4.22 Intellectual The Intellectual
IPCC AR6 lists the collapse of the Atlantic Ocean's warped circulation as a low-probability, high-impact event | Image source: pixabay.com
Introduction
Today (April 22) is the 53rd World Earth Day, the theme is "Cherish the Earth, Man and Nature Live in Harmony", the initiators of the event aim to evoke the awareness of human beings around the world to love the earth and protect their homeland.
The famous science fiction disaster film "The Day After Tomorrow", released in 2004, tells the story of the sudden collapse of the Atlantic ocean's warped circulation due to global warming, the surrounding regions of Europe and North America near the middle and high latitudes of the North Atlantic, and the world is about to fall into the second ice age. Is it really possible for this scene to be repeated in the future?
This article will explore this issue from scientists' monitoring of the Atlantic Ocean Current System in recent years.
Written by | Tang Haosu
Editor-in-charge | Feng Hao
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In London, England, the average temperature in January in the past 30 years is generally 2 to 7 °C, compared with the average temperature in January in Mohe, Heilongjiang, China, which is at the same latitude, as low as -36 to minus 20 °C. An important reason for this east-west latitude difference is precisely because the Warm North Atlantic Current continues to deliver warm, moist air to Western Europe in winter.
Figure 11948 to 2010 after removing the average east-west latitude of the winter (December to February) average, red indicates warmer, the more red color, the warmer the warmer, blue is the opposite. The North Atlantic Current plays an important role in the warming of Western Europe in winter, and the warming of Western Europe is also affected by the mid-latitude westerly wind belt, so the western part of the northern hemisphere continent is always warmer than the eastern part of the | source[1]
Not only in Western Europe, but also in winter, the North Atlantic Warm Current can also regulate the weather and climate system thousands of miles away and far away in China - the warm and humid air brought by the North Atlantic Current penetrates deep into the Arctic region through the North Atlantic, driving the cold air masses such as the Arctic vortex that originally hovered there to Siberia, the Mongolian plateau and other regions, thereby strengthening the Siberian high pressure, and eventually exacerbating the occurrence of extreme cold and warm events in northern China in winter [2].
Several recent studies [3-5] have shown that the Atlantic Current System is at its weakest state in nearly a thousand years, which may mean that the system is losing stability [6].
The potential collapse of the Atlantic Ocean Current system could have serious consequences on a global scale [7].
First and foremost were the sharp cooling in Western Europe and North America and the rise of sea levels in the Atlantic Ocean. Subsequently, it may trigger a series of chain reactions around the world, or even interrupt the current global warming, so that the entire middle and high latitudes of the northern hemisphere are plunged into a low-temperature glacial period of hundreds or even thousands of years.
Figure 2 The surface temperature anomaly from January to October 2015, with a red (blue) shading indicating that it was hotter (colder) than the average in previous years. The whole world is warming, only the southern part of North Atlantic Greenland is getting colder, a phenomenon known as the "North Atlantic Cold Spot", which may be caused by the weakening of the Atlantic Current System| Source: The Hadley Centre of the British Meteorological Service
The "Achilles Heel" of the Global Climate System
Global ocean circulation can be divided into two main categories: wind circulation and hot salt circulation.
Aeolian circulation is driven by the wind field at the surface of the sea and can affect the flow of seawater as deep as 400 meters below the surface.
The deep circulation driven by the density gradient is called the thermohaline circulation, the density of seawater is determined by temperature and salinity, and the lower the temperature and the greater the salinity, the greater the density. Further, differences in seawater density cause deep-sea flows on a global scale, such as low-temperature, high-salt water masses on the surface of the sea that sink due to their density [8]. Thus, the thermohaline circulation, also known as The Great Ocean Conveyor,[9] controls about 90% of the world's ocean and is a key link in regulating the global climate system.
Figure 3 Schematic diagram of the ocean circulation belt. The world's oceans circulate with each other, and its circulation path is like a "conveyor belt" through the ocean| source: NASA, translated by the author of this article
The Atlantic Ocean Current System is an important part of the Ocean Circulation Belt, and its more academic name is the Atlantic Meridional Overturning Circulation (AMOC), and the "warp direction" in the name is the north-south direction. In the process of flowing northward, the North Atlantic Warm Current will gradually release heat and water vapor to the atmosphere, and it will become a low temperature and high-salt water body. Seawater sinks in the northern North Atlantic as its density increases (becomes heavier), forming deep water in the North Atlantic. The body of water moves southward in the middle and deep oceans, crosses the equator and rises in the southern hemisphere, eventually merging into the Warm North Atlantic Current to form a closed loop [10].
The surface waters of the Atlantic Ocean's warped inverted circulation flow northwards will transport equatorial heat to the middle and high latitudes of the North Atlantic; this process is like the "Achilles heel" of the global climate system, once weakened or even collapsed, the heat transported to the middle and high latitudes of the North Atlantic becomes less, the first to become cold is the nearest Western Europe and North America, and then the entire northern hemisphere will also become cold together, involving the whole body, "the world is cool and hot".
