When people think about the risks of climate change, the idea of drastic changes is quite frightening. Movies like The Day After Tomorrow fuel this fear, with visions of unimaginable storms and populations fleeing to escape rapidly changing temperatures.
While Hollywood is apparently taking liberties with the speed and scale of disasters, several recent studies have raised real-world concerns that a crucial ocean current that circulates heat northward could bring this century to a halt with potentially catastrophic consequences.
This scenario has occurred in the past, most recently more than 16,000 years ago. However, it is counting 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 amounts of ice right now, it may not last long enough to stop the flow on its own. A closer look at the 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 around the body.
Warm water from the tropics circulates northward along the Atlantic coast of the United States 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 colder, denser water circulates back southward at depth. 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 sheets are made of fresh water, so the rapid release of icebergs into the Atlantic Ocean can reduce the ocean’s salinity and slow the heart. 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 will become drier, and the warming and melting of Antarctica will 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 may be headed for a climate tipping point this century. But is this concern justified?
To answer this, we need to look back in time.
Radioactive discovery
In the 1980s, a junior 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 deep 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 with many mineral grains and rock fragments from the land.
The sediment grains were too large to be carried to the mid-ocean by wind or ocean currents alone. Heinrich realized that they must have been brought there by icebergs that lifted the rock and mineral when the icebergs were still part of the glaciers on land.
The layers with the most rock and mineral debris, from the time when the icebergs must have come to light, coincided with a strong weakening of the Atlantic current system. These periods are now known as the Heinrich events.
As paleoclimate scientists, we use natural records like sediment cores to understand the past. By measuring uranium isotopes in the sediments, we were able to determine the deposition rate of sediments released from 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 Atlantic Current System headed for a climate tipping point due to Greenland melting? We think this is unlikely in the coming decades.
Although Greenland is losing massive amounts of ice right now—disturbingly comparable to a medium-frequency Heinrich event—the ice loss likely won’t last long enough to stop the flow on its own.
Icebergs are much more effective at interrupting currents than meltwater from land, in part because icebergs can carry fresh water directly to where the current sinks. However, future warming will force the Greenland ice sheet to retreat from the coast too soon to supply enough fresh water via an iceberg.
The strength of the Atlantic Meridional Overturning Circulation, or AMOC, is projected to decrease by 24% to 39% by 2100. By then, Greenland iceberg formation will be closer to the weakest Heinrich events of the past. Heinrich’s events, in contrast, spanned about 200 years.
Instead of icebergs, meltwater pouring into the Atlantic Ocean at the edge of the island is expected to become the leading 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 refreshing the open ocean, as floating icebergs do.
This does not mean that electricity is not at risk
The future trajectory of the Atlantic current system is likely to be determined by a combination of the slowing but more effective icebergs and the accelerating but less influential surface outflow. This will be compounded by rising ocean surface temperatures, which may further slow currents.
So the pumping heart of the Earth may still be at risk, but history shows that the risk is not as immediate as some people fear.
In “The Day After Tomorrow”, the slowdown of the Atlantic Current system has frozen New York. Based on our research, we can take comfort in knowing that such a scenario is unlikely in our lifetime. Nevertheless, decisive efforts to halt climate change remain necessary to ensure the protection of future generations.
Yusin Zhou, postdoctoral fellow in Earth Science, University of California, Santa Barbara and Jerry McManus, professor of earth and environmental sciences, Columbia University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
