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The hot magma comes into direct contact with the icy seawater, creating a violent explosion that tears the magma further | Image source: pixab
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Introduction
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At 17:27 local time on January 15, 2022, an undersea volcano (Hunga Tonga-Hunga Ha'apai Volcano) located near the South Pacific island nation of Tonga erupted violently. The eruption affected the entire Trans-Pacific Coast, and many countries issued tsunami warnings.
Written by | Tang Haosu Wei Ke Xu Lulu Wang Yuchen
Editor-in-charge | Feng Hao
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1
How powerful are submarine volcanoes?
Volcanic eruptions are a manifestation of the movement of the Earth's crust and the release of energy from the Earth's interior on the surface. The eruption of The Tonga Volcano on January 15 is probably the strongest eruption of the 21st century to date [1]. Satellite imagery from space captured the process in real time, with huge amounts of volcanic ash and steam being ejected into the atmosphere for up to 20 km (about 17 to 18 km at the top of the tropical troposphere), temperatures at the top of the clouds falling below -100°C, and tsunami waves triggered threatening the trans-Pacific rim [2]. Fiji, Samoa, Vanuatu, Australia around Tonga, Japan on the west coast of the Pacific, the United States, Chile and other countries on the east coast of the Pacific Have issued tsunami warnings.
Figure 1 Explosive eruptions of submarine volcanoes in Tonga, South Pacific | Source: National Oceanic and Atmospheric Administration (NOAA)
Figure 2 RGB image of a volcanic eruption in Tonga, using a satellite infrared channel to detect volcanic ash and sulfur dioxide gas| Source: National Oceanic and Atmospheric Administration (NOAA)
Why did the Tonga volcano suddenly erupt? At present, information is still very limited. But what is certain is that the submarine volcano near Tonga, located in the Pacific Ring Volcano-Seismic Belt (part of the volcanic arc of the Tonga-Komadek Islands), has erupted on several small scales over the past few years (e.g., 2009, 2014/15, 2021), but only recently has it attracted global attention.
The volcano erupting in Tonga is an undersea volcano, so why can't the sea water pour out the magma from the eruption? Normally, if the magma slowly rises into the seawater, a thin layer of steam is formed between the magma and the seawater, and the presence of this insulating layer will allow the outer surface of the magma to cool. However, when magma erupts violently from submarine volcanoes, the process does not work. The hot magma will come into direct contact with icy seawater, creating a violent explosion that tears the magma further, further exposing its inner surface to cold seawater and creating cascading explosions that eventually erupt into the sky, forming huge clouds known as the "fuel-coolant interaction" [3].
Why did local volcanic eruptions, thousands of miles away in the southern hemisphere, cause tsunamis in the East Asian region where we live? This may be due to submarine landslides and pressure disturbances caused by submarine volcanic eruptions in Tonga, which led to local tsunamis, and because the tsunami has a very long wavelength (up to the order of 100 kilometers, part of gravity longwaves), it has been able to spread to East Asia. In addition, the propagation speed of tsunami waves is proportional to the depth of the seawater, when the tsunami propagates to the coast, the depth of the seawater becomes shallower, the propagation speed is reduced, and the back wave catches up with the front wave, which will multiply the wave height and form a huge wave tens of meters high.
Figure 3 Several causes of tsunamis caused by volcanic eruptions: a) underwater eruptions; b) volcanic explosions; c) pyroclastic flows; d) crater collapse; e) near-surface destruction; f) seabed destruction | Source: Literature[4]
2
Will the global climate be affected?
Some people think that volcanic eruptions will reduce global temperature, but this statement has some prerequisites. To affect the global climate, volcanic eruptions require injecting large amounts of sulfur dioxide or other sulfate substances, such as hydrogen sulfide, into the stratosphere. There, they will be converted into sulfuric acid (or sulfate) aerosols, a process that often lasts weeks or months.
