Volcanism – volcanic eruptions and climate change – huge sulfur throwers

When the Mount Pinatubo volcano erupted in the north of the Philippines on June 15, 1991, it not only had devastating consequences for thousands of people in its neighborhood. The consequences of the world’s strongest eruption in a long time were felt around the globe. 17 million tons of sulfur dioxide were thrown high into the atmosphere and spread around the globe. This was the largest amount of sulfur ever observed in a volcanic eruption. The violence of the eruption caused ash and gases to be ejected far into the stratosphere, the layer of the atmosphere that begins in the tropics at an altitude of around 18 kilometers above the ground.

Unlike in the lower atmosphere, it is very dry there and the aerosols, the small suspended particles that form from sulfur dioxide, are not washed out by rain. Accordingly, they can stay in the air for up to four years and absorb some of the sunlight. The planet gets a pair of sunglasses, so to speak. The effect of this reduced solar radiation was felt worldwide: in 1992, global temperatures fell by around half a degree Celsius lower. That may not sound dramatic in times of global warming, but cooler temperatures and less sunlight had a noticeable effect on crops. The global corn harvest was nine percent lower, while wheat and soybean yields each fell by five percent.

That could have been a wake-up call. Since humans have been living on agriculture and livestock farming, they have been dependent on stable climatic conditions, and in fact, nature has usually treated them very leniently. At least compared to other eras. The last ten thousand years, the so-called Holocene, had very few climatic surprises compared to the previous ice and warm ages.

Santorini eruption with consequences

Now humans are in the process of leaving the Holocene comfort zone with their greenhouse gas emissions, but that’s another story. The point here is rather that even in the stable Holocene, civilizations have repeatedly collapsed over the last 4,000 years due to climate changes. For example, the Indus culture and the Khmer empire or the Mayan empires and the ancestors of the Pueblo culture in Central and North America. Sometimes there were self-made environmental changes, sometimes droughts with natural causes and sometimes severe volcanic eruptions, such as the Santorini eruption on the Greek island of the same name in the 16th or 17th century before the beginning of the common era, which is probably associated with the decline of the Minoan culture on Crete can be related.

In the fast-moving media age, Mount Pinatubo has long since been forgotten and volcanic eruptions are usually only perceived as local disasters, such as the 2021 eruption at Cumbre Vieja on the Canary Island of La Palma, whose lava slowly escaping over several months destroyed 1,600 buildings. Now the volcanoes of the Canary Islands are so-called hotspot volcanism. Its lava rises from deep within the earth, contains few gases and is therefore usually not particularly explosive. However, exceptions prove the rule, as Eyjafjallajökull in Iceland showed in 2010. This ejected volcanic ash high enough into the atmosphere to show the world that a local event can quickly have global effects. Air traffic was disrupted throughout the northern hemisphere and was initially completely paralyzed in Europe for several weeks, then for days in various regions.

Island arches with risk of explosion

However, volcanoes like Mount Pinatubo play in a different league, one far more explosive than the hotspot volcanoes or those that gently push the plates of the earth’s crust apart on the oceanic ridges beneath the sea surface. These are pushed together elsewhere, with either large mountains like the Alps and the Himalayas rising up, or oceanic plates being pushed under continental plates. The latter is the case on the island arcs of Japan, the Philippines and Indonesia or along the western coasts of the two Americas. The more explosive volcanoes can be found there.

Experts call these regions subduction zones, which are characterized by particularly active and generally disastrous volcanism. Along with the rocks of an oceanic plate, large amounts of moist sediments also descend into the Earth’s interior. Extreme heat caused by friction and high pressure causes this mixture, which has been pushed several dozen kilometers underground, to partially melt and collect in magma chambers, sometimes just a few kilometers below the surface. From time to time these then empty, with the trapped super-hot water vapor making the eruptions particularly explosive.

A year without summer

Mount Pinatubo is one such explosive island-arc volcano, but its eruption was by no means the most severe in historical times. Events like the one in the Philippines in 1991 can happen every few decades. The eruption of Mount Tambora in neighboring Indonesia in 1815 was ten times as powerful. At the time, this ejected so much sulfur into the atmosphere that the northern hemisphere cooled by an average of around one degree Celsius. It is said that 1816 was a year without summer. The year 1817 was also very cold in North America and Europe and brought correspondingly poor harvests. Food prices doubled, which in turn led to social unrest in various European countries. The young USA plunged into its first recession, and in India, fueled by growing need, a cholera epidemic broke out that became a global pandemic. In total, several tens of millions of people probably died as a result of the indirect consequences of the volcanic eruption, i.e. several percent of the world population at the time, which was around a billion people at the time.

Recently, a commentary in the scientific journal Nature recalled the Tambora eruption and pointed out that there is a one in six chance that an eruption of this magnitude could happen again this century. If you consider the serious consequences that this would have in today’s already crisis-ridden world that is highly dependent on international food trade, this is not exactly a small risk. Especially since food production is already under considerable pressure due to the climate crisis and the ongoing loss of biological diversity – just think of the dangerous decline in bees, which are important for pollination.

What makes matters worse is that the risks have so far only been estimated very imprecisely. The authors of the commentary, who work in climate science in Geneva, Grenoble and London, urgently need to do more research in various fields. So it is not necessarily clear how much sulfur and the aerosol formed by it should be expected. Atmospheric observation data has only been available since 1991. Older data must be determined from deposits and inclusions in the ice at the poles and also linked to climate data from the same time. Even for the Tambora eruption, it is not known for sure in what quantities and in what sizes aerosols were formed. Information about size is important because smaller particles deflect sunlight much better and therefore cool the earth more.

Volcanism in a changing climate

What is unclear and by no means trivial is what effect volcanism will have in a warmer world. For example, global warming means that stratification in the oceans is increasing. Since the warm water lies on top of the colder water, the warming reduces the exchange. In the event of an abrupt cooling due to a volcanic eruption, this would probably result in less cool water sinking and the atmosphere therefore cooling down somewhat more, the authors write.

It can also be assumed that the ejection from the volcanoes will rise higher if it reaches the stratosphere. This is because it cools down in a warmer world, meaning the hot gases and ash would have more buoyancy. Changing flow conditions between the lower troposphere and the stratosphere will probably shorten the residence time of aerosols on the one hand, but will favor smaller particles on the other. Finally, changes in air chemistry caused by climate change can also contribute to the formation of aerosols. Some work suggests that the sum of these factors could mean that large volcanic eruptions lead to greater cooling in the future. But in order to say this with greater certainty, more simulations with climate models and more data are urgently needed, the authors appeal.

After the eruption of Tambora in 1815, the northern hemisphere cooled by around one degree Celsius.

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