Tonga volcano, large eruptions and climate cooling

We're six months after Tonga and yet there aren't signs that it affected the climate. But weren't large volcanic eruptions and their aerosols supposed to have a worldwide cooling effect? Tonga was indeed the largest eruption ever recorded by instruments. It is hence no surprise that people have been speculating whether it would have an impact on the climate. But it didn't, and the much-needed cooling was just wishful thinking. Let's find out how the Tonga eruption differs from those of other large volcanoes.

On January 15, 2022, a then completely unknown volcano in the South Pacific blew up violently. Its name is Hunga Tonga-Hunga Ha'apai. The ash column rose to a height of 48 km, tsunamis rolled across the ocean, the roar was heard as far as Alaska and an atmospheric pressure wave hit monitoring stations worldwide.

Ring of Fire

This submarine volcano is located in the island nation of Tonga, at about 2000 km of New Zealand. The volcano is largely submarine, and only a few small islets showed where the crater's rim is (was). The eruption was extremely violent due to the sudden evaporation of large volumes of seawater and to the chemistry of the magma. Tonga is located in a very active part of the Earth's crust, where one tectonic plate dives below another, forming a so-called subduction zone. The Kermadec-Tonga subduction zone is dotted by volcanoes and links up with the entire Ring of Fire in the Pacific Ocean.

Tonga was an extremely large volcanic eruption. The magnitude of an eruption is expressed by the Volcanic Explosivity Index or VEI, ranging from 1 to 8. Tonga was 5 or 6 depending on the source. Pinatubo in 1991 was a 6. In a heavily populated continent, it would have caused total chaos. At open sea, Tonga caused destruction on the surrounding islands, including the main capital island of Tonga. Several people were killed by the tsunami, e.g. in Peru. Since, tsunami research and preparedness gained momentum again, as described in 'Three ways to survive a tsunami', by geologist Roseanne Chambers, who lives near San Francisco, a geologically sensitive area.

Frankenstein

But beyond the immediate and local effects such as pyroclastic clouds, ashfall and tsunamis, large volcanic eruptions sometimes have a global, long-term impact. Major upheavals in history sometimes synchronized with natural disasters. 

For example, Neanderthals may have disappeared from southern Italy due to an eruption of the Campi Flegrei supervolcano nearly 40 thousand years ago, as described in a previous GondwanaTalks article. There are presumed links between the collapse of Egypt at the time of Cleopatra and an eruption in Alaska. The French Revolution may be an outcome of the Icelandic volcano Laki. Tambora in 1815 was responsible for a year without a summer and a 2 degree drop in temperature in Europe. The horrendous European weather prompted Mary Shelley to write Frankenstein.

All these eruptions had something in common, they caused great disruption, not only locally, but globally.

Predictable

A massive eruption can cause global cooling, eclipse the sun, cause changes in seawater temperature, in the El Niño weather phenomenon and in the water cycle. This in turn has far-reaching consequences, especially because humans, and most lifeforms, prefer predictable weather. 

Since the dawn of humanity and the rise of civilizations after the Last Ice Age, we have been spoiled with weather that is predictable most of the time. We've counted on and prayed for a faithful succession of seasons, year after year. In fact, all of civilization has been based on this predictability since agriculture was invented. 

But when it was too cold, or there was no summer, or no rain, the crops would fail. A few difficult years can be overcome. People can resort to stored food and other food sources. But after several years, unusual weather (especially drought) led more than once to famine and war, migration and major shifts in power, as explained in detail in one of my favourite books: Collapse by Jared Diamond.

Wah Wah

Of all tectonic activities that make earth an active, living, moving planet with earthquakes, tsunamis, mudflows, etc., volcanic eruptions potentially have a much greater impact than all the other disasters. Only an asteroid impact would be worse, but is much less likely to occur. While Tonga was already destructive enough, modern humans have yet to witness the most cataclysmic of volcanic eruptions. I.e., those with a VEI 7 or VEI 8, like Yellowstone, that are several orders of magnitude larger than Tonga, Pinatubo or Vesuvius.