The stratosphere, as the name suggests, the air is predominantly horizontal and very stable. With this property of the stratosphere, aerosols formed by local volcanic eruptions will spread across the globe. Sulfate aerosols, which can stay in the stratosphere for years, will enhance planetary albedo, causing more of the Sun's incident short-wave radiation to be reflected back into space, further causing the global average surface temperature to drop, also known as the "umbrella" effect.
In addition to the direct radiative cooling effect described above, sulfate aerosols in the stratosphere can also warm the stratosphere by absorbing surface long-wave radiation and part of the solar short-wave radiation, thereby regulating the atmospheric temperature gradient and further affecting atmospheric circulation (an indirect advection effect). The aerosol's radiative cooling effect interacts with the flat flow effect, resulting in regional cooling and warming of the surface.
The intensity of the two effects also varies in different seasons. For example, in winter in the mid-latitudes of the Northern Hemisphere, the advection effect tends to dominate, and winter warming on the northern hemisphere continents tends to last about 2 years after large tropical volcanic eruptions [5].
Figure 4 Schematic diagram of the climatic impact of large volcanic eruptions| Source: Literature[6], figure Chinese translated by Tang Haosu
Figure 5 In the image above, the black curve represents the optical thickness of the global stratospheric aerosol from 1979 to 2018, which is the most commonly used physical quantity to measure aerosol concentration; the red dot represents the annual emissions of sulfur dioxide from volcanic eruptions. In the image below, the circle represents the sulfur dioxide emissions and estimated eruption altitude measured by volcanic eruption satellites since October 1978| Source:Literature[7]
Overall, the degree of global cooling after a volcanic eruption is directly proportional to the intensity of the eruption. One of the strongest volcanic eruptions in modern times was the 1815 eruption of Tambora in Indonesia, which directly caused the "summerless year" in Europe and North America in 1816, and the subsequent Great Famine in Europe.
In the early days, due to the lack of direct observational means, volcanic eruptions were mainly estimated by rock evolution [8] or ice core acidity measurements [9]. Simulations have estimated that the global cooling from the eruption of Mount Tambora has reached a height of 1°C [10]. In 1978, a satellite-mounted total ozone spectrometer (TOMS) was used to monitor global sulfur dioxide levels, and since then, the sulfur dioxide emissions from volcanoes have been quantified.
From toms instrumental observations[11], the 1982 eruption of El Chichón transported nearly 7 million tons of sulfur dioxide to the atmosphere, causing the global average temperature to drop by 0.3°C [12]. The most recent major volcanic eruption was in Pinatubo, Philippines, in 1991, which delivered nearly 20 million tons of sulfur dioxide to the atmospheric stratosphere, directly contributing to a 0.3–0.5°C drop in global temperatures [14].
In fact, people can not only feel the climatic impact of volcanic eruptions thousands of miles away, but even see it with their own eyes. William Turner, a famous British Romantic landscape painter, once used his brush to record the sky before and after the volcanic eruption. This difference is that after large volcanic eruptions, aerosols in the stratosphere are scattered around the world, and they scatter incident sunlight, giving the sky an orange-red color, which cannot be washed away by even the heavy rain in the troposphere [13].
On the left, Turner's painting The Harbor of Dieppe (1826), which shows the normal sunsets of Europe, and on the right, Turner's Painting The Lake, Petworth: Sunset, Fighting Bucks (1829), showing scarlet skies under the influence of volcanic aerosols | Source: J.M.W. Turner）
So, will the eruption of the Tonga volcano drag the world into a "summerless year" and cause the world to cool? The answer is basically no.
Although the intensity of this eruption is relatively strong, in order to affect the global climate pattern, the amount of sulfur dioxide entering the stratosphere must reach at least millions of tons. Current satellite monitoring data show that the Tonga volcano injects about 400,000 tons of sulfur dioxide into the stratosphere [15], which is not enough to have much of an impact.
