Link traced between tropical temperatures and volcanoes
22 December 2008
Volcanoes in the tropics have influenced the climate over the last four hundred years, according to new research.
Scientists already believed volcanic activity at high latitudes is linked to changes in the climate at similar latitudes. But this is the first research to show that volcanoes in the tropics - the area within 30° North and South of the equator - have a comparable effect on the climate there.
In a paper published yesterday in Nature Geoscience, scientists from Lamont-Doherty Earth Observatory in the US, and the Scottish Universities of Edinburgh and St Andrews argue that volcanic eruptions in the tropics are associated with periods of lower sea surface temperatures in the Indian and Pacific oceans. These in turn affect the wider climate.
They compared historical data on sea surface temperatures going back to around 1550, derived from corals, tree rings and ice cores, with established indexes of volcanic activity. The results showed the link between tropical volcanic activity and lower sea surface temperatures in the region.
The longest sustained cold snap in recent centuries happened early in the nineteenth century, following the eruptions of Mount Tambora in Indonesia in 1815 and another unidentified volcano in 1808 or 1809; the researchers believe that though the relationship between volcanoes and temperatures is highly complex, this is unlikely to be a coincidence.
They note that 'Tambora may have cooled the tropics within two to three months', though they add that its strongest cooling effects were felt at higher latitudes, and that at the tropics its effects may partially have been masked by the 1817 El Nino effect, which caused warming sea surface temperatures.
They conclude that 'the tropical ocean-atmosphere system has been sensitive to changes in radiative forcing caused by tropical volcanism over the last several centuries', implying that particles released into the atmosphere by erupting volcanoes cause less sunlight to make it to sea level.
The effects of volcanic activity are harder to spot in the twentieth century, whether because there were fewer eruptions big enough to have a measurable effect or because general climate change caused by human greenhouse gas emissions has dampened the effect of volcanoes.
The researchers studied nineteen sets of samples from tree rings, corals and ice cores for information about past climate in the particular places in the tropics. All three substances form in layers, with each layer containing information about climatic conditions in the year over which it formed.
For tree rings, the crucial component is the width of each ring. A tree can grow more in a warmer year than in a colder one, so warm periods appear as sections of widely-spaced rings and cold ones as bands of tightly-packed rings.
The team drew in particular on analysis of specimens of Bristlecone Pine, probably the planet's longest-living tree species with some specimens thought to have lived for almost 5,000 years.
The samples were taken from locations throughout the tropics, ranging from Indonesia and Nepal to Peru. Five sets of new samples were used to improve the global coverage of old data series for sea surface temperatures, which relied on samples from just 14 places.
Sandy Tudhope from the University of Edinburgh was funded by NERC.
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