Wednesday, March 6, 2013

Vulcan’s whimper unsettles the Earth

For centuries, people have noticed that volcanic eruptions can cause regional, if not worldwide, changes in weather (“climate forcing”). Big volcanic eruptions can eject dust and sulfuric acid aerosol high into the atmosphere. Droplets of these sulfates remain suspended in the stratosphere and reflect sunlight back into space, acting like a global parasol. A “volcanic winter” may ensue, while the aerosol persists aloft. Famous examples include 1816: the year that had no summer (thanks to stratovolcano Mt. Tambora’s eruption in Java). Since then, Krakatau (1883), Santa Maria (1902), Agung (1963), El Chichan (1982) and, most recently, Pinatubo (1991) have been the five biggest eruptions and each has perturbed global climate for a short while, typically resulting in a drop of around 0.25 ˚C for a couple of years after eruption.

Up to now, the influence of smaller volcanoes on changes to the solar heating (insolation) of the Earth has been largely ignored. Recently, however, a combination of observations and modelling by a team of scientists from NASA, CU Boulder, and MIT, indicates that repeated and unusual pattern of eruptions by smaller volcanoes in the tropics can also force climatic perturbations over relatively short timescales. They found that medium-sized eruptions of tropical volcanoes, occurring between 2000 and 2010, have led to an increase in stratospheric aerosols, and can be linked to global cooling.

This is an important observation. Firstly, it demonstrates that stratospheric aerosols that had been noticed to be on the increase during this period are not primarily the result of anthropogenic coal-burning in Asia, as had previously been assumed. Secondly, it may explain the recent slowdown in global temperature rise seen around 2010 to 2012. More disturbingly, the stratospheric input from volcanoes is fickle and random, and cannot be relied upon to mitigate against anthropogenic climate change. Past evidence suggest that it may (at best) induce a temporary short-term blip in the rising trend of global temperature, akin to that seen a couple of centuries ago in that year that had no summer.

Image: eruption of Tarvurvur, a medium-sized tropical stratovolcano, Papua New Guinea, in November 2008 (Image credit: Taro Taylor)


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