Using various instruments, scientists measure the amount and type of particles and gases ejected by volcanoes. Some scientists take ground-based measurements of the amount of carbon dioxide given off by volcanic vents. Others use ‘Light Detection and Ranging’ instruments (LIDARs), carried on board scientific aircraft or satellites – the aircraft are sometimes flown right into the plume of erupting debris to take measurements. LIDARs are able to record the amount of volcanic ash given off by a particular eruption, as well as gases such as sulphur dioxide, which forms aerosols in the atmosphere. Examining LIDAR measurements helps scientists to understand the effects of volcanic eruptions on global temperature.

Volcanic eruptions pump out clouds of dust and ash, which block out some sunlight. Because the ash particles are relatively heavy, they fall to the ground within about three months, so their cooling effect is very short-lived. But volcanic debris also includes sulphur dioxide. This gas combines with water vapour and dust in the atmosphere to form sulphate aerosols, which reflect sunlight away from the Earth’s surface. These aerosols are lighter than ash particles and can remain in the atmosphere for a year or more. Their cooling effect outweighs the warming caused by volcanic greenhouse gases – the eruption of Mount Pinatubo in 1991 caused a 0.5 °C drop in global temperature.

When volcanic eruptions have a significant effect on global temperature, they usually cause temporary cooling by putting aerosol particles into the atmosphere which block out some sunlight. But volcanoes also release carbon dioxide, a greenhouse gas which warms the Earth by trapping heat in the atmosphere. However, the amount of carbon dioxide emitted by today’s volcanoes is far too small to outweigh the cooling effect of volcanic aerosols reflecting some sunlight away from the Earth’s surface. Measurements show that on average the amount of carbon dioxide given off by volcanoes is less than 1% of the amount emitted by human activities.

Not all volcanic eruptions cause significant cooling. ‘Explosive’ eruptions eject aerosols into the atmosphere, which cool the planet by reflecting some sunlight. But lava from ‘flood basalt’ volcanoes leaks out in a continuous stream, with no explosive blast. Even some explosive eruptions fail to cool the planet. Mount St Helens in 1980 ejected its debris sideways, so most of the aerosols remained in the lower atmosphere, where they soon fell in raindrops. But in vertical blasts, such as Mount Pinatubo in 1991, large amounts of aerosols can reach the stratosphere, where there are no rain clouds and they can stay aloft reflecting sunlight for a year or more, causing significant temporary cooling.

In recent centuries most volcanic eruptions have been of 'explosive' rather than 'flood basalt' type, so their main effect on the climate has been short-term global cooling. A temporary decrease in volcanic activity probably contributed to the warming trend in the first half of the 20th century. But since the 1960s volcanoes have been erupting frequently. The 1991 eruption of Mount Pinatubo in the Philippines caused a 0.5 °C drop in global temperature the following year. The eruptions of Gunung Agung in Indonesia in 1963 and El Chichón in Mexico in 1982 also caused significant temporary cooling. These volcanic effects contribute to the short-term fluctuations in the long-term global warming trend.

Beneath the Earth’s surface, it gets progressively hotter with increasing depth until – between about 600 and 1300 °C – it’s hot enough to melt rock. This molten rock, called magma, is heated by the Earth’s radioactive core. It’s also in constant motion, causing the slow tectonic drift of the continents floating on top. Pockets of magma reach the surface during volcanic eruptions, becoming lava, which cools and solidifies, releasing some of its energy into the atmosphere. But as hot as the lava is, it covers far too small an area to have any significant effect on the climate. This geothermal energy makes up just 0.03% of the Earth’s surface energy balance.

Many different factors influence the Earth’s climate. Professor Graf’s work focuses on a range of these factors, from human activity to volcanic eruptions. ‘Understanding what drives our climate and its variability is both fascinating (just look at clouds or volcanic eruption plumes) and essential to the society.’ Professor Graf’s work has resulted in the first successful simulation of the climate effects of a big volcanic eruption. However he’s quick to emphasise the complexity involved in such work. ‘There are always struggles with the big computer models and our work would benefit from more reliable observations. However, our understanding of the basic physics involved in the field is quite good.’

• NERC: Do Volcanoes Trigger Climate Change
• NASA: Volcanoes and Climate Change