of debris allowed us to estimate how much fresh water these icebergs added to the ocean and compare it to today to assess whether history could repeat itself in the near future.
Why a shutdown won’t happen anytime soon
So is the Atlantic Current system heading towards a climate tipping point due to the melting of Greenland? We consider this unlikely in the coming decades.
Although Greenland is currently losing huge amounts of ice – disturbingly similar to a mid-range Heinrich event – the ice loss is unlikely to continue long enough to shut off the flow on its own.
Icebergs are much more effective at disrupting currents than meltwater from land, in part because icebergs can transport fresh water directly to the locations where the current sinks. However, future warming will cause the Greenland ice sheet to retreat from the coast too quickly to provide sufficient fresh water via an iceberg.
The strength of the Atlantic Meridional Overturning Circulation, or AMOC, is expected to decline by 24% to 39% by 2100. By then, Greenland’s iceberg formation will be closer to the weakest Heinrich events of the past. Heinrich events, on the other hand, lasted about 200 years.
Instead of icebergs, meltwater flowing into the Atlantic Ocean at the island’s edge is expected to be the main cause of Greenland’s thinning. Meltwater still sends fresh water into the ocean, but it mixes with seawater and tends to move along the coast rather than freshening the open ocean directly, as floating icebergs do.
That does not mean that the current is not in danger
The future trajectory of the Atlantic Current system will likely be determined by a combination of the slowing but more effective icebergs and the accelerating but less influential surface runoff. That will be exacerbated by rising ocean surface temperatures, which could further slow the current.
So the beating heart of the Earth could still be in danger, but history shows that the risk is not as great as some people fear.
In “The Day After Tomorrow,” a slowdown in the Atlantic Current system froze New York City. Based on our research, we can take some comfort in knowing that such a scenario is unlikely in our lifetimes. Nevertheless, strong efforts to halt climate change remain necessary to ensure the protection of future generations.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Yuxin Zhou, University of California, Santa Barbara and Jerry McManus, Columbia University
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Yuxin Zhou received funding from the International Ocean Discovery Program Schlanger Fellowship.
Jerry McManus receives funding from the United States National Science Foundation.