Figure 4 Global ocean energy transfer. A positive abscissa indicates the transport of energy to the northern hemisphere, a negative value indicates a transport to the southern hemisphere, and the ordinate is latitude. The blue segment represents the overall energy transfer of the global ocean, while the purple, red, and green segments represent the contribution of the Atlantic, Pacific, and Indian Oceans, respectively. Due to the existence of the Atlantic Ocean's warped inversion circulation, the Atlantic ocean basin always transmits energy from south to north to the northern hemisphere. For comparison, the world's population consumes about 0.015 PW of energy a year, which is about 1% of the energy transported by the world's oceans| source[11]
Because of its important role in global energy transfer, the Atlantic Ocean's warped loop has also been listed by climate scientists as one of nine key climatic "tipping points" affecting the world.
Figure 5 Nine key critical points affecting the Earth's climate system, with the Atlantic Ocean's entropically flipped circulation as the source of the | shown at point C in the figure[12]
The so-called climate "tipping point", that is, once this point is crossed, major and irreversible climate change is likely to occur. Just as a flat boat with the same leaf floating on the surface of the water, it can still maintain its balance when it first tilts into the water; but when the tilt reaches a certain level, the boat will overturn - the angle of inclination that causes this irreversible consequence is the "tipping point".
Figure 6 The collapse of the tipping point is like a building block game, you don't know which block will be the "last straw to crush the camel" | Source: "Green Sprint"
The "acquired" of humanity
If the Atlantic Ocean's inverted circulation suddenly crossed the climatic "tipping point", what would the world look like?
The famous science fiction disaster film "The Day After Tomorrow" tells such a story: due to global warming, the Atlantic Ocean warped circulation suddenly collapsed, Europe and North America near the high latitude of the North Atlantic were surrounded by glaciers, and the world was about to fall into the second ice age.
Figure 7 Due to the sudden collapse of the Atlantic Ocean's warped circulation, New York was frozen overnight, and the torch in the hands of the Statue of Liberty became a popsicle| Source: The movie "The Day After Tomorrow"
Admittedly, there are many exaggerations in the plot of the movie, and the plot of the overnight arrival of the Ice Age in the film is unlikely to happen in today's real world. However, in geological history, the change of the Atlantic Ocean's inverted circulation did cause dramatic environmental changes like the scene of "The Day After Tomorrow".
About 12,000 years ago, the Earth was in a warming period, the Atlantic Ocean's warped inverted circulation suddenly weakened, the ocean circulation belt closed, global warming was interrupted, and the temperature dropped sharply. The entire event lasted about 1200 years, during which the overall global average temperature dropped by about 6 °C. This global sudden cooling event has been dubbed the "Younger Dryas Event" by posterity.
Figure 8 The "fairy wood" flower, which was originally only able to grow in the polar region, competed to open on the European continent in this global sudden cooling event, indicating that Europe was once as cold as the Arctic during this time, which is why the event is also called the "New Fairy Wood Event" | Source: National Oceanic and Atmospheric Administration
Figure 9 Mammoth restoration, climate cooling is often associated with the extinction of large mammals| Source: Science cover
Using climate models and numerical simulations on supercomputers, contemporary climate scientists have analyzed in detail the "sudden cooling in the context of global warming" dating back to 12,000 years ago.
The mainstream view (ice sheet melting theory, the other is comet impact theory) believes that due to global warming, the high-latitude ice layer in the northern hemisphere (such as the Laurenteide ice sheet on the North American continent) melted in large quantities, resulting in a large amount of fresh water being injected into the North Atlantic; and because the density of fresh water is less than that of seawater, it is difficult to sink to the bottom of the sea, the vertical layer of the ocean is increased, the high-latitude sinking branch of the Atlantic Ocean meridian in the North Atlantic Ocean weakens, further driving the weakening of the entire ocean circulation belt, and the warm seawater in the equatorial region is no longer transported northward, which eventually causes global cooling - "The New Fairy Wood Incident" [13].
Figure 10 simulates global average annual precipitation changes during the "New Nymph Event" simulated by the climate model (compared to its previous warm period), with green indicating wetness and brown indicating drying. The northern hemisphere cooled violently during the "New Fairywood Incident", further causing a decrease in overall rainfall| Source: Literature[14]
In addition, the Arctic warming in the context of global warming at that time was faster than in other parts of the world, resulting in a decrease in the temperature difference between the polar regions and the tropics, which further caused the weakening of the high-altitude westerly currents in the northern hemisphere, which may have exacerbated the sharp drop in temperature in Western Europe and eastern North America during the "New Fairywood Event".
Figure 11 Under the simulated scenario of global average temperature rise of 1.5 °C, 2 °C and 4 °C, the deeper the red color, the stronger the warming, and the Arctic temperature rise is significantly faster than in other parts of the world, which is also known as the "Arctic amplification effect" | source[14]
Not sure now
The Atlantic Ocean's warped circulation spans the entire Atlantic Ocean, making direct observations extremely challenging, and much of the research on it relies heavily on climate model simulations or the use of proxy data. To this end, since April 2004, climate scientists have conducted continuous observations in the 26°N section of the Atlantic Ocean, known as the Rapid Climate Change Programme. The results show that the Atlantic ocean meridian reversal circulation has basically shown a slowing trend during the observation time period.