The scale of eruptions is put into perspective in the article Something Important We Know About Volcanoes, also by Roseanne Chambers, in which she mentions eight, still active volcanoes in the world that could cause global disruption far beyond Tonga's. She mentions one of the greatest eruptions ever, based on volume of released magma, Wah Wah Springs in the United States. I had never heard of it but a quick search shows it happened 30 million years ago in Oligocene times, and its magnitude was discovered only in 2013.

Hollywood asteroid

While the possibility of great, unimaginable disasters makes us feel uncomfortable, and it is easier to pretend they won't happen during our lifetime, it is better to be prepared. As Roseanne says in her article: "Only when we face facts do we have a chance of being ready for those events that we hope will never happen, but history tells us they could"; and she cites Benjamin Franklin: "By failing to prepare, you are already preparing to fail".

Hollywood movies show how to kick away asteroids, and space agencies receive huge amounts of money to maybe, one day, do the same. Yet no agency receives money to study the impact of mega-eruptions and design post-catastrophe plans. Similarly, we're not acting fast enough to reverse climate change, which is much easier and cheaper to do than dealing with the aftermath of asteroids or super-eruptions.

Climate model

Tonga was, in terms of explosivity, 500 times more violent than Hiroshima. When scientists realized what the scale of that eruption was, they started to hypothise on its impact on the climate.

Chinese scientists from Beijing (see source below) used a climate model in which they used the latitude of the volcano and the volumes of aerosols (mixture of ash, gas and dust particles, e.g. fog, clouds, mist, smog, smoke, ... ), and their chemical composition, as variables. To test the model, they also used data from 70 eruptions over the last millennium and their climate effects.

Sulfur dioxide in the aerosols

The results show that Tonga's climate impact appears to be negligible. The climate in the northern hemisphere would have dropped only 0.004 degrees Celsius, and in the southern hemisphere a bit more, 0.018 degrees, still not much.

The reason for this is the composition of the stratospheric aerosols. Volcanoes emit a lot of gases, including sulfur dioxide (SO2). When sulfur dioxide comes in contact with oxygen it oxidizes and then sulfuric acid forms. This binds with water and forms droplets that reflect some of the sun's radiation back into space. The planet hence receives less sunlight and cools, albeit temporarily.

Tonga vs Pinatubo

In Tonga, a mass of 0.4 Tg SO2 was released (1 Tg is a billion of kilograms), which is apparently not enough to generate a noticeable effect. By comparison, Pinatubo emitted fifty times as much sulfur and caused a 0.6-degree (Celsius) drop in global average temperature. The effect lasted two years.

Tonga's location also plays a role in its limited impact. Volcanoes in the southern hemisphere mainly affect the climate on that side of the globe, while volcanoes in the northern hemisphere and the tropics have a more generalized impact, as stated in the article by the Chinese climatologists. 

Yet, we should take the general results with a grain of salt. Climate calculations are very complex and there is not enough data regarding volcanoes to make statements with a very high degree of reliability. Yet, Tonga will probably not affect climate in a way that would be of benefit in our battle to curb climate change.

Aerosol injection

On another note, aerosols (human-made) are being investigated as a possible tool to reduce global warming. In theory, the effect of large volcanoes can be mimicked by the engineering technique known as stratospheric aerosol injection. Yet it involves many risks and unpredictable outcomes on the weather. It is interesting however how much we can learn from volcanoes, their impact and their unpredictable behavior. 

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More volcanoes: 

• Campi Flegrei supervolcano: Yellowstone in Europe?
• Mount Saint Helens erupted 40 years ago.
• Geology and dangers of La Palma.
• La Palma: the announced eruption of Cumbre Vieja.
• Etna volcano: an explosive start for 2021.

Article by Kathelijne Bonne, geologist and soil scientist. I also write on Good Climate News, and the Planeetzusjes (Dutch).

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