A shutdown or slowdown of the thermohaline circulation is a hypothesized effect of global warming on a major ocean circulation.
A 2015 study suggested that the AMOC has weakened by 15-20% in 200 years.
Don Chambers from the University of South Florida College of Marine Science mentioned, "The major effect of a slowing AMOC is expected to be cooler winters and summers around the North Atlantic, and small regional increases in sea level on the North American coast." James Hansen and Makiko Sato stated,
AMOC slowdown that causes cooling ~1°C and perhaps affects weather patterns is very different from an AMOC shutdown that cools the North Atlantic several degrees Celsius; the latter would have dramatic effects on storms and be irreversible on the century time scale.
A 2017 review concluded that there is strong evidence for past changes in the strength and structure of the AMOC during abrupt climate events such as the Younger Dryas and many of the Heinrich events.
Lohmann and Dima 2010 found a weakening of the AMOC since the late 1930s. Climate scientists Michael Mann of Penn State and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research suggested that the observed cold pattern during years of temperature records is a sign that the Atlantic Ocean's Meridional overturning circulation (AMOC) may be weakening. They published their findings in 2015, and concluded that the AMOC circulation showed exceptional slowdown in the last century, and that Greenland melt is a possible contributor, with the slowdown of AMOC since the 1970s being unprecedented over the last millennium.
A study published in 2016 found further evidence for a considerable impact from sea level rise for the U.S. East Coast. The study confirms earlier research findings which identified the region as a hotspot for rising seas, with a potential to divert 3–4 times in the rate of rise, compared to the global average. The researchers attribute the possible increase to an ocean circulation mechanism called deep water formation, which is reduced due to AMOC slow down, leading to more warmer water pockets below the surface. Additionally, the study noted, "Our results suggest that higher carbon emission rates also contribute to increased [sea level rise] in this region compared to the global average."
Global warming could, via a shutdown of the thermohaline circulation, trigger cooling in the North Atlantic, Europe, and North America. This would particularly affect areas such as the British Isles, France and the Nordic countries, which are warmed by the North Atlantic drift. Major consequences, apart from regional cooling, could also include an increase in major floods and storms, a collapse of plankton stocks, warming or rainfall changes in the tropics or Alaska and Antarctica, more frequent and intense El Niño events due to associated shutdowns of the Kuroshio, Leeuwin, and East Australian Currents that are connected to the same thermohaline circulation as the Gulf Stream, or an oceanic anoxic event — oxygen (O
2) below surface levels of the stagnant oceans becomes completely depleted — a probable cause of past mass extinction events.
Hansen et al. 2015 found, that the shutdown or substantial slowdown of the AMOC, besides possibly contributing to extreme end-Eemian events, will cause a more general increase of severe weather. Additional surface cooling from ice melt increases surface and lower tropospheric temperature gradients, and causes in model simulations a large increase of mid-latitude eddy energy throughout the midlatitude troposphere. This in turn leads to an increase of baroclinicity produced by stronger temperature gradients, which provides energy for more severe weather events.
Many of the most memorable and devastating storms in eastern North America and western Europe, popularly known as superstorms, have been winter cyclonic storms, though sometimes occurring in late fall or early spring, that generate near-hurricane-force winds and often large amounts of snowfall. Continued warming of low latitude oceans in coming decades will provide more water vapor to strengthen such storms. If this tropical warming is combined with a cooler North Atlantic Ocean from AMOC slowdown and an increase in midlatitude eddy energy, we can anticipate more severe baroclinic storms.
Hansen et al. results at least imply that strong cooling in the North Atlantic from AMOC shutdown does create higher wind speed. The increment in seasonal mean wind speed of the northeasterlies relative to preindustrial conditions is as much as 10–20 %. Such a percentage increase of wind speed in a storm translates into an increase of storm power dissipation by a factor ∼1.4–2, because wind power dissipation is proportional to the cube of wind speed. However, the simulated changes refer to seasonal mean winds averaged over large grid-boxes, not individual storms.
In April 2004, the hypothesis that the Gulf Stream is switching off received a boost when a retrospective analysis of U.S. satellite data seemed to show a slowing of the North Atlantic Gyre, the northern swirl of the Gulf Stream.
