On the precipice

A major ocean current system that helps regulate climate and weather patterns around the world may come to a halt this century, much sooner than earlier believed. The Atlantic Meridional Overturning Circulation (AMOC), which moves warm ocean water from the tropics to the northern Atlantic Ocean basin, could collapse between 2025 and 2095 due to the impact of anthropogenic emissions, says a July 25, 2023 paper by scientists from the University of Copenhagen, Denmark. Based on the current rate of emissions, the collapse is likely to occur in the 2050s, the scientists say with 95 per cent confidence in the study published in Nature Communications.

If this prediction deems true, AMOC, which plays a crucial role in moderating the climate of Europe and North America and influences temperatures near the Equator, may be the first of the 16 climate tipping elements to be breached. Tipping elements are large-scale systems that influence the planet’s climate and ecology, which are undergoing changes due to warming and accumulation of greenhouse gases in the atmosphere from anthropogenic emissions. If these elements cross certain thresholds due to rise in temperature, changes in them will become irreversible. AMOC’s collapse could also have a cascading impact on the stability of other tipping elements and climate systems in a wider geography.

Slowed cycle 

AMOC transports warm salty surface waters from the tropics to the northern regions. The waters then cool down, increase in density and sink in the North Atlantic Ocean, in a phenomenon called downwelling or overturning. Cold, fresh water from the melting ice of the Arctic is added to the cooler ocean that moves towards the equator. This cycle, therefore, acts like a heat conveyor belt, warming the northern latitudes and cooling the southern latitudes. But for the past few decades, more fresh cold water is added to the North Atlantic Ocean due to increased precipitation and rapid melting of the Greenland ice sheet, which is reducing salinity and density of the water. In other words, the colder layer of the ocean is expanding and the warmer layer is contracting. This is slowing down AMOC and will lead to its collapse.

For the latest study, the authors have assessed sea surface temperatures (SSTs) of a particular region in the North Atlantic Ocean known as the Subpolar gyre region, which is close to the southern coasts of Greenland and Iceland and the northeastern coast of Canada. Collating data for SSTs between 1870 and 2020, they have found that early warnings of the collapse are significant in this “fingerprint” region. Historically, collapse of AMOC has been associated with warming events. According to paleoclimatic data, in the past before the start of the planet’s glacial periods or ice ages, there was abrupt warming. Scientists term these incidents Dansgaard-Oeschger events, which occurred over a few decades before gradual cooling heralds an ice age.

Over the last few decades, AMOC has already slowed down by about 15 per cent and is at its slowest in 1,600 years, a team led by a researcher from Maynooth University in Ireland says in a 2021 study published in Nature Geoscience. Another paper published in Nature Climate Change in September 2021 by Niklas Boers from Potsdam Institute for Climate Impact Research in Germany states that AMOC has lost its stability in the last century and might be much closer to collapse than previously thought. The study concludes this from analysis of eight datasets of temperature and salinity in AMOC dating back 150 years.

The paper also says that according to palaeoclimate data from the last 100,000 years, AMOC has two states: a fast and strong state that has been active for the past few thousand years and a slow and weak one towards which the system now seems to be turning. The analysis shows AMOC could switch between these two states in a few decades.

However, the UN Intergovernmental Panel on Climate Change (IPCC) in its sixth Assessment Report series estimates that AMOC is unlikely to see a halt this century.

The 2023 study cautions that earlier assessments on the collapse may not be accurate, as research on current and future behaviour of the AMOC has a few model biases. These include overestimation of AMOC’s stability and poor representation of the cold deep water current, salinity and water input from melting glaciers in the Arctic region. “There is uncertainty related to the extent of the SST proxy ‘fingerprint’ that we used to get information before direct measurements (which have only been available since 2004). But the fingerprint is in some sense the best we can do,” Peter Ditlevsen, a professor of physics at the Niels Bohr Institute, University of Copenhagen and one of the study’s authors, tells Down To Earth (DTE). He adds that the exact time and nature of the collapse depends upon the world’s success in reducing greenhouse gas emissions. “Reductions will most likely postpone a collapse”, he says.

Climate ripples

A collapsed AMOC would cause widespread cooling across the northern hemisphere and less precipitation in places such as Europe, North America, China and some parts of Russia in Asia. “AMOC is a kind of ‘switch’ for climate in the northern hemisphere, especially Europe. In its current state, with warm water flowing far to the north in the Atlantic Ocean, Europe is warm and the ice sheets over Scandinavia and North America are small. However, if polar water (fresher, cooler water) expands due to more freshwater input (for example, from the melting ice sheets in Greenland), it can initiate a sudden change to a much cooler European climate. This can be self-sustaining because of an expansion of sea ice in the Arctic, keeping the warm water below the surface and covered,” says Ted Scambos, a climate scientist at the Co-operative Institute for Research in Environmental Science, US, who was not invovled in the latest study. The cooling of western Europe and changed precipitation patterns may especially impact food production.

Ditlevsen adds that the collapse could also impact climate over a wider geography. “The heat not transported to the North will also stay in the tropics,” he says. This could disrupt other tipping elements like the Amazon rainforest, West Antarctic ice sheet, and the West African monsoon, as well as the Indian monsoon (see ‘Domino effect’).

First, excess heat due to a collapsed AMOC could lead to less rainfall over the Amazon rainforest and make it drought prone and dry, says Ditlevsen. “There is fear that the collapse could change the hydrological cycle of the Amazon and potentially transform it to a savannah state. This is, however, still quite speculative,” he says. Earlier in June 2021, a paper published in the European Physical Journal claimed that the weakening or collapse of AMOC would lead to stabilisation of the southern Amazon rainforest. It explained that the accumulation of heat post AMOC collapse would make the Inter Tropical Convergence Zone (ITCZ), a band of clouds and storms that circles the Earth near the Equator and is responsible for most of the rainfall in the tropical regions of Africa and Asia, shift southwards and bring more rainfall to southern Amazon.

More ocean heat in the southern hemisphere may also impact the stability and melting of the West Antarctic ice sheet. However, there are still uncertainties to the potential of this impact, says Ditlevsen. A slowdown of AMOC could hinder monsoon formation and rainfall in different regions. “Rainfall in the Sahel region (the West African monsoon) could reduce, the summer monsoon circulation in South Asia and India could weaken; and there might be more winter storms in Europe,” Swapna Palickal, scientist F at the Centre for Climate Change Research, Indian Institute of Tropical Meteorology, Pune, tells DTE.

Palickal has co-authored an April 2020 research study published in Climate Dynamics that says the northern latitudes may, along with the less ocean heat due to AMOC collapse, see more transport of atmospheric heat. This would decrease the large-scale temperature gradient between the northern and southern hemispheres and the regional land-sea thermal gradient. “Weakening of the land-sea thermal gradient weakens the sea level pressure gradient and the summer monsoon circulation over Indian region,” Palickal tells DTE. “However, the projected changes are for the later years of the 21st century, when more warming leads to more availability of moisture,” she adds. As per IPCC’s estimates, moisture in the atmosphere will increase 7 per cent with every 1°C rise in temperature.

“The increase in moisture can counteract the decrease in rainfall by the weakening of AMOC,” she adds. It is therefore uncertain how AMOC’s early collapse, if it occurs, could impact rainfall.

This was first published in the 16-31 August, 2023 print edition of Down To Earth

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