It’s happened before in the Earth’s past. The Gulf Stream is slowing down.
This NASA image depicts the current which is part of a tangle of currents collectively called the Atlantic Meridional Overturning Circulation, or AMOC. If the slowing trend continues it may have major consequences.
The northern branch of the Gulf Stream is a giant heat engine, transporting tropical waters’ heat to northern latitudes. It’s estimated this current delivers to Scandinavia 78,000 times its annual energy use. Its moderating impact is greatest for Greenland and western Europe. The transport of the tropical warmth makes London far more temperate, at a latitude of 51 degrees north. Oslo has cold winters, but imagine a city at more than 59 degrees north without help from the Gulf Stream. (By comparison, Buffalo is at just under 43 degrees north.)
Evidence is strengthening that the AMOC and its Gulf Stream tentacle are slowing down. The culprit appears to be a warming climate, with the fastest warming occurring at high Arctic latitudes as predicted even in early climate models. The Arctic warms faster because the thinning and disappearing Arctic sea ice, which reflects much of the sun’s heat energy back into space, gets replaced by dark ocean waters which instead absorb the energy which used to be reflected. This rapid warming is sending glacier freshwater runoff into the sea as well as Greenland ice cap melting runoff. The huge influx of freshwater dilutes the salinity of North Atlantic water. How much dilution occurs is critical.
Paleoclimatologists, using ice core and sediment samples, know the Gulf Stream slowed down drastically several times after the Earth’s last ice age, which peaked around 22,000 years ago. There is also evidence the weakening occurred over a period of just a few decades. A new study published last week in Nature Geosciences focuses on the current apparent weakening, and is one of several newer studies gauging the speed and amplitude of the weakening. There is much new data because oceanographers have been able to string a virtual necklace of sensors around the Atlantic to get a far more detailed fix on the state of the AMOC and its many tentacle currents.
Some – not all – leading oceanographers are worried the newer higher resolution data suggests the weakening is on a trend of acceleration. As described by scientists in a New York Times article, the weakening could lead to faster sea level rises in the Atlantic basin, as well as a buildup of excess heat energy in tropical Atlantic waters. That could intensify hurricanes and make them more likely in the southeast U.S. It could also spread more severe drought and famine in the Sahel region of Africa. In the Times, one such oceanographer commented: “We’re all wishing it’s not true,” Peter de Menocal, a paleoceanographer and president and director of the Woods Hole Oceanographic Institution, said of the changing ocean currents. “Because if that happens, it’s just a monstrous change.”
The diluted salinity from fresh meltwater brings the risk of slowing or stopping the turnover in North Atlantic currents which sends sinking cooler waters back south to the tropics to be reheated and transported north again. Less salty water is more buoyant, and therefore doesn’t sink as readily. When this happens, the formation of a “cold blob” in the North Atlantic becomes more apparent, as seen in this NASA image.
If the cold blob sits in place instead of feeding the return current to the tropics, the entire AMOC slows down or may even stop. That would be part of a climate tipping point which produces regional cooling in an otherwise warming world. Such cooling would be far more than merely ironical. It would spell harsh cold returning to western Europe. The basic mechanism is well illustrated in this AccuWeather diagram.
A primary point of concern for many climate and ocean scientists is the rapidity with which a sharp slowdown or shutdown has occurred in the past, 12,800 years ago. The paleoclimate data shows a rapid period of warming (obviously not caused by humans such as is the current warming) took just a few decades for glaciers to retreat, forming a blocking cold blob. In just a few decades, Greenland and much of western Europe was plunged back into Arctic cold, with mean temperatures dropping 18 degrees Fahrenheit for about 1,300 years. This drastic regional cooling then retreated even more rapidly than it had advanced.
Until the 1990s, it was largely thought such drastic changes took much longer to progress, but the abundant paleoclimate data proves there are some mechanisms which can set up with startling and dangerous speed. That such a dramatic cooling is triggered by a warming climate is a point of alarm. Greenland’s ice cap is now melting at six times the upper range of projected melting, according to NASA.
There are caveats to these scenarios. There is disagreement as to how much AMOC and the Gulf Stream are actually slowing. Data in 2009-2010 detected an important increase in the rate of slowing, and alarm bells went off because that rate of slowing, if it continued, would cause huge climate impacts more quickly. However, the rate of AMOC slowing, in fact, itself slowed down in the following decade for several years. It now appears to be accelerating again. This may be because the subpolar North Atlantic is now less salty than at any time in the last 120 years, according to a study in Nature last year.
While there are some skeptics on the rate of AMOC slowing, there is also evidence of probable impacts from the already-present cold blob which were not predicted. The U.K. National Oceanography Centre linked the cold blob with extreme heat waves in Europe during 2015 and 2018 because, they stated, the jet stream was dipping south and detouring around the blob. That dip delivered North African and tropic heat to the continent. None of this had been showing up in climate models.