Is the Earth's "Top Stream" about to "collapse"? This time it may be true

In recent years, people have paid more and more attention to global warming. Under the influence of global warming, extreme weather of varying degrees has occurred all over the world. These extreme weathers have not only had a great impact on human production and life, but also posed a great threat to other organisms, the earth's environment and the operation of the earth itself.

Recently, Peter Ditlevsen, a physicist at the University of Copenhagen, and Susanne Ditlevsen, a statistician, published a new study in Nature Communications, stating that if greenhouse gas emissions continue, the Earth's Atlantic Meridional Overturning Circulation (AMOC) - a huge ocean current system carrying tropical warm water northward to the North Atlantic - may collapse in the middle of this century or at any time after 2025, and no later than 2095.

However, the most recent Intergovernmental Panel on Climate Change (IPCC) assessment suggests that the Atlantic Meridional Overturning Circulation is unlikely to collapse completely during the 21st century.

Seeing this, you may wonder, what exactly is the Atlantic Meridional Overturning Circulation? What impact will its collapse have on us? Today we will talk about the Atlantic Meridional Overturning Circulation.

What is the Atlantic Meridional Overturning Circulation?
What impact does it have on the earth's environment?

The Atlantic Meridional Overturning Circulation is an important component of the global "Thermohaline Circulation" and an important component of the climate system. Changes in its intensity can directly affect the heat distribution between the northern and southern hemispheres.

What is the Thermohaline Circulation?

Thermohaline circulation is a type of ocean current caused by the difference in density between different areas of seawater. The density of seawater is determined by its temperature and salinity. When the sea surface is heated unevenly or evaporation and precipitation are uneven, the temperature and salinity of the seawater will change, resulting in uneven distribution of seawater density. The resulting ocean current is called the thermohaline circulation.

There is another type of ocean current, which is the "wind-driven current" that we learned about in high school geography. It is a current formed by wind blowing the sea water.
Thermohaline circulation conveyor belt in the ocean. The red box in the upper left corner is the Atlantic Meridional Overturning Circulation. Image source: National Geographic

Specifically, the Atlantic Meridional Overturning Circulation can transport warm surface water from the Earth's low latitudes to high latitudes, allowing these "warm waters" to cool and infiltrate the cold North Atlantic deep water areas, and then flow back south to complete the heat exchange. Changes in the Atlantic Meridional Overturning Circulation will have a profound impact on the heat transfer in the northern oceans, and therefore will also have a certain impact on the climate of Europe and the North Atlantic.

Large-scale ocean circulation is the main way of redistributing the earth's matter and energy. Cold, salty seawater in high-latitude areas flows to low-latitude areas, and high-temperature ocean currents in low-latitude areas flow to high-latitude areas, redistributing the earth's heat. Among them, the Atlantic Meridional Overturning Circulation sends "hot water" to the North Atlantic, playing a huge role in regulating the climate in Europe and the United States. Thanks to this, the average temperature in Europe in January is 15℃ to 20℃ higher than that in Asia and the east coast of North America at the same latitude.

What are the impacts of the collapse of the Atlantic Meridional Overturning Circulation?

The Atlantic Meridional Overturning Circulation is one of the most critical elements of the Earth's climate system (i.e., a subsystem that switches to an irreversible state). Its possible collapse is one of the most serious concerns for humanity because it could have huge impacts on the climate in the North Atlantic region and around the world.

The last time this kind of sudden climate change occurred was during the last ice age, also caused by the collapse and recovery of the Atlantic Meridional Overturning Circulation, which caused the average temperature in the Northern Hemisphere to fluctuate by 10 to 15°C within a decade, far greater than the current change of 1.5°C within a century.

The latest research results found that the minimum temperature rise that may activate the critical point of collapse of the Atlantic Meridional Overturning Circulation is 1.4°C, the estimated average value is 4°C, and the range is 1.4 to 8.0°C. If the temperature rise exceeds the maximum value of 8.0°C, the Atlantic Meridional Overturning Circulation will completely collapse, which is an irreversible collapse.

By then, the average winter surface temperature in the North Atlantic will drop by 2-3°C within 10 years. The number of storms in Europe will increase, and the average winter temperature will drop as the North Atlantic cools down. The Atlantic sea level in eastern North America will rise, and the Amazon rainforest, which has become a carbon source, will face greater danger. The melting of the Antarctic ice sheet will accelerate. The Earth will usher in a new Little Ice Age.
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Relevant research shows that the Atlantic Meridional Overturning Circulation may collapse in the middle of this century or sometime after 2025. The disaster movie "The Day After Tomorrow" is based on the Atlantic Meridional Overturning Circulation being blocked by melting glaciers as the main plot line, which is chilling. If the Atlantic Meridional Overturning Circulation is closed, India, West Africa, South America and other places will have a very terrible consequence - rainfall in these areas will be interrupted, not only may agriculture collapse, but the land may even turn to arid desertification. This collapse may trigger rapid changes in weather and climate in the United States, Europe and other regions. If this happens, it may cause sea levels to rise in cities such as Boston and New York in the United States, and more severe storms and hurricanes will occur along the East Coast of the United States.

