Volcanoes and climate

List of contents

• General

• Tambora 1815

• Krakatau 1883

• Mount Pinatubo 1991

• The spatial effect of the 1991 Pinatubo eruption on surface air temperature

• The effect of the 1991 Pinatubo eruption on the mass balance of the Greenland Ice Sheet

• 2007: Scientists find evidence for subsea volcanic blast of CO₂

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General

Volcanoes are generally found where tectonic plates are pulled apart or come together. Or they may be caused by deep seated mantle plumes. These so-called hotspots, for example at Hawaii, can occur far from plate boundaries.

Volcanic eruptions can alter the climate of the earth for both short and longer periods of time. For example, average global temperatures dropped about 0.5^oC for about two years after the eruption of Mount Pinatubo in 1991, and low air temperatures caused crop failures and famine in North America and Europe for two years following the eruption of Tambora in 1815. Click here to see a list of volcanic eruptions.

Volcanoes affect the climate through the gases and dust particles thrown into the atmosphere during eruptions. The effect of the volcanic gases and dust may warm or cool the earth's surface, depending on how sunlight interacts with the volcanic material.

Volcanic dust blasted into the atmosphere causes temporary cooling. The amount of cooling depends on the amount of dust, The duration of the cooling depends on the size of the dust particles. Particles the size of sand grains usually fall out of the air in a matter of a few minutes and stay close to the volcano, and therefore have little effect on global climate.

Dust-size ash particles will float around in the lower atmosphere for hours or days, causing darkness and cooling directly beneath the ash cloud, but these particles are quickly washed out of the air by rain. However, dust reaching the dry upper atmosphere, the stratosphere, can remain for weeks or months before they settle back to the planet surface. These particles block sunlight and cause cooling over large areas.

Volcanoes that release large amounts of sulphur compounds like sulphur oxide or sulphur dioxide affect the climate more strongly than other volcanoes mainly ejecting dust. The sulphur compounds usually rise easily into the stratosphere. There they combine with water vapour to form a haze of tiny droplets of sulphuric acid. These tiny droplets are very light in colour and reflect a great deal of incoming sunlight. Although the sulphuric acid droplets eventually may grow large enough to fall to the earth, the stratosphere is so dry that this process usually takes months or even years. Consequently, reflective hazes of sulphur droplets can cause significant global cooling for 2-3 years after a major sulphur-bearing volcanic eruption.

Sulphur hazes are believed to have been the primary cause of the global cooling that occurred after the large Pinatubo (1991; see below) and the Laki (1783-1785) and Tambora eruptions (1815).

Volcanoes also release large amounts of water and carbon dioxide, the two most important greenhouse gasses. From an isolated point of view, a volcanic eruption might thus be expected to result in atmospheric warming. However, there are large amounts of water and carbon dioxide in the atmosphere already, and even a large eruption will not be able to change the atmospheric composition of these compounds much. In addition, the water generally condenses out of the atmosphere as rain within a few days, and the carbon dioxide quickly dissolves in the ocean or is absorbed by plants. Usually, the volcanic sulphur compounds therefore have a greater effect, and cooling typically dominates following volcanic eruptions.

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Tambora 1815

The 1815 eruption of Tambora was probably the largest eruption in historic time. About 150 cubic kilometres of ash were erupted. This is about 150 times more than the 1980 eruption of Mount St. Helens in USA. Ash fell as far as 1,300 km from the volcano. In central Java and Kalimantan, 900 km from the eruption, one centimetre of ash fell. The eruption column is estimated to have reached a height of about 45 km.

An estimated 92,000 people were killed by the eruption. About 10,000 direct deaths were caused by bomb impacts, tephra fall, and pyroclastic flows, the rest indirectly by starvation, disease, and hunger. The eruption apparently lowered average world temperature by about 0.5-0.7°C over a period of 2-3 years. The 1815 eruption of Tambora was followed in North America and Europe by what was called "the year without a summer". London experienced snow in August.

Central England temperature series 1770-1840. The length of the cooling effect of the Tambora 1815 eruption is indicated by the blue bar. These graphs have been prepared using the composite monthly meteorological series since 1659, originally painstakingly homogenized and published by the late professor Gordon Manley (1974). The data series is now updated by the Hadley Centre and may be downloaded from there by clicking here. A graph showing the entire Central England temperature series since 1659 can be seen by clicking here. 

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Krakatau 1883

The explosive Krakatau eruption in Indonesia 27 May 1883 released huge amounts of ash into the atmosphere, giving rise to spectacular sunrise and sunset phenomena for a couple of years. Several painters have recorded this effect in their artwork.

Painting of the Krakatau eruption 27 May 1883 (left). Oil painting 'Sunset' by Thames 23 Nobember 1883 (centre). The painting 'Skrik' (the Scream) 1893 by Edvard Munch (right). The dramatic skyline in this painting is thought to have been inspired by the global optical effects caused by the 1883 Krakatau eruption as seen over Oslofjord in the years thereafter.

