Volcanic eruptions are more than a light show for us or a disaster for people living nearby.

When volcanic gases only reach the lower atmosphere, the effects are local and for a short term, because the material is wiped out by the rain. But eruptions that reach higher, up to the stratosphere, can have a global impact. "In the stratosphere, sulfur dioxide that was originally in the magma gets oxidized and forms droplets of sulfuric acid," said Joël Savarino, a researcher at the University of California, San Diego, the National Center for Scientific Research (CNRS) and the University of Grenoble.

"This layer of acid can stay for years in the stratosphere because no liquid water is present in this part of the atmosphere. The layer thus acts as a blanket, reflecting the sunlight and therefore reducing the temperature at ground level, significantly and for many years."

So, an American-French team has developed a method to see the influence of past volcanic eruptions on the climate, in order to understand how volcanoes could influence climate change. "The work is particularly relevant because the effect of atmospheric particles, or aerosols, is a large uncertainty in models of climate," said Mark Thiemens, Dean of UCSD's Division of Physical Sciences and professor of chemistry and biochemistry.

"Now for the first time, we can account for all of the chemistry involving sulfates, which removes uncertainties in how these particles are made and transported. That's a big deal with climate change."

To make a difference between eruptions that made it to the stratosphere from those that did not, the scientists examined the chemical fingerprint of sulfur isotopes in fallout preserved in the ice in Antarctica. Ice samples were achieved from snow pits near the South Pole and Dome C, the new French/Italian inland station. The volcanic gases are carried by air currents to the stratosphere, above the ozone layer, and exposed to short wavelength ultraviolet (UV) light, which creates a unique ratio of sulfur isotopes.

The sulfur isotope ratio in fallout reveals whether or not an eruption was stratospheric. Two volcanic eruptions - the 1963 eruption of Mount Agung in Bali and the 1991 eruption of Mount Pinatubo (photo) in the Philippines - were found to be stratospheric. "Young volcanoes have the advantage of having been documented by modern instruments, such as satellites or aircraft," said Savarino.

"We could therefore compare our measurements on volcanic fallout stored in snow with atmospheric observations."

The method also allowed the researchers to distinguish the Pinatubo eruption from the eruption of Cerro Hudson that occurred the same year, sulfur isotope ratio revealing that the second one did not reach stratosphere. Even if the volcanic material from more ancient eruptions is preserved in Antarctica, the older, deeper seasonal layers of ice are extremely thin as a result of the pressure from the overlying ice, thus, this method cannot be applied to them.

Some researchers have proposed that if global warming becomes severe, sulfates could be injected into the stratosphere to reduce the temperature. "Sulfates can cause warming or cooling depending on how they are made," said Thiemens. "They are usually white particles, which tend to reflect sunlight, but if they are made on dark particles like soot, they can absorb heat and worsen warming."