by When people think about the risks of climate change, the thought of sudden changes is scary. Movies like “The Day After Tomorrow” fuel that fear, with scenes of unimaginable storms and populations fleeing to escape rapidly changing temperatures.
While Hollywood obviously takes liberties with the speed and scale of disasters, several recent studies have raised warnings that a vital ocean current that spreads heat to northern countries could shut down this century, potentially disastrous.
That situation happened in the past, more recently than 16,000 years ago. However, it depends on Greenland putting a lot of ice into the ocean.
Our new research, published in the journal Science, suggests that while Greenland is indeed losing large and alarming volumes of ice at present, this may not continue long enough to stem the flow on its own. A closer look at evidence from the past shows why.
Blood and water
The Atlantic current system distributes heat and nutrients on a global scale, much like the human circulatory system distributes heat and nutrients throughout the body.
Warm water from the tropics spreads north along the US Atlantic coast before crossing the Atlantic. As some of the warm water evaporates and the surface water cools, it becomes saltier and denser. Water sinks more densely, and this cooler water spreads more densely to the southwest at depth. The variations in heat and salinity fuel the pumping heart of the system.
A weakening of the Atlantic circulation system could lead to world climate chaos.
Ice sheets 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 core. If the surface water is no longer able to go deep and the circulation collapses, a dramatic cooling is likely to occur across Europe and North America. Both the Amazon rainforest and the Sahel region of Africa would collapse, and the warming and melting of Antarctica would accelerate, all in a few years to decades.
Today, the Greenland ice sheet is melting rapidly, and some scientists are concerned that the current Atlantic system may be approaching this century’s climate crisis. But is that concern justified?
To answer that, we have to look back in time.
Radioactive discovery
In the 1980s, a junior scientist named Hartmut Heinrich and his colleagues removed a series of deep-sea sediment cores from the ocean floor to study whether nuclear waste could be safely disposed of in the deep North Atlantic.
Sediment cores contain the history of everything that has accumulated on that part of the ocean floor over hundreds of thousands of years. Heinrich found several layers with many mineral grains and rock fragments from the ground.
The sediment grains were too large to be carried 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 containing most of the rock and mineral debris were from a time when the icebergs must have come out in force, at the same time as a sharp weakening of the Atlantic current system. These periods are now known as Heinrich events.
As paleoclimate scientists, we use natural records 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 from falling icebergs. The amount of debris allowed us to estimate the amount of fresh water those icebergs added to the ocean and compare it to today to see if history could repeat itself in the near future. .
Why a shutdown is unlikely anytime soon
So, is the current Atlantic system headed for climate slippage due to the melting of Greenland? We think it is unlikely in the coming years.
Although Greenland is currently losing massive amounts of ice – very comparable to the average Heinrich event – it is likely that the ice loss will not continue long enough to close the flow on its own.
Icebergs are much more effective at disrupting the flow than meltwater from the ground, in part because icebergs can carry fresh water directly out to the locations where the flow disappears. However, future warming will force the Greenland ice sheet to retreat from the coast too soon to deliver enough fresh water through an iceberg.
It is projected that by 2100 the strength of the Atlantic Amphibious Cyclone, or AMOC, will decrease by 24% to 39%. By then, the formation of Greenland’s icebergs will be closer to the weakest Heinrich events of the past. In contrast, Heinrich’s events spanned 200 years or so.
Instead of icebergs, meltwater pouring into the Atlantic at the edge of the island is thought to be the main cause of Greenland’s thinning. Meltwater still sends freshwater into the ocean, but it mixes with seawater and tends to move along the coast rather than freshen the open ocean in the same way that drifting icebergs do.
That does not mean that the current is not at risk
The future path of the Atlantic current system is likely to be determined by a combination of the accelerating but more effective icebergs and the accelerating but less influential surface runoff. That amount will be increased by a rise in the surface temperature of the ocean which could further slow down the flow.
So, the pumping heart of the Earth may still be at risk, but history suggests that the risk is not as imminent as some fear.
In “The Day After Tomorrow”, the Atlantic current system slowed down and froze New York City. Based on our research, we may know that such a situation is unlikely to exist in our lifetime. However, strong efforts to stop climate change are still needed to ensure the protection of future generations.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of 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 Schlanger Fellowship of the International Ocean Discovery Program.
Jerry McManus receives funding from the US National Science Foundation.
