Aerosols from an eruption scatter the Sun’s rays and cool the Earth.© Nature

Large volcanic eruptions can have such an extreme effect on world climate that sea levels dip right across the globe, taking years to recover.

When Mount Pinatubo in the Philippines blew its top in 1991, it triggered a cascade of climate events that caused the global average sea level to fall by about 6 millimetres over a single year. So say researchers led by John Church, a marine and atmospheric researcher with Antarctic Climate and Ecosystems Cooperative Research Centre in Hobart, Tasmania.

The blast released huge amounts of microscopic particles, called aerosols, into the atmosphere, scattering the Sun's rays and leading to a drop in the amount of heat reaching the planet's surface. As a result, the seas cooled and contracted.

The ocean level then crawled back by about 0.5 millimetres per year over the next decade or more.

Fighting the tide

The sea has risen at an average rate of 1.8 millimetres a year since 1950, but if one looks at the shorter time period of 1993 to 2000 the average is more like 3.2 millimetres per year. So about half of the difference between these rates can be accounted for by the ocean's gradual rebound after the Pinatubo eruption.

The other half of this increase is attributed to ice melting and increased glacier flow at the poles, and the expansion of the sea as it is heated by rising global temperatures.

By comparison, experts have predicted that a global temperature change of 2 to 6 °C over the next hundred years could cause a sea-level climb of a whopping 25 centimetres, thanks to both expanding waters and melting ice (see 'Oceans extend effects of climate change' ). So volcanoes might not make a big difference compared with changes expected from global warming. But they can delay sea level rises and confuse past records of sea-level change.

Volcanic spate

The researchers chose to study Pinatubo's eruption because of the wealth of data on both the event and on sea levels during the years that followed. Using a computer climate model, they gauged the effects of the aerosols released into the atmosphere, and the oceans' subsequent behaviour.

They calculate that the cooling effects lasted for around 18 months after the eruption, after which the seas began once again to warm. Their results are reported in this week's Nature¹.

"I was surprised," Church told news@nature.com. "I had not realized how strong the cooling of the ocean was following a volcanic eruption."

The Pinatubo event was part of a spate of large eruptions that began with the 1963 blast at Mount Agung in Indonesia. Church's team argues that global rises in sea level during this period would have been even more severe were it not for these events pushing it back down.

"The series of eruptions offset the acceleration in sea-level rise that would otherwise have been present from increasing greenhouse gases in the atmosphere," Church says. 

Antarctic Climate and Ecosystems Cooperative Research Centre, Tasmania

• References

  1. ChurchJ. A., WhiteN. J. & ArblasterJ. M. Nature, 438. 74 - 77 (2005). | Article | PubMed |