When people think about the risks of climate change, the idea of ​​sudden changes is quite scary. Movies like “The Day After Tomorrow” feed this fear with visions of unimaginable storms and populations fleeing to escape rapidly changing temperatures.
While Hollywood apparently indulges in the speed and scale of disasters, several recent studies have raised alarm in the real world that a crucial ocean current that circulates heat to northern lands could shut down this century, with potentially catastrophic consequences.
This scenario has happened in the past, most recently over 16,000 years ago. But it relies on Greenland dumping a lot of ice into the ocean.
Our new research, published in the journal Science, suggests that while Greenland is indeed losing huge and alarming volumes of ice right now, it may not continue long enough to stop the flow on its own. A closer look at the evidence from the past shows why.
Blood and water
The current Atlantic 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 circulates northward 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. The denser water sinks and this cooler, denser water circulates back south at depth. Changes in heat and salinity drive the pumping heart of the system.
If the Atlantic circulation system were to weaken, it could lead to global climate chaos.
IPCC 6th Assessment Report
Ice sheets are made of fresh water, so the rapid release of glaciers into the Atlantic Ocean can reduce the ocean’s salinity and slow the pumping heart. If surface water were no longer able to sink deeply and the circulation collapsed, dramatic cooling would likely occur in Europe and North America. Both the Amazon rainforest and Africa’s Sahel region would become dry, and the warming and melting of Antarctica would accelerate, all within years to decades.
Today, the Greenland ice sheet is melting rapidly, and some scientists fear that the current Atlantic system may be headed for a climate tipping point this century. But is this concern justified?
To answer that, we have to look back in time.
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.
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 lots of mineral grains and rock fragments from the ground.
The sediment grains were too large to be carried into the mid-ocean by wind or ocean currents alone. Heinrich realized that they must have been brought there by glaciers that scooped up the rock and minerals while the glaciers were still part of the glaciers on land.
The layers with the greatest amount of stone and mineral debris from the time when the glaciers must have appeared in force coincided with a serious weakening of the Atlantic current system. These periods are now known as the Heinrich events.
As paleoclimatic scientists, we use natural records such as sediment cores to understand the past. By measuring the uranium isotopes in the sediments, we were able to determine the rate of deposition of the sediments that fell from the glaciers. The amount of debris allowed us to estimate how much fresh water these glaciers added to the ocean and compare it with today to assess whether history may repeat itself in the near future.
Why the shutdown probably won’t come soon
So is the current Atlantic system headed for a climate tipping point due to the melting of Greenland? We think this is unlikely in the coming decades.
While Greenland is losing huge volumes of ice right now—worryingly comparable to a mid-range Heinrich event—the ice loss is unlikely to continue long enough to shut down the current on its own.
Glaciers are much more effective at disrupting the flow than meltwater from the ground, in part because glaciers can carry fresh water directly to where the flow is falling. However, future warming will force the Greenland ice sheet to move away from the coast too soon to provide enough fresh water for the glacier.
NASA
The strength of the Atlantic Meridional Overturning Circulation, or AMOC, is projected to decrease by 24% to 39% by 2100. By then, glacier formation in Greenland will approach the weakest Heinrich events of the past. Heinrich’s events, on the other hand, spanned approximately 200 years.
It is believed that the main cause of Greenland’s thinning will be meltwater instead of glaciers entering the Atlantic at the edge of the island. Meltwater still sends fresh water into the ocean, but it mixes with seawater and tends to move along the coast rather than directly refreshing the open ocean, as drifting glaciers do.
This does not mean that the current is not at risk
The future trajectory of the current Atlantic system is likely to be determined by a combination of slowing but more effective glaciers and accelerating but less influential surface runoff. This will be compounded by rising ocean surface temperatures, which could further slow the current.
So the pumping heart of the Earth may still be at risk, but history suggests the risk is not as imminent as some people fear.
In “The Day After Tomorrow”, New York City is frozen by the slowdown of the Atlantic Current System. Based on our research, we can take some comfort in knowing that such a scenario is unlikely in our lifetimes. However, significant efforts to stop climate change are still necessary to ensure the protection of future generations.