- Scientists warn that the Atlantic Meridional Overturning Circulation, a system of ocean currents crucial for global climate stability, is weakening faster than previously thought.
- The weakening of the AMOC could trigger extreme weather, disrupt rainfall patterns and harm marine ecosystems.
- Urgent global action to reduce emissions is needed to prevent catastrophic tipping points, writes the author of this commentary.
- The views in the commentary are that of the author.
In October this year, a group of climate scientists wrote an open letter to the Nordic Council of Ministers, expressing their concerns about the serious risk of a major ocean circulation change in the Atlantic ocean. This letter published online and signed by eminent climate scientists, argues that the danger of a weakening ocean circulation in the Atlantic, greatly underestimated even by the Intergovernmental Panel on Climate Change (IPCC), warrants urgent action.
The Atlantic Meridional Overturning Circulation (AMOC), the dominant mechanism of northward heat transport in the North Atlantic, is critical to life conditions for all people in the Arctic region and beyond. The scientists worry that a shift in this circulation could cause major cooling in Nordic countries and affect the entire globe, disrupting tropical rainfall in the Indian Ocean and reducing the ocean’s ability to absorb carbon dioxide. This, they believe, calls for greater urgency in the global effort to reduce emissions as early as possible.
Climate scientists have known long that the Arctic region is the “ground zero” for many global changes. The U.S. government’s National Oceanic and Atmospheric Administration’s (NOAA) 2021 Arctic Report Card showcases the numerous ways climate change continues to alter the Arctic, which recalls the image of an ever-frozen landscape.
The report by NOAA mentions how the increasing heat and the ice loss are transforming this land into a warmer, less frozen one with an unpredictable future trajectory. Studies from another part of ground zero – Greenland – reveal a risky proposition that by 2100, this part of the Arctic will be shedding ice at its fastest rate in the past 12,000 years. To sum up, the Greenland Ice Sheet, the Barents Sea ice, drainage of lakes in the Arctic and boreal lowland permafrost regions, the sub polar deep-water formations and the AMOC, are all becoming increasingly vulnerable to major, interconnected, but non-linear changes.
What is AMOC?
It was probably Wallace S. Broeker, in his now famous 1987 commentary in the journal Nature, Unpleasant Surprises in the Greenhouse?, first mentioned the “flip-flops” of the “Atlantic Conveyor Belt”. Now referred to as the AMOC, it is a system of ocean currents that circulates water within the Atlantic Ocean, from north to south and back, distributing heat, moisture, and nutrients throughout the world’s oceans. We must recognise the ocean as water on the move, controlling our climate, ecosystem, seafood, and ocean currents operating both on the ocean’s surface and in its depths. Winds, water density, and tides drive ocean currents. Earth’s rotation results in the Coriolis Effect, which influences ocean currents.
The AMOC brings warmth to various parts of the globe and plays a crucial role in ocean life. Surface currents are formed by wind and tides in the upper 100 metres of the sea. The slower currents arise from the surface and travel deeper to the ocean floor (average depth 4.6 km). These deep-ocean currents are driven by differences in the water’s density, controlled by temperature (thermos) and salinity (haline) – a process known as thermohaline circulation. It is initiated in the Earth’s Polar Regions where ocean water gets cold, forming sea ice.
Consequently, the surrounding seawater gets saltier because as the sea ice forms, salt from the water is left behind. As the seawater gets saltier, its density increases and starts to sink. Gradually, surface water begins to flow in to occupy the space left by the sinking water. This renewed water supply would eventually become cold, salty and dense enough to sink. As it begins to sink, the deep-ocean currents are initiated, akin to a global conveyor belt that starts in the Norwegian Sea. As warm water is transported to the north, the cooler water sinks and moves south to make room for the incoming warm water. The cold bottom current flows south of the equator down to Antarctica and eventually returns to the surface as warm and dense through mixing and wind-driven upwelling, continuing as a conveyor belt that encircles the globe.
Although the process sounds very dynamic, the entire circulation cycle of the AMOC and the movement of the conveyor belt is quite slow. It is estimated that a parcel of a cubic metre of water would take about 1,000 years to complete its journey along the belt. What makes the AMOC newsworthy is the likelihood of its slowing down further, which will have major global impacts on the climate system. Even though the processes leading to the ‘slowdown’ of AMOC take time because of compounding and interacting effects, like most Earth processes, its non-linear progression to dysfunctionality is less felt until it reaches the tipping point.