The Volcanic Explosivity Index (VEI) of Mount Pinatubo in 1991 was 6, and at least for now, the eruption of Tonga volcano is far from reaching this magnitude (VEI is about 5), and if there is no stronger eruption in the later period, its impact will be more local. The Volcanic Eruption Index is the most commonly used indicator of the size and intensity of volcanic eruptions, its numerical range is between 0-8, and it is a logarithmic indicator, and for each additional unit of the index, the eruption power is 10 times that of the previous level.
Figure 7 Volcanic eruption index| Source: Literature[16]
Another controversial but debatable point is whether this eruption will have an impact on the El Niño-Southern Oscillation (ENSO) event in the equatorial Middle East Pacific.
At the end of the last century, after the eruption of two large tropical volcanoes (Pinatubo in 1991 and Elchjoan in 1982), the world cooled down the following year, and the equatorial Middle East Pacific Ocean turned into an El Niño event a year later. A study using climate models suggests that large volcanic eruptions appear to be able to divert the equatorial Middle East Pacific ocean into An El Niño event by weakening tropical east winds [18].
The relevance of the two remains controversial [19]. First, the uncertainty of the climate model paleoclimate experiment simulation is very large; second, the factors that trigger El Niño events are complex, and various factors are modulated to each other. Therefore, whether the equatorial Middle East Pacific Ocean will enter El Niño at the end of 2022 remains to be further observed in the future.
3
Using "man-made" volcanic eruptions to mitigate global warming?
Since large volcanic eruptions tend to reduce global temperatures, can the increasingly severe global warming be mitigated by injecting sulfate particles into the stratosphere by aircraft to "artificially" create climate effects after volcanic eruptions? Based on this idea, various techniques of Solar Radiation Management (SRM) came into being. These technologies seek to limit or even reverse human-caused global warming by reflecting more solar radiation back into space.
The Idea of solar radiation intervention, commented by the United Nations Intergovernmental Panel on Climate Change (IPCC) in its recently released Sixth Assessment Report, does indeed offset some of the effects of increased greenhouse gases on global and regional climates (high reliability). However, on a regional or seasonal scale, there may be significant uncertainty about the cooling potential of climate change that may be excessively compensated (e.g., areas that would otherwise be warmed due to the extreme weakening of incident solar radiation) or residual (because the area will not warm enough from incident solar radiation). In addition, the stratospheric technical assumptions cannot prevent humans living in the troposphere from continuing to emit carbon dioxide, and thus cannot slow down secondary ocean acidification, sea level rise, etc. (high confidence).
Another visible moral dilemma is that if humanity does one day implement solar radiation intervention, then who will be the "sword bearer" who controls solar radiation? Just as a global nuclear war breaks out, the black smoke (black carbon) from the explosion of nuclear weapons will be injected into the stratosphere, making the surface of the earth where human beings live as cold as winter. If solar radiation interventions are used indiscriminately, the worst consequence may be to bring a "nuclear winter" to the world in another way.
Real-life examples of past eruptions tell us that the "umbrella" effect after a volcanic eruption can only last for a few years, after which global temperatures will gradually recover. Solar radiation interventions therefore require a continuous injection of aerosols into the stratosphere; climate change will occur rapidly and irreversibly if the injection is abruptly terminated in future high-carbon dioxide emission scenarios.
In short, the lack of solar radiation interventions for emission reductions and Carbon Dioxide Removal (CDR) cannot lead humanity out of the current climate dilemma.
Figure 8 Possible consequences of solar radiation intervention| Source: Literature [21], figure Chinese translated by Tang Haosu
Figure 9 observes anomalies in global annual average surface temperature (black line) and land surface temperature (red line), with solid blue lines indicating warming in the early 20th century (1908-1945), mid-20th century cooling (1940-1976), rapid warming at the end of the 20th century (1975-1998) and early 21st century warming stagnation (2001-2012), and purple vertical lines indicating large volcanic eruptions that did not change long-standing global warming trends | Source: Literature[22]. ]
4
Construction of scientific early warning system
After the eruption of the Tonga volcano, the countries along the Trans-Pacific coast issued tsunami warnings for the first time.