However, we cannot conclude from this that global warming caused by human activities is responsible for this change, because the directly observed weakening is also affected by the variability of the circulation system itself. It is because of uncertainty that the conclusion that "the Atlantic ocean's inverted circulation changed in the 20th century" was classified as "low confidence" by the Sixth Assessment Report of the United Nations Intergovernmental Panel on Climate Change (IPCC), Climate Change 2021: Natural Science Foundations.
Fig. 12 Schematic diagram of the composition of the RAPID observation array at 26°N in the Atlantic Ocean| source[15]
Figure 13 In addition to the RAPID observation array, there is another international observing system located in the subpolar region of the North Atlantic, called osnap (Overturning in the Subpolar North Atlantic Program) observation project (shown in the red line in the figure) | source[16]
It has also been suggested that, in the current climate context, the weakening of the Atlantic ocean's inverted circulation is not causing global cooling, but is more likely to exacerbate global warming [17].
The study believes that when the Atlantic Ocean is stronger in the inverted circulation, it will transport more low-temperature and high-salt seawater to the high latitude area of the North Atlantic, and the subsequent local sinking will bring more artificial heat on the surface into the deep sea, which will weaken the artificial warming on the surface and act as a "buffer" for global warming. When the Atlantic ocean's warped circulation is weaker, less heat enters the deep ocean, and more heat stays on the surface of the ocean, heating the atmosphere and causing its temperature to rise rapidly, thereby exacerbating global warming.
Figure 141945 to 2017 The change in the intensity of the Atlantic ocean meridian inversion circulation went through the following stages: the strong period from 1945 to the mid-70s, the weakening period from the mid-1970s to the late 1990s, and another strong period from the late 90s to the mid-10s. These three phases coincide with periods of acceleration and stagnation of global warming, suggesting that the Atlantic ocean's warped inversion circulation may also be inhibiting or enhancing global warming| source[17]
However, due to issues such as the reliability of the data sources used on proxy data, this conjecture remains controversial [18]. In the future, climate scientists will need to determine the relative contribution of internal variability, natural forcing, and anthropogenic forcing to the atlantic ocean's warped inversion circulation through direct observations on longer-scale scales.
Determine the future
Although there is still significant uncertainty in the study of the Atlantic warped circulation, one conclusion is certain – if greenhouse gas emissions continue to increase, the system will weaken in the future.
The conclusion that the Atlantic warped reversal circulation will weaken this century under all future emission scenarios projected by climate models is classified as "highly reliable" by the IPCC's Sixth Assessment Report, meaning that the results of all climate simulation tests are highly consistent.
Specifically, assuming global temperature rises stabilize at 1.5 °C, 2.0 °C, and 3.0 °C compared to 1850 to 1900, the Atlantic warped inverted circulation will weaken by 15%, 20%, and 30% over several decades [14], and the complexity of the climate system dictates that the weakening of this local ocean circulation is likely to lead to a wide range of global climate anomalies.
Figure 15 The intensity change of the Atlantic ocean warped circulation under different future warming scenarios simulated by the climate model, the abscissa is time, the ordinate is the intensity, the black and gray lines represent the historical period simulation, and the colored lines represent the simulated | source under different future scenarios[14]
Although the system will weaken, a sudden collapse (moderate reliability) is unlikely to occur in the next hundred years. On the other hand, such a collapse, if it occurs, will most likely lead to a sudden shift in regional weather patterns and water cycles, such as the southward shift of tropical rain bands and the weakening of the African and Asian monsoons. Therefore, the IPCC's Sixth Assessment Report lists the collapse of the Atlantic Ocean's warped circulation as a low probability, high-impact event (black swan event).
Figure 16 Comparison of Atlantic warped circulation in the context of current and future temperature rises, the left chart shows the current climate conditions, and the right figure shows the weakening Atlantic warward flip circulation in the context of future global warming| source[14]
Even if the Atlantic Ocean's warped circulation weakens to collapse in the future, the Earth can still reach a new climate equilibrium. But whether humans can adapt again, we don't know.
"The day after tomorrow" may not be far away, and we need to pay enough attention to the current global warming and put it into action to slow this change by reducing greenhouse gas emissions. After all, the earth does not need to be saved by human beings, and it is human beings who want to save themselves.
Note: Wallace Broecker, the author of the "Ocean Circulation Belt" theory, who died in 2019, was one of the first proponents of the term "global Warming",[19] and is also recognized as one of the greatest earth scientists.
Thanks
Thanks to Zhou Tianjun, a researcher at the Institute of Atmospheric Physics of the Chinese Academy of Sciences, for providing academic guidance for this article.
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Plate editor| Ginger Duck