The effect of volcanic eruptions on climate has been one of the more hotly contested topics in the global warming debate. Seized upon briefly by climate skeptics as an alternative to human-caused warming, eruptions are now understood by mainstream science to result most often in net cooling for a period of up to several years.

Few researchers, however, have considered that an inverse relationship might also exist -- that over time, climate might have an effect on the planet's igneous activity.

Yet those are precisely the findings of new research from the GEOMAR Helmholtz Center for Ocean Research Kiel in Germany. Operating over a long enough time scale, sustained warmer temperatures lead to increased frequency of volcanic activity, the research finds.

Building off research by the Collaborative Research Center, which spent 10 years exploring volcanic regions, the GEOMAR team analyzed the layers of ash left in sea beds in Central America. From these, the team was able to reconstruct a history of eruptions dating back some 460,000 years, said volcanologist Steffen Kutterolf.

In analyzing that historical record, the researchers saw a distinct pattern emerge, he said. "There were periods when we found significantly more large eruptions than others," he said.

When they overlaid those patterns with climate data for the same period, it became clear that periods of enhanced volcanic activity corresponded with periods of sharp temperature rise and accelerated glacier melt.

To corroborate their theory, the team needed to look beyond Central America, and core samples collected by the International Integrated Ocean Drilling Program (IODP) provided an expedient way to do so. Those samples, taken from seafloor sediments and rocks, constitute one of the most complete geological records available to scientists. And in that record, the team's theory appeared to be reaffirmed.

"We found the same pattern from these cores as in Central America," said GEOMAR geophysicist Marion Jegen.

Pushing apart tectonic plates
But how to explain the correlation? Volcanoes are products of geologic forces operating in the Earth's lower strata; climate plays out most visibly in weather patterns and surface temperatures. How could these seemingly separate spheres interact?

Through collaborative work with researchers at Harvard University and extensive computer modeling, the GEOMAR team arrived at a possible explanation. A warming climate would eliminate a large portion of the ice massed at the northern and southern poles, they hypothesized, lessening the pressure on continents and allowing them to spread.

In the fissures between geologic plates, lava would then find its way to the surface.

"In times of global warming, the glaciers are melting on the continents relatively quickly," said Jegen. "At the same time, the sea level rises. The weight on the continents decreases, while the weight on the oceanic tectonic plates increases."

"Thus, the stress changes within the Earth to open more routes for ascending magma," he added.

Periods of planetary cooling take much longer than warming phases, meaning a more gradual adjustment period for the Earth's tectonic plates. Accordingly, volcanic activity during these periods tends to be more subdued, Jegen said.

While the researchers are unsure what effects the current phase of warming, accelerated by human-induced greenhouse gas emissions, might have on volcanic activity, it is unlikely that that activity will increase in any noticeable sense for at least a millennium, said Kutterolf.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500