Can the Atlantic Meridional Overturning Circulation be saved?

The Atlantic Meridional Overturning Circulation is key to warming the Northern Hemisphere. It plays a role in the northward transfer of global atmosphere-ocean heat, helping to avoid a sudden drop in temperatures in northwestern Europe. The pace of the possible activation of the climate tipping point of the collapse of the Atlantic Meridional Overturning Circulation is getting closer and faster.

The current global warming has reached 1.1°C, and according to the trend, the United Nations Environment Program (UNEP) has concluded that the temperature will rise by 2.8°C. As pointed out in the 2022 Emissions Gap Report, as climate impacts intensify, the Earth is closing its window - insufficient progress in climate action has forced the urgent transformation of the whole society to become the only option. The world has not yet reached the Paris climate goals, and there is no credible path to reach 1.5°C. Only an urgent system-wide transformation can avoid an accelerated climate disaster. This transformation can only be achieved through action in the power supply, industry, transportation and construction sectors, as well as the food and financial systems, to avoid the closing of the final window.
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Actions that are consistent with global goals are always more powerful and effective than endless arguments and stagnant mentality. In the face of the coming of climate disasters, humans do not need to panic! Today's human civilization will still stick to two lines of defense, scientific defense and cognitive defense. Science and rationality tell people that the earth's carrying capacity limits require the management of the earth's public domain and the need to manage our common home; humans also have clear cognition and can rely on knowledge and technology to do things that are good for both society and the economy. Therefore, using actions to reverse or delay the activation of climate tipping points is a line of defense to defend human civilization.

In addition to the Atlantic Meridional Overturning Circulation, what other climate tipping points are there?

In 2022, the research results of David Armstrong McKay, a climate researcher at the University of Exeter, and others found that there are 16 global climate tipping points, including 9 global tipping points that affect the world and 7 regional tipping points that may have serious regional impacts. Global tipping points may lead to global chain effects, accompanied by additional carbon emissions and higher rates of sea level rise. Regional tipping points may have serious regional or local impacts, such as extreme temperatures, more frequent droughts, forest fires, and unprecedented weather.

Climate tipping point thresholds and their impacts

Mackay et al. calculated the temperature rise required to trigger each critical point, and found that five critical points are already in the danger zone and have been or will soon be broken. The five dangerous critical points are the collapse of the Greenland ice sheet, the collapse of the West Antarctic ice sheet, the collapse of the Arctic permafrost, the disappearance of tropical coral reefs, and the collapse of the Labrador Sea subpolar convection. The critical points for the collapse of the Greenland ice sheet and the West Antarctic ice sheet have been broken. The critical point that triggers the complete disappearance of the Greenland ice sheet is a temperature rise of 0.8°C, while the critical point that triggers the complete disappearance of the West Antarctic ice sheet is a temperature rise of 1°C. In addition to these five critical points, the remaining 11 critical points require a temperature rise of more than 1.5°C to be triggered and are classified as critical points that may be activated.

Global warming tipping point distribution map, image compiled from: David Armstrong Mackay

Can these 'climate tipping points' be changed?

It is not easy to change the current climate tipping points because they have two important characteristics: irreversibility and unpredictability.

1 Irreversibility

Global warming has caused the "climate tipping point" to be breached, further triggering a domino-like positive feedback effect, which may push the world's forests, oceans, ice sheets and other systems into an irreversible death abyss. After crossing the tipping point, climate change may turn into a steeper nonlinear exponential series change. The cumulative time to reach the tipping point may be very long. During this cumulative period of time, efforts to avoid triggering the tipping point are meaningful. Once the tipping point is triggered, the system may quickly flip into a bad result - after the tipping point, the system will enter a new balance, but it will no longer be the original state.

2 Unpredictability

Unpredictability is the most dangerous part - although people know that danger will come, they cannot accurately foresee when the tipping point will arrive. This is a bit like the definition of "disaster system" by anthropologists - we can often predict that dangerous situations will occur, but we cannot predict the exact time of the disaster. When we realize that the tipping point is coming, the tipping point has actually been triggered. You may ask, after the "climate tipping point" is activated, what will the world become after crossing the "climate tipping point"? Our answer is "I don't know".

References
[1] GB 5749-2022 Sanitary Standard for Drinking Water
[2] GB 19298-2014 National Food Safety Standard for Packaged Drinking Water
[3] GB 8537-2018 National Food Safety Standard for Drinking Natural Mineral Water
[4] GB/T 13727-2016 Specification for Geological Exploration of Natural Mineral Water Resources
[5] GB/T 20349-2006 Geographical Indication Product - Jilin Changbai Mountain Drinking Natural Mineral Water
[6] GB/T 10789-2015 General Rules for Beverages
Author: Zhou Bing, Chief Reviewer of Climate Services of China Meteorological Administration; Wang Changke, Researcher of National Climate Center of China Meteorological Administration