A team of atmosphere scientists from the Tokyo Institute of Technology and the University of Copenhagen have come up with a more precise method to show which historic episodes of global cooling were caused by volcanic eruptions. The team looked at patterns caused by stratospheric photochemistry in sulphur isotopes in volcanic sediments trapped in ice cores. Matthew Johnson, an associate professor at the Department of Chemistry, University of Copenhagen, pointed out that although historical records can be inaccurate, as some are written down long after the fact, ‘the chemistry does not lie’.
Large volcanic eruptions can eject gases through the atmosphere and into the stratosphere; these can affect the global climate for a year or more. Eruptions of less power can also impact the climate, but only locally and over shorter time frames. High plumes of material spend longer in the stratosphere, where the sunlight interacts with the sulphur in the plume and changes its chemical signature.
Johnson and his team can use their method to determine whether an historical eruption was powerful enough for the plume to reach the stratosphere and thus affect global climate. Material from ancient eruptions can be used to get an accurate record of global volcanic events over the last hundreds of thousands of years. The best place to find such material is within the ice cores taken from the ice shields of Greenland and Antarctica.
Ice cores show layers of material; each layer is a year of compacted snow. If volcanic material is found within a layer, that shows there was a significant eruption that year. The method described by Johnson and his team can be used to analyse just how powerful each of these eruptions was; whether they reached the stratosphere or were confined to the troposphere.
This method could also aid in the settling of some controversial eruptions, for example the eruption of the island of Santorini that caused the end of the Minoan culture. 1601 was known as the 'year without a summer' however identifying the volcano that caused this has remained elusive.
The collaboration between the Tokyo Institute of Technology and the University of Copenhagen combined the specific resources of each institute. The Tokyo group was able to synthesise the isotopically labelled samplesand the laboratory measurements were carried out in Copenhagen. The two groups worked together to build the atmospheric chemical model that demonstrated the stratospheric photoexcitation mechanism.
The image is of a cloud of smoke and ash produced by the eruption of the Chaiten volcano on Tuesday, May 6, 2008, in southern Chile. The eruption ejected incandescent material and ash about 30 kilometres (20 miles) into the sky.
Image: AP Photo/La Tercera