ATLANTIC MERIDIONAL OVERTURNING CIRCULATION
A study published in Nature Climate Change notes that the Atlantic Meridional Overturning Circulation (AMOC) is losing its stability. According to the IPCC’s Report (AR6), it is very likely that AMOC will decline over the 21st century. AMOC is at its weakest in over a millennium. The team studied the evolution of AMOC over the past 1600 years.
About AMOC
- Largest water circulation: AMOC, which is sometimes referred to as the “Atlantic conveyor belt,” is one of the Earth’s largest water circulation systems.
- North-South water movement: In the AMOC system, ocean currents move warm, salty water from the tropics to regions further north, such as western Europe, and send colder water south.
Working Mechanism
- Driven by water density: The AMOC is a large system of ocean currents, like a conveyor belt, driven by differences in temperature and salt content—the water’s density. As warm water flows northwards, it cools, and some evaporation occurs, which increases the amount of salt.
- Deep ocean upwelling: Low temperature and a high salt content make the water denser, and this dense water sinks deep into the ocean. The cold, dense water slowly spreads southward.
- Global distribution: This global process ensures that the world’s oceans are continually mixed, and that heat and energy are distributed around the Earth. This, in turn, contributes to the climate we experience today.
Historical Weakening of AMOC
- Noted in Pleistocene: AMOC and Thermohaline Circulation strength has always been fluctuating. This is noted by looking at the late Pleistocene time period (last 1 million years). The extreme glacial stages have seen weaker circulation and a slowdown in AMOC, while glacial terminations have shown a stronger AMOC and circulation.
- Paleoclimate study: These past fluctuations are known by studying paleoclimate proxies such as sea surface temperatures (SST), salinity, and isotope signatures from single-celled organisms called foraminifera.
- Stable until 19th Century: The team that studied the evolution of AMOC over the past 1600 years suggested that AMOC has been relatively stable until the late 19th century.
- Decline in ocean current: With the end of the Little Ice Age in about 1850, the ocean currents began to decline, with a second, more drastic decline following since the mid-20th century.
- Today its anthropogenic factor: The changes experienced in the last 100-200 years are anthropogenic, and these abrupt changes are destabilizing the AMOC, which could collapse the system.
Reason for weakening of AMOC
- Global warming: Climate models suggest that the AMOC will weaken over the 21st century as greenhouse gases increase. This is because as the atmosphere warms, the surface ocean beneath it retains more of its heat.
- Melting glaciers: The freshwater from melting ice reduces the salinity and density of the water. As this water is unable to sink as it used to, it weakens the AMOC flow.
- Weakening of Gulf Stream: According to some estimates, global warming can lead to weakening of the Gulf Stream System by 34 to 45 percent by 2100.
- Increasing precipitation in the IOR: As the Indian Ocean is warming at a faster rate, it generates additional precipitation. This draws more air from other parts of the world to the Indian Ocean, including the Atlantic. High precipitation in the Indian Ocean thus leads to less precipitation in the Atlantic Ocean. This causes higher salinity in the waters of the tropical portion of the Atlantic. This saltier water in the Atlantic, as it comes north via AMOC, gets cold much quicker than usual and sinks faster.
Importance of AMOC
- Driver of climate system: The AMOC is an important component of the Earth’s climate system and results from both atmospheric and thermohaline drivers.
- Aid in distribution of heat and energy: It aids in distributing heat and energy around the Earth, as the warm water it carries releases heat into the atmosphere and helps in absorbing and storing atmospheric carbon.
- Climate moderator: Western Europe’s climate is less harsh even in winter due to AMOC.
- Warming of Northern Hemisphere: The net northward heat transport in the Atlantic is unique among global oceans and is responsible for the relative warmth of the Northern Hemisphere.
Impact of slowing down of AMOC
- Rainfall decline: Modelling studies have shown that an AMOC shutdown would cool the northern hemisphere and decrease rainfall over Europe.
- Colder Europe: Without a proper AMOC and Gulf Stream, Europe will be very cold.
- El-Nino trigger: It can also have an effect on the El Nino.
- Cooling of Atlantic: AMOC collapse could bring about large, markedly different climate responses: a prominent cooling over the northern North Atlantic and neighboring areas.
- Sea level rise: The northward surface flow of the AMOC leads to a deflection of water masses to the right, away from the US east coast. As the current slows down, this effect weakens, and more water can pile up at the US east coast, leading to an enhanced sea level rise.
- Weaker thermohaline: Freshwater from melting Greenland ice sheets and the Arctic region can make circulation weaker as it is not as dense as salt water and doesn’t sink to the bottom.
- Socio-economic impacts: On agriculture, wildlife, transport, energy demand, and coastal infrastructure.
- For example: One study showed a 50% reduction in grass productivity in major grazing regions of the western UK and Ireland due to weakening of AMOC.
- Severe consequences for Atlantic marine ecosystems: The North Atlantic ecosystems are adapted to the overturning circulation, which sets the conditions for the seasonal cycle, temperature, and nutrient conditions. Changes in these conditions would disrupt fish populations and other marine life.
- Tipping point: If we continue to drive global warming, the Gulf Stream System will weaken further—by 34 to 45 percent by 2100, according to the latest generation of climate models. This could bring us dangerously close to the tipping point at which the flow becomes unstable.
Way Forward
- Evidential study: We urgently need to reconcile our models with the presented observational evidence to assess how far or how close the AMOC really is to its critical threshold.
- Anomalies in study: Moreover, it isn’t clear whether the slowdown of AMOC is caused by global warming alone or if it is a short-term anomaly related to natural ocean variability.
- Better understanding of past AMOC changes: Through the collection and analysis of proxy records that most effectively document AMOC changes and their impacts in past climates.
- Development of climate system models: Incorporating improved physics and resolution, and the ability to satisfactorily represent small-scale processes that are important to the AMOC.