Volcanic Blast Location Influences Climate Reaction

If the volcanic eruption is strong enough it will inject materialinto the stratosphere, more than 10 miles above the Earth's surface.Here, tiny particles called aerosols form when the volcano's sulfurdioxide combines with water vapor. Despite their size, these aerosolswork to alter interactions between the atmosphere and sun, affectingclimate patterns.

Now, new research funded by NASA and the National ScienceFoundation, focusing on the eruption of Mount Katmai, Alaska, in June1912, shows that location is also important, as major volcaniceruptions far north of the equator affect the world's climate muchdifferently than volcanoes in the tropics.

The Mount Katmai eruption was the one of the largest in the worldduring the 20th century. It actually refers to the eruption of Katmaiand the larger explosion of Novarupta, just west, that spewed tons ofsulfur dioxide gas into the atmosphere. Novarupta also released anincredible amount of molten rock (magma) that drained under Katmaicausing its summit to collapse, forming a massive crater (caldera). Theash fall from the eruption covered an area of more than 3,000 squaremiles to a depth of a foot or more, while its ash cloud, carried bywinds high in the atmosphere, spread a haze as far away as Africa.

"Studying such events will help us be better prepared for the nextmajor eruption while giving scientists clues on the type of climateshifts and changes to expect," said Luke Oman, a researcher at RutgersUniversity's Department of Environmental Sciences, New Brunswick, N.J.,and lead author of the study that appeared in the July 2005 issue ofthe Journal of Geophysical Research-Atmospheres.

By using one of the most modern General Circulation computer climateModels (GCMs) at NASA's Goddard Institute for Space Studies (GISS), NewYork, N.Y., the researchers studied the Mount Katmai volcanic eruption.They made a computer simulation of Katmai's eruption and an eruptionthree times as large to study their climate impacts.

Unlike earlier studies on volcanic eruptions in the tropics, thisresearch did not show a change in an important climate pattern calledthe "Arctic Oscillation" (AO) following the Mount Katmai eruption.

AO is a climate pattern defined by winds circulatingcounterclockwise around the Arctic at about 55 degrees north latitude(about even with Moscow). The air can spin more slowly and spill coldair down toward the equator into the mid-latitudes, or it can spinfaster and keep the cold up north.

"Large tropical volcanic eruptions tend to spread aerosols aroundthe globe, but with high-latitude eruptions like Katmai, they remainnorth of 30°N latitude, where they are heated less efficiently byoutgoing, or longwave radiation," said Oman. "As a result, the lowerstratosphere does not warm enough to influence the AO."

Eruptions in the tropics, like Mount Pinatubo in 1991, createaerosols that block heat from the sun in the lower atmosphere, ortroposphere, cooling temperatures in the subtropics. In turn, thereduced north to south temperature gradient results in a "positive"phase of the AO, with generally warmer winters over the NorthernHemisphere.

"In our research, although the Mount Katmai eruption was found tohave some role in the winter climate, including distinct cooling insouthern Asia, the most significant climate effect was during thesummer when strong cooling over the Northern Hemisphere landmassescaused a decrease in the Asian monsoon circulation," said Oman.

Normally, northern India experiences large amounts of cloudiness andrain due to the summer monsoon, the seasonal shift in winds thatdevelops out of the temperature contrast between the Indian Ocean andAsia. But, the eruption worked to lessen this gradient, weakening themonsoon, bringing reduced cloudiness, warmer temperatures and lessprecipitation across northern India west into the Persian Gulf.

"This study not only offers further evidence that the location andintensity of an eruption largely determine the Earth's overall climaticresponse, it also helps us see how well our computer models perform,"said co-author Gavin Schmidt of NASA GISS.

To verify our knowledge about the effects of volcanoes, researchersoften investigate historic intense eruptions that brought major climateswings, like the Laki, Iceland eruption of 1783 that dimmed andreddened the sun while resulting in a very warm summer in Europe andone of the coldest winters on record for the Northeast United Statesand Europe in 1783-1784. Using a variety of techniques, including themeasurement of acid fallout over polar areas from ice cores andanalyzing annual growth rings in trees, researchers can confirm firsthand the impact of such ancient eruptions.