Pinatubo Volcano Research Boosts Case For Human-Caused Global Warming

The Pinatubo research also has improved scientists' ability to forecast the impact of future volcanoes on weather and climate, says the paper's author, Alan Robock of the university's Center for Environmental Prediction in the Department of Environmental Sciences, Cook College.

According to Robock, the eruption on Luzon Island in the Philippines on June 15, 1991 produced the largest volcanic cloud of the 20th century and caused changes in worldwide climate and weather that were felt for years.

The changes wrought by Pinatubo's sulfuric acid cloud, which blocked a large percentage of sunlight from reaching the earth, initially included cooler summers and warmer winters, an overall net cooling at the earth's surface and altered winds and weather patterns, Robock said.

In certain areas such as the Middle East, it produced a rare snowstorm in Jerusalem and led to the death of coral at the bottom of the Red Sea, he noted.

The cloud also caused depletion of the ozone layer over Temperate Zone regions of the Northern Hemisphere where much of the world's population resides, in addition to the regular ozone "hole" which appears in October over Antarctica, the researcher said.

Most significant, the scientist said, Pinatubo helped validate computer-generated climate models that demonstrate human-caused global warming.

Using computer modeling, said Robock, scientists have been able to account for natural warming and cooling, as found in Arctic and Antarctic ice core samples and tree rings covering hundreds of years up to the last century.

"If you plug in volcanic eruptions, El Niños, solar variations and other natural causes and try to simulate past climate changes, you can do a pretty good job of modeling climate change until the end of the 19th Century," the researcher said.

After that period, he said, natural causes alone don't account for the amount of warming, about 0.6 degrees Celsius (1.1 degrees Fahrenheit), that has taken place in the last century.

"But when you factor in Pinatubo and other eruptions along with anthropogenic (human-caused) emissions," said the scientist, "it accounts for the observed record of climate change for the past century, including the overall warming and episodic cooling, and validates the climate models."

In addition to improving understanding of global warming, scientists will be able to develop better seasonal forecasts after the next major eruption occurs, he noted. "Although overall the planet cools after volcanic eruptions, over Northern Hemisphere continents it actually gets warmer in the winter. This is because the wind patterns change in response to heating of the stratosphere by the volcanic aerosols."

The ozone layer, which protects against the sun's life-damaging ultraviolet radiation, developed a hole over the South Pole during the last century due to human activity -- chlorofluorocarbons (freons) from refrigeration, air conditioning, and industrial processes, noted the researcher.

Because the Antarctic region is so cold, clouds are able to form in the stratosphere, Robock said. The cloud particles serve as surfaces to allow sunlight to catalyze chemical reactions involving chlorine and bromine pollutants that