For example, Tonga's neighbor, Australia, issued a tsunami warning covering most of Australia's eastern coast in a timely manner. Such a rapid response relies heavily on the Australian Meteorological Agency's Tsunami Monitoring System, which uses tsunami Buoys to monitor tsunami waves from seafloor earthquakes, volcanic eruptions or landslides.
Each sea meter buoy is fixed to an anchor chain and connected to an observation platform thousands of meters below sea level, which can monitor the depth of sea water through pressure sensors and report to the buoy through an acoustic system. After the buoy receives the report, it transmits the signal to land via artificial satellite. These buoys are able to observe and record changes in sea level in real time in the deep ocean, greatly enhancing Australia's ability to monitor and report tsunamis in advance of their borders.
Figure 10 | of the Australian Tsunami Monitoring System Source: Literature[23]
Our neighbor, Japan, which has the highest number of earthquakes/volcanic tsunamis in the world and the worst victims, has established one of the most complete disaster early warning systems in the world. For example, after the Great East Japan Earthquake in 2011, Japan established two dense seafloor observation networks along the Pacific coast: DONET and S-NET. These two submarine observation networks can observe and record tsunami altitudes in real time, enabling tsunami warning through data assimilation.
When an earthquake occurs, the Japan Meteorological Agency estimates the likelihood of a secondary tsunami based on seismic observations. If a catastrophic tsunami is expected to occur in coastal areas, the Japan Meteorological Agency issues a tsunami warning for each area expected to be affected based on the estimated height of the tsunami [24]. In addition, in response to possible volcanic eruptions on Japanese soil, the Japan Meteorological Agency will also issue four types of early warnings according to the severity of the situation. When a person's life has been or may be lost due to volcanic eruption activity, the highest level of "volcanic alert" is issued.
Figure 11 The tsunami signal monitored by the DONET Seabed Observation Network in Japan has an abscissa coordinate of time (January 15, Japan local time) and a vertical coordinate of the height of the tsunami. Source: Japan Disaster Prevention Science and Technology Research Institute | Source: Wang Yuchen
Fig. 12 Japan's most famous ukiyo-e print, "Kanagawa Surf", which is set against the backdrop of Mount Fuji (one of the largest active volcanoes in the world), depicts the huge waves of "Kanagawa Rush" rolling up fishing boats, and the boatmen struggling to survive | Source: Katsushika Hokusai
The mainland is also building its own tsunami warning system. The State Oceanic Administration's Tsunami Warning Center is a national forecasting center responsible for monitoring and forecasting tsunamis and issuing tsunami warnings. In terms of numerical calculation, the early warning center has established a new generation of tsunami numerical forecasting system for the Pacific Ocean and South China, and the numerical forecast of tsunami values in the Pacific, Northwest Pacific and South China Sea can be completed in 5 minutes, 1 minute and 30 seconds respectively. In terms of observation networks, the Early Warning Center recently conducted an in-depth study on the possibility of a tsunami triggered by the Manila Trench earthquake and analyzed the feasibility of tsunami warning buoys.
epilogue
Science fiction writer Liu Cixin wrote in the novel "The Wandering Earth": "At first, no one cared about this disaster, it was just a volcano, a tsunami, the extinction of a species, the disappearance of a city. Until this disaster is relevant to everyone. ”
While the eruption of The Tonga volcano is not expected to have much impact, the succession of extreme weather events around the world is worth pondering. We humans are also part of the Earth system and need to learn to revere nature.
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About the Author
Haosu Tang, PhD candidate, Institute of Atmospheric Physics, Chinese Academy of Sciences; Wei Ke, Associate Researcher, Institute of Atmospheric Physics, Chinese Academy of Sciences; Lulu Xu, Ph.D. student, Department of Earth System Science, Tsinghua University; Yuchen Wang, Researcher, Japan Marine Research and Development Institution (JAMSTEC).
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References: (Swipe up and down to browse)
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Plate Editor | Lucas
This article is reprinted with permission from the WeChat public account: Intellectuals (ID: The-Intellectua), written by Tang Haosu