Th implications of a slower AMOC
The AMOC carries heat all around the globe and redistributes it from the tropics to the Arctic. Therefore, it has a large role in the evolution of the global climate system. The heat carried by the Gulf Stream to Iceland and Northern Europe results in a more temperate climate in these regions, which does not happen without the AMOC. Similarly, the deep southbound current brings cold water to the Antarctic, and as it wells up to the surface from the depths, it gets enriched in nutrients – a boon for marine life.
These processes are sustainable only if the wholeness of the AMOC is left unimpaired. The scientists suspect that the AMOC will not stay inviolable if any causative parameters change fundamentally. The problem emanates from the rapid melting of ice in the Polar Regions. As the melting of Greenland ice sheets releases large quantities of freshwater, this water is less dense than saltwater would join the North Atlantic Current. As the region gets flushed with water that cannot sink easily, the downward flow towards the ocean floor slows down, resulting in a slower conveyor belt motion that redistributes lesser heat in the “no melt” scenario. This would affect the ocean temperature and the air, and the consequent changes would upset the rainfall patterns.
In effect, a weakened AMOC could lead to colder temperatures in Europe and greater snowfall there in winter. Shifts in precipitation patterns could lead to drought in some areas and flooding in others, impacting agriculture. A weaker AMOC could result in depleted nutrients in seawater and disruption in their distribution- both in content and spatial patterns. This could affect sea life, from plankton and sea birds to fish and whales. Scientists predict far-reaching and unexpected fallouts from the weaker AMOC that might affect the Amazonia, in the far south. Although the impact on vegetation is not well constrained, models predict drier conditions in the north and wetter in the south.
Some researchers have used the silty particles (their deposition implies speed of movement of silt in sediment cores) to reconstruct the flow speed of the AMOC in the past 1,600 years and others have analysed temperature anomalies in the North Atlantic sub polar region, to infer changes in AMOC flow in the past century. These studies conclude that the AMOC has weakened by about 15% since the 1950s, although there is no clarity on when the decline would have started.
What about the future in the age of global warming? Climate models predict that if greenhouse gas emissions continue unabated and the global temperatures keep rising, AMOC could weaken by 30-50 percent at the end of the 21st century. This transformation can generate extreme weather conditions in Europe, alter tropical rainfall patterns, and regional changes in sea levels. A more catastrophic future may lie ahead, even with only a small amount of climate warming, the AMOC may switch from strong to very weak or shut down in decades. This is called a tipping point, and models suggest it could lead to the severe climate impacts outlined earlier. However, the 2021 IPCC consensus view is more optimistic, and their models indicate that a complete AMOC collapse before 2100 is unlikely.
How will a slower AMOC impact Asian weather and the Indian Monsoon, which are linked with the Intertropical Convergence Zone (ITCZ)? The position of the ITCZ location affects the distribution of tropical rainfall and is a fundamental component of the global monsoon. A strong AMOC, as it happens in normal conditions, transports more heat northward in the ocean and consequently controls atmospheric energy flow across the equator shifts southward, pushing the ITCZ north of the equator.
New research from IIT Bhubaneswar comments on the implications of a slowing AMOC on the ITCZ and the Indian Monsoon. The study concludes that as the climate warms and the AMOC is expected to slow down, it would reduce the northward movement of warm water and energy. This would push the ITCZ to shift southward, potentially weakening the Indian summer monsoon in the future.
Returning to Waly Broeker’s paper, he warns, “We play Russian roulette with climate, hoping that the future will hold no unpleasant surprises. No one knows what lies in the active chamber of the gun, but I am less optimistic about its contents than many.”
The restoration course is amply clear: reduce emissions as quickly as possible to stay close to the 1.5ºC target set by the Paris Agreement. Are the decision makers at COP29 listening?
The author is an adjunct professor at the National Institute of Advanced Studies, Bengaluru, and the director of the Consortium for Sustainable Development, Connecticut, U.S.A.
Banner image: Scientists place a portable climate observatory, across an open-water lead in the Arctic in December 2020. Image by Lianna Nixon-Alfred-Wegener Institut via Wikimedia Commons (CC BY 4.0).
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