Atlantic Meridional Overturning Circulation
The Atlantic Meridional Overturning Circulation (AMOC) is an ocean current system transporting warm, salty tropical waters northwards towards Europe and the Arctic, while cooler deep waters flow southwards in a slower, deep ocean conveyor belt. This heat and water exchange powers the Gulf Stream, regulating vital weather and climate patterns across the North Atlantic region and beyond.
However, human-induced climate change is severely weakening the AMOC system through warming and freshening North Atlantic waters. Scientists warn that the current is at its feeblest in over 1000 years, risking major disruptions to weather systems and ecosystems if it shuts down altogether.
Mechanisms Driving AMOC
The Atlantic overturning circulation is driven by colder, saltier and therefore denser waters sinking in the high latitudes of the North Atlantic around Greenland and Scandinavia. As these deep cold waters sink, they drive compensating flows of warm and saline surface waters moving northwards from the tropics and Southern Hemisphere.
The density contrast caused by temperature and salinity variations is thus central to powering the oceanic conveyor belt. But increased ice melt and warming interfere with these processes – freshwater lowers density while heat reduces sinking.
Significance of AMOC
The northward transport of heat by AMOC warms Western Europe, maintaining a temperate climate much farther north than expected at such high latitudes. Rainfall patterns from the Amazon to Sahel also rely on equatorial heat and water flows powered by sinking cold waters in northern regions.
Additionally, the overturning circulation oxygenates deeper oceans and helps sustain marine ecosystems through nutrient mixing. So a weakening current deprives oceans of oxygen essential to aquatic life.
Evidence of Decline
Analyzing indicators from surface temperatures to deep ocean sediment cores, multiple independent studies have confirmed that AMOC flows are now at their feeblest in over 1000 years.
Observed surface temperatures and salinity changes reveal significantly reduced overturning circulation since the mid-20th century. Paleoclimate reconstructions from marine sediments and Greenland ice cores also track this human-induced decline as unprecedented over millennia.
Risk of Shutdown
If the overturning continues weakening, scientists warn it can reach an irreversible tipping point into collapse. The exact threshold is uncertain but the current decline puts it on the edge.
Once the circulation shutdown tipping point passes, models show a likely sudden steep decline over one to two decades culminating in a near-total halt. This will profoundly disrupt weather systems and ocean ecosystems.
Expected Impacts
A complete AMOC collapse will radically alter rainfall patterns and air temperatures worldwide. Decreased heat transport can shift Europe’s climate towards ice age-like conditions. Tropical rainfall systems may also catastrophically reorganize, with the Amazon forest turning to savanna grasslands which substantially worsens global warming.
Additionally, stalled circulation will prevent ocean mixing and lead to widespread deoxygenation, killing marine life en masse. Sea levels are also projected to rise substantially around American coastlines as currents shift direction.
Such fundamental oceanographic and ecological changes risk triggering irreversible, accelerating feedback loops – including potential tipping points for ice sheet disintegration and mass aquatic extinctions.
Timeframes
While the AMOC tipping point threshold could breach between 2050 and 2070, an eventual complete circulation shutdown has high probability of occurring before 2100 if emissions remain very high. This will unleash the worst-case scenarios of a largely uninhabitable Earth.
Mitigation Efforts
Urgent and cooperative international efforts to radically curb greenhouse gas emissions can delay the tipping point by many decades, providing time to adapt. But governments must act swiftly on evidence that business-as-usual pathways likely sentence the AMOC to collapse by mid-century, precipitating extreme climate chaos.
