8,200 years ago, even as the world was warming after the last ice age, the Northern Hemisphere experienced a sudden and dramatic cooling. Temperatures dropped up by to 14 degrees Fahrenheit in less than two decades, affecting weather patterns worldwide and sea levels in the Atlantic Ocean. We know this from a 2003 study commissioned by the Pentagon. U.S. Geological Survey (USGS) researchers analyzed Greenland ice cap cores and sediments in the Chesapeake Bay. They showed that, paradoxically, a warming earth triggered rapid global cooling. They attributed this to melting glaciers discharging so much fresh water into the sea that ocean currents were disrupted.

Could this happen again? Some scientists believe we’re already seeing something similar, based on recent changes to the Gulf Stream. 

The Gulf Stream is a key component of the Atlantic Meridional Overturning Circulation (AMOC). The AMOC is like an oceanic conveyor belt, transporting cold, deep water from the North Atlantic Ocean toward the equator, and warm surface water — in the form of the Gulf Stream — from equatorial to high latitudes. This cyclical current flow redistributes heat and helps keep northern land masses, especially in Europe, warmer than they would otherwise be. The circulation is enhanced by seasonal changes around Greenland and northern Canada. When the high-latitude seas freeze, the cold, salty, dense water below the ice sinks and flows toward the south. The northward-flowing Gulf Stream completes the cycle.

If anything were to disrupt this circulation, the Gulf Stream’s transport of warm water to higher latitudes could slow and be redirected. The 2003 study hypothesized that massive melting of the Greenland ice cap and Antarctic glaciers from human-caused climate change could trigger such an outcome, similar to the events of 8,200 years ago. In fact, subsequent research has shown that the AMOC is now weakening and that the Gulf Stream is indeed slowing. The presumed culprit: runoff from melting Greenland glaciers. That fresh, low-density water forms a layer above the denser seawater and freezes sooner than seawater. This impedes the sinking and southward movement of cold, salty water and, in return, the northward flow of the Gulf Stream. 

Until very recently, most scientists believed that a collapse of the AMOC wouldn’t occur in this century. But a 2023 paper in Nature Communications predicted that the AMOC is near a tipping point and that its collapse will likely occur in the next few decades — and perhaps even in the next few years. If so, the consequences for Europe and North America could be catastrophic. They could include plunging temperatures in Europe, rapidly rising sea levels in eastern North America and Chesapeake Bay, and agriculture-threatening weather changes worldwide.

The Nature paper is by no means the last word. Many scientists are not convinced that the AMOC will collapse in this century — if at all. Others are persuaded that the circulation is weakening, but not on the brink of collapse. Some oceanographers believe that recent changes in the Gulf Stream’s flow reflect natural variability, not changing climate. One thing is certain: the causes and potential consequences of a weakening or collapsing AMOC are still poorly understood. 

We must gain much greater understanding of the links between Atlantic Ocean circulation and climate, and we must do it soon. This is particularly important for Chesapeake Bay. Already the sea is rising faster in the mid-Atlantic region than almost anywhere else in the country. Scientists estimate that Chesapeake Bay’s mean sea level will increase by at least a foot in the first half of the 21st Century and by as much as four feet by 2100. Eighteen million people now reside in the estuary’s watershed; 10 million of those live close to the shore. If the Bay’s waters continue to rise as predicted, much of that inhabited area will be flooded. If water levels rise even faster, because of an AMOC collapse, … well, you can finish the sentence.

Henry S. Parker is a writer and scientist. He can be reached at [email protected].

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Henry S. Parker is an adjunct associate professor at Georgetown University. He can be contacted at [email protected]