When people think about the dangers of climate change, the idea of sudden changes is very scary. Movies like The Day After Tomorrow feed into that fear, with visions of unimaginable storms and populations fleeing rapidly changing temperatures.
While Hollywood clearly takes liberties with the speed and magnitude of disasters, several recent studies have raised real-world alarms that a crucial ocean current that circulates heat to northern countries could shut down this century, with potentially catastrophic consequences.
This scenario has occurred in the past, most recently more than 16,000 years ago. However, 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 large and worrisome volumes of ice now, it may not continue long enough to shut down the current on its own. A closer look at evidence from the past shows why.
Blood and water
The current Atlantic system distributes heat and nutrients on a global scale, just as the human circulatory system distributes heat and nutrients around the body.
Warm water from the tropics circulates 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. The denser water sinks and this cooler, denser water circulates southward at depth. Changes 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.
IPCC 6th Assessment Report
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 heart’s pumping. If surface water is no longer able to sink deeply and circulation collapses, dramatic cooling is likely to occur in Europe and North America. Both the Amazon rainforest and the Sahel region of Africa 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 current Atlantic system may be heading for a climate tipping point this century. But is this concern justified?
To answer this, we need to look back in time.
A radioactive discovery
In the 1980s, a young scientist named Hartmut Heinrich and his colleagues retrieved a series of deep-sea sediment cores from the ocean floor to study whether nuclear waste could be safely buried in the depths of the North Atlantic.
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 of many 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 icebergs, which had picked up the rock and ore when the icebergs were still part of the glaciers on land.
Layers of more rock and mineral debris, from a time when icebergs should have emerged, coincided with severe weakening of the current Atlantic system. Those periods are now known as the Heinrich events.
As paleoclimate scientists, we use natural data like sediment cores to understand the past. By measuring uranium isotopes in the sediments, we were able to determine the rate of deposition of the sediments dumped by the 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 might repeat itself in the near future.
Why a shutdown isn’t likely anytime soon
So is the current Atlantic system headed for a climate tipping point due to the melting of Greenland? We think it is unlikely in the coming decades.
While Greenland is losing large volumes of ice right now—disturbingly comparable to a mid-Heinrich event—the ice loss likely won’t continue long enough to shut down the current on its own.
Icebergs are much more effective at disrupting currents than meltwater from the ground, in part because icebergs can carry fresh water directly to places where currents sink. Future warming, however, will force the Greenland ice sheet to retreat too quickly from the coast to deliver enough fresh water from the iceberg.
NASA
The strength of the Atlantic Meridional Overturning Circulation, or AMOC, is projected to drop 24% to 39% by 2100. By then, Greenland’s iceberg formation will be closer to the weaker Heinrich events of the past. Heinrich’s events, in contrast, spanned about 200 years.
Instead of icebergs, meltwater flowing into the Atlantic at the edge of the island is predicted to become the main cause of Greenland’s thinning. The meltwater still sends fresh water into the ocean, but it mixes with seawater and tends to move along the coast instead of directly cooling the open ocean as drifting icebergs 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 icebergs and accelerated but less influential surface flow. This will be compounded by rising ocean surface temperatures that could further slow the current.
So the pumping heart of the Earth may still be in danger, but history suggests the danger is not as imminent as some people fear.
In “The Day After Tomorrow”, a slowdown in the Atlantic Current system freezes New York City. Based on our research, we can take comfort in knowing that such a scenario is unlikely in our lifetime. However, strong efforts to stop climate change remain necessary to ensure the protection of future generations.