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Mount Pinatubo 1991

Mt. Pinatubo ash plume June 15, 1991 (left). Red haze in atmosphere caused by the Mt. Pinatubo eruption, as seen from Space Shuttle on August 8, 1991 (right). Pinatubo aerosol clouds (dark) are seen above high cumulonimbus tops. 

The 1991 Eruption of Mount Pinatubo, Philippine Islands, was the 2^nd largest volcanic eruption during the 20th century. The eruption produced high-speed avalanches of hot ash and gas, giant mudflows, and a huge cloud of volcanic ash hundreds of kilometres across.

On July 16, 1990, a magnitude 7.8 earthquake (comparable in size to the great 1906 San Francisco earthquake) struck about 100 kilometres northeast of Mount Pinatubo on the island of Luzon in the Philippines. At Mount Pinatubo, this earthquake caused a landslide and a short-lived increase in steam emissions from a pre-existing geothermal area, but otherwise the volcano seemed to be continuing its 500-year-old period of inactivity.

In March and April 1991, however, magma rising toward the surface from more than 30 kilometres beneath Pinatubo triggered small earthquakes and on 2 April 1991 caused powerful steam explosions that blasted three craters on the north flank of the volcano. Thousands of small earthquakes occurred beneath Pinatubo through April, May, and early June, and many thousand tons of noxious sulphur dioxide gas were also emitted by the volcano.

From June 7 to 12, the first magma reached the surface of Mount Pinatubo. Because it had lost most of the gas contained in it on the way to the surface, the magma oozed out to form a lava dome but did not cause an explosive eruption. However, on June huge amounts of gas-charged magma reached the surface and exploded in a spectacular eruption.

When even more highly gas charged magma reached Pinatubo's surface on June 15, the volcano exploded in a cataclysmic eruption that ejected more than 5 cubic kilometres of material. The ash cloud from this climactic eruption rose nearly 40 kilometres into the air, high into the stratosphere. At lower altitudes, the ash was blown in all directions by the intense cyclonic winds of a coincidentally occurring typhoon, while winds at higher altitudes blew the ash south-westward. A blanket of volcanic ash and larger pumice lapilli blanketed the landscape around the volcano.

HadCRUT3 global monthly temperatures illustrating the effect of the June 1991 volcanic eruption of Mount Pinatubo in the Philippines. The temperature effect apparently lasted to the end of 1992, as indicated by the shaded area.

Some of the tephra fell on Singapore, over 2000 km away to the southwest. Satellites tracked the ash cloud several times around the globe. At the same time, nearly 20 million tons of sulphur dioxide were injected into the stratosphere by the eruption, and dispersal of this gas cloud around the world caused global temperatures to drop by about 0.4°C from 1991 to December 1992. Sulphur dioxide oxidised in the atmosphere to produce a haze of sulphuric acid droplets. The large stratospheric injection of aerosols resulted in a 5 percent reduction in sunlight reaching the earth's surface. Global temperatures dropped about 0.35^oC during the next couple of years, as can be seen from the diagram above.

Diagram showing monthly variations in total global ice and snow cover since July 1983. During the period of observations, the total amount of clouds has varied slightly, reaching a minimum in 1990, followed by a rise presumably caused by the Mount Pinatubo volcanic eruption in 1991. From about 1995, the global ice and snow cover has again decreased slowly. The time labels indicate day/month/year. Data source: The International Sattelite Cloud Climatology Project.

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The spatial effect of the 1991 Pinatubo eruption on surface air temperature

The clickable diagrams in the table below visually display the spatial change 1991-1993 of monthly surface air temperature following the June 1991 Pinatubo eruption, compared to average conditions during the previous 5 yr period 1987-1991.

The cooling effect of the June 1991 Mount Pinatubo volcanic eruption is especially clear during the period November 1991 - December 1993. It should be noted, however, that there always will be regional temperature variations from month to month, so not all temperature changes following June 1991 can be ascribed to the eruption. In addition, it should also be noted that even during the period where the volcanic cooling effects apparently are at their maximum (November 1991 - December 1993), there can be found a number of regions with a positive temperature anomaly. Data source: NASA Goddard Institute for Space Studies (GISS).

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The effect of the 1991 Pinatubo eruption on the mass balance of the Greenland Ice Sheet

The reduced air temperatures during the 1992 northern hemisphere summer had effects on the mass balance of glaciers. This is well illustrated by a diagram in the 2005 ACIA report (see below), where 1992 becomes an outlier in a statistical sense. Unfortunately, the special background for the extraordinary low summer melting 1992 appears not to have been recognised by the authors of the report. Consequently, the year 1992 and 2002 (another outlier) were erroneously presented in the ACIA report as visually demonstrating the effect on the Greenland Ice Sheet of late 20th century warming in the Arctic (the two maps to the right). Click the 1992 June-July-August diagrams in the table above to inspect the general temperature conditions in Greenland during the 1992 ablation period.