When people think about the risks of climate change, the idea of sudden change sounds pretty scary. Movies like “The Day After Tomorrow” feed into this fear with images of populations fleeing to escape unimaginable storms and rapidly changing temperatures.
While Hollywood clearly takes liberties with the pace and magnitude of disasters, some recent studies have raised real-world alarms that a key ocean current that disperses heat to northern countries could shut down this century, with potentially disastrous consequences.
This scenario has played out in the past, most recently more than 16,000 years ago. But it depends on Greenland dumping too much ice into the ocean.
Our new research, published in the journal Science, suggests that although Greenland is indeed losing large and alarming amounts of ice now, this may not continue long enough to seal 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, just as the human circulatory system distributes heat and nutrients throughout 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 hot water evaporates and the surface water cools, it becomes saltier and denser. The denser water sinks, and this colder, denser water circulates deeply to the south. Changes in temperature and salinity feed the pumping heart of the system.
If the Atlantic circulation system weakens, this could lead to climate chaos.
Ice sheets consist of fresh water, so rapid release of icebergs into the Atlantic Ocean can reduce the salinity of the ocean and slow heart rate. If surface water can no longer go deep and the circulation collapses, dramatic cooling could occur in Europe and North America. Both the Amazon rainforest and Africa’s Sahel region will become drier, and the warming and melting of Antarctica will accelerate within a few years or decades.
Today, the Greenland ice sheet is melting rapidly, and some scientists worry that the Atlantic current system could be headed for a climate tipping point this century. So is this concern justified?
To answer this, we need to look to the past.
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 examine whether nuclear waste could be safely buried in the deep North Atlantic.
Sediment cores contain the history of everything that accumulated on that part of the ocean floor over hundreds of thousands of years. Heinrich found several layers containing numerous mineral grains and rock fragments from the land.
The sediment grains were too large to be transported to the middle of the ocean by wind or ocean currents alone. Heinrich realized that it must have been brought there by icebergs that collected rocks and minerals during the time when the icebergs were part of the glaciers on land.
The layers, which contain the most rock and mineral debris since a period when icebergs appeared, coincided with a severe weakening of the Atlantic current system. These periods are now known as the 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 accumulation rate of sediments dropped by icebergs. The amount of debris allowed us to estimate how much freshwater these icebergs contributed to the ocean and compare it to today to assess whether history might repeat itself in the near future.
Why is there no chance of closure anytime soon?
So is the Atlantic current system heading towards a climate tipping point due to melting Greenland? We think this is unlikely in the coming years.
While Greenland is currently losing large amounts of ice (comparable to an alarmingly medium-scale Heinrich event), the ice loss will likely not continue long enough to seal the flow on its own.
Icebergs are much more effective at blocking currents than meltwater coming from land; This is partly because icebergs can carry fresh water directly into areas where the current sinks. But future warming will force the Greenland ice sheet to move away from the coast too early to supply enough fresh water with icebergs.
The strength of the Atlantic Meridional Overturning Circulation (AMOC) is projected 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. The Heinrich events, on the contrary, lasted about 200 years.
Instead of icebergs, meltwater pouring off the edge of the island into the Atlantic is predicted to be the primary cause of Greenland’s dilution. Meltwater still sends freshwater into the ocean, but it mixes with seawater and tends to move along the coast rather than directly cooling the open ocean as drifting icebergs do.
This doesn’t mean the stream isn’t at risk
The future trajectory of the Atlantic current system will likely be determined by a combination of slowing but more effective icebergs and accelerating but less effective surface flow. This will be exacerbated 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 shows that the risk is not as imminent as some people fear.
In “The Day After Tomorrow”, the slowing of the Atlantic current system froze New York. Based on our research, we can take some comfort in knowing that such a scenario is unlikely in our lifetimes. However, strong efforts to stop climate change are still necessary to ensure the protection of future generations.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and authoritative analysis to help you understand our complex world. Written by: Yuxin Zhou, University of California, Santa Barbara and Jerry McManus, Colombia University
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Yuxin Zhou received funding from the International Ocean Exploration Program Schlanger Fellowship.
Jerry McManus receives funding from the United States National Science Foundation.