What happens to the atmosphere when volcanoes erupt?
Can volcanic eruptions lead to climate change?

In 2010 the eruption of Eyjafjallajökull in south Iceland brought air traffic in northern Europe to a standstill for almost seven days. Now, the Bárðarbunga volcano, which has erupted beneath the Dyngjujökull glacier in central Iceland, is being intensively studied by scientists.

On average, there are around 50-60 volcanic eruptions around the world each year. When volcanoes erupt, they can emit huge volumes of gases, aerosols, and volcanic ash into the stratosphere (part of the atmosphere at around 10-45 km altitude http://climatica.org.uk/climate-science-information/earth-system). With all of this material, we might expect there to be major changes in atmospheric composition and air circulation. A logical question, then, is whether these injections of volcanic material can contribute to climate change. To test this out, we can explore the different types of material that are emitted in a volcanic eruption. We can also look to past examples of volcanic eruptions, where scientists have collected detailed measurements of environmental conditions before and after an eruption event.

 

Volcanic material

Volcanic ejecta (the umbrella term used to refer to all matter emitted from a volcano) comes in a number of forms, and is often classified on the basis of size:

 

  • Large boulder-sized material is often called a ‘block’ or a ‘bomb’,
  • Pebble-sized material is called ‘lapilli’,
  • And very fine material is called ‘ash’.
  • Gases are also emitted during the eruption process.

 

The large material (blocks/bombs and lapilli), which is made of molton or solid rock, is dense and quickly falls back to earth. It is the finer material that we are interested in here.

 

The fine ash is projected up into the stratosphere, but eventually falls back to earth – after several days or weeks. On the other hand, volcanic gases such as sulphur dioxide and carbon dioxide, which are also injected high into the stratosphere can lead to major changes in the atmosphere. It is these gases that have the potential to contribute to global climate changes. In particular, when sulphur dioxide is introduced into the atmosphere it combines with water and oxygen to form sulphuric acid aerosols. When these aerosols are present in the atmosphere, they reflect more of the Sun’s radiation back in to space, and can cause the atmosphere to cool for several years. This mechanism is one of the most important impacts of volcanos on the earth’s climate.  A number of eruptions over the last century or so have caused such volcano-induced cooling. Let’s take a look at some examples…

 

 

Volcanic eruptions

 Krakatau, Indonesia – 1883

This eruption remains one of the largest eruptions ever documented. The explosions were so violent that they were heard over 4,500 km away (that’s the same as the distance between New York and Italy). In fact, when the eruption had finished, only a third of Krakatau remained above sea level. Fine ash and aerosols are thought to have been thrown as high as 50 km up into the atmosphere, and spread around the equator in two weeks. Diaries and record books from that time report spectacularly bright red an orange sunsets for several years following the eruption. All of this material caused more solar radiation to be reflected back into space, lowering  global temperatures by up to 1.2°C. Temperatures did not return to their pre-eruption levels for around 5 years.

Mount Pinatubo, The Phillippines – June 15, 1991

This was one of the largest eruptions of the twentieth century. Estimates suggest that around 15-20 million tons of sulphur dioxide were ejected into the stratosphere. The cloud was the largest ever observed by satellites since satellite observation began in 1978. In fact, it had covered the entire planet within a year. All of this gas caused one of the largest transformations in the stratospheric composition in recent times, and a reduction in global temperatures of around 0.7°C.

 

 

How do volcanic gases compare to anthropogenic emissions?

Estimates suggested that human activity releases around 35 billion tons (Gigatons – Gt) of CO2 into the atmosphere each year (based on data from 2010). Volcanic gases contribute only a fraction of this, at 0.26 Gt per year (see table below). This is 135 times lower than human CO2 sources. Using these estimates, we would need hundreds of extra volcanoes on earth to match the annual anthropogenic CO2 emission levels. Or to put it another way, in any one year, we would need 700 Mount Pinatubo eruptions to match the human CO2 output. In short, the quantities of gas released during volcanic eruptions do not come close to human outputs of CO2. They do, however, have the capability to have a profound effect on the earth’s climate in the short-term. These are natural events, and the earth system can reorganise itself back to pre-eruption conditions within just a few years.

 

 

Annual CO2 source (for 2010) Billion tons per year (Gt/yr)
Anthropogenic CO2 35.0
Global volcanic emissions (maximum estimate) 0.26
CO2 source (per event) Billion tons (Gt)
Mount Pinatubo, 15th June 1991 0.05
Number of eruptions to match human sources 700

From the USGS: http://volcanoes.usgs.gov/hazards/index.php

 

 

For further information on volcanic eruptions and climate change, take a look at:

The US Geological Survey (USGS) section on volcanoes

And the American Geophysical Union (AGU) circular Eos – on volcanic gases vs anthropogenic gases