New Study Improves Understanding of Climate Impacts of Volcanic Eruptions

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Volcanic eruptions leave traces in the planet's ice sheets. Image: Mike Doukas (USGS)

Volcanic eruptions leave traces in the planet’s ice sheets. Image: Mike Doukas (USGS)

It’s well known that volcanic eruptions, with their emissions of (sometimes vast) quantities of gas, can have a significant impact on climate. Scientists are able to use aspects of their knowledge of such eruptions to assist in modelling future climate change.

The problems of such models lie in the uncertainties associated with the past record, where measurement, if it exists at all, is based upon contemporary, and probably incomplete, observations.

Now a new study from a multi-national group of scientists led by Dr Michael Sigl, of the United States’ Desert Research Institute (DRI), has looked in more detail at the record – with potential benefits to future modelling.

Volcanoes and Climate

Large-scale volcanic emissions of gases such as sulphur dioxide are capable of causing marked climate variability on a global scale (Indonesia’s Tambora volcano, for example, erupted in 1815 and poured out so many gases that the climate was affected worldwide and 1816 became known as ‘the year without a summer’).

Much further back in the geological timescale, several of the world’s massive flood basalt eruptions are associated with mass extinctions and are even implicated in the demise of the dinosaurs.

The mechanism for change is the emission of sulphur dioxide, which forms tiny particles know as sulphate aerosols. These reflect sunlight so that, rather than reaching the earth’s surface, it is reflected back into space. The result is that temperatures at the planet’s surface are lower than normal. Some eruptions are thought to have produced significant levels of volcanic forcing.

Identifying the Scale of Past Eruptions

So how do we know how big an eruption was? Historic records go back only as far as humans have been able to write down what they saw, or as long as an area has been settled. Many eruptions in remote areas may have gone unrecorded altogether.

Layers in ice cores such as this give clues to past atmospheric composition. Image credit: NOAA

Layers in ice cores such as this give clues to past atmospheric composition. Image credit: NOAA

The key lies in ice cores. As snow falls it traps minute quantities of atmospheric gases and preserves them for as long as it lies. In glaciers and ice caps, especially those in Greenland and the Antarctic, they may be preserved for thousands of years and have provided scientists with vital information on atmospheric composition.

The technique is commonly used and many climate models have been built upon a record of sulphate composition, drawn from ice cores, which goes back for about 1,500 years. But there are problems with the approach as not all eruptions lead to the same pattern of deposition, meaning that one ice core may show evidence of elevated sulcate levels while another may not.

The most extensive study so far undertaken, the new research from the DRI used 29 ice cores from 19 sites and correlated them. This allowed the researchers to refine and extend the existing data in terms of its extent (it goes back a further 500 years) and its resolution, with increasing numbers of eruptions.

When we started developing this array in Antarctica we had only a few records extending beyond 2,000 yrs and even less were available for Greenland,” Dr Sigl told Decoded Science. “We have been analyzing new records since, and will extend the record to longer timescales covering the Roman period in the near future.”

The record is complete for the Southern hemisphere,” he went on. “But of course for a complete picture of global volcanic impact the Northern Hemisphere is equally important. We are working on a similar array for Greenland is, however, more challenging to interpret, as there are many more “local” sources (e.g. Iceland) that would produce large sulfate in the ice core although on a global scale sulfate concentrations and thus climate impact are limited.”

Ice Core Results and the Future

So what can we learn from the increased information in the ice? The models which scientists have developed differ from observed patterns and that has been a cause of concern. “Scientists debate … why the climate model simulated cooling following some of the largest events was stronger than the reconstructed cooling from tree rings, some even blaming that tree ring chronologies might be wrong,” explained Dr Sigl.

The reconstructed and refined ice cores allowed them to correct this discrepancy. The key result was that the new data suggest that previous cooling calculated from computer models was in fact less than thought. Perhaps most notably, the researchers found that two key examples, “the two largest volcanic eruptions in recent Earth history (Samalas in 1257 and Kuwae in 1458) deposited 30 to 35 percent less sulfate in Antarctica, suggesting that these events had a weaker cooling effect on global climate than previously thought.”

Better quality of data leads, in theory at least, to more accurate results from modelling and to a greater understanding of how major events such as volcanic eruptions can have an impact upon the wider Earth system. Our understanding of these mechanisms and impacts can be crucial to informing approaches to environmental policy and combating global warming.

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