In May 2005, Peter Wadhams reported in The Times (London) about the results of investigations in a submarine under the Arctic ice sheet measuring the giant chimneys of cold dense water, in which the cold dense water normally sinks down to the sea bed and is replaced by warm water, forming one of the engines of the North Atlantic Drift. He and his team found the chimneys to have virtually disappeared. Normally there are seven to twelve giant columns, but Wadhams found only two giant columns, both extremely weak.
In 2005 a 30% reduction in the warm currents that carry water north from the Gulf Stream was observed from the last such measurement in 1992. The authors noted uncertainties in the measurements. Following media discussions, Detlef Quadfasel pointed out that the uncertainty of the estimates of Bryden et al. is high, but says other factors and observations do support their results, and implications based on palaeoclimate records show drops of air temperature up to 10 °C within decades, linked to abrupt switches of ocean circulation when a certain threshold is reached. He concluded that further observations and modelling are crucial for providing early warning of a possible devastating breakdown of the circulation. In response Quirin Schiermeier concluded that natural variation was the culprit for the observations but highlighted possible implications.
In 2008, Vage et al. reported "the return of deep convection to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008," employing "profiling float data from the Argo program to document deep mixing," and "a variety of in situ, satellite and reanalysis data" to set the context for the phenomenon. This might have a lot to do with the observations of variations in cold water chimney behaviour.
In January 2010, the Gulf Stream briefly connected with the West Greenland Current after fluctuating for a few weeks due to an extreme negative phase of the Arctic oscillation, temporarily diverting it west of Greenland.
Heat is transported from the equator polewards mostly by the atmosphere but also by ocean currents, with warm water near the surface and cold water at deeper levels. The best known segment of this circulation is the Gulf Stream, a wind-driven gyre, which transports warm water from the Caribbean northwards. A northwards branch of the Gulf Stream, the North Atlantic Drift, is part of the thermohaline circulation (THC), transporting warmth further north to the North Atlantic, where its effect in warming the atmosphere contributes to warming Europe.
The evaporation of ocean water in the North Atlantic increases the salinity of the water as well as cooling it, both actions increasing the density of water at the surface. Formation of sea ice further increases the salinity and density, because salt is ejected into the ocean when sea ice forms. This dense water then sinks and the circulation stream continues in a southerly direction. However, the Atlantic Meridional Overturning Circulation (AMOC) is driven by ocean temperature and salinity differences. But freshwater decreases ocean water salinity, and through this process prevents colder waters sinking. This mechanism possibly caused the cold ocean surface temperature anomaly currently observed near Greenland (Cold blob (North Atlantic)).
An AMOC shutdown may be able to trigger the type of abrupt massive temperature shifts which occurred during the last glacial period: a series of Dansgaard-Oeschger events – rapid climate fluctuations – may be attributed to freshwater forcing at high latitude interrupting the THC. 2002 model runs in which the THC is forced to shut down do show cooling – locally up to 8 °C (14 °F).
Recent studies (2017) suggest potential convection collapse (heat transport) of the subpolar gyre in the North Atlantic, resulting in rapid cooling, with implications for economic sectors, agriculture industry, water resources and energy management in Western Europe and the East Coast of the United States. Frajka-Williams et al. 2017 pointed out that recent changes in cooling of the subpolar gyre, warm temperatures in the subtropics and cool anomalies over the tropics, increased the spatial distribution of meridional gradient in sea surface temperatures, which is not captured by the AMO Index.
Based on coupled Atmosphere-Ocean General Circulation Models from 2001, the THC tends to weaken somewhat rather than stop, and the warming effects outweigh the cooling, even over Europe. In the IPCC Fifth Assessment Report, it was reported that it is very unlikely that the AMOC will undergo a rapid transition (high confidence).
The film The Day After Tomorrow exaggerates a scenario related to the AMOC shutdown.
Kim Stanley Robinson's science-fiction novel Fifty Degrees Below, a volume in his Science in the Capital series, depicts a shutdown of thermohaline circulation & mankind's efforts to counteract it by adding great quantities of salt to the ocean.
In Ian Douglas' Star Corpsman novels, an AMOC shutdown triggered an early glacial maximum, covering most of Canada and northern Europe in ice sheet by the mid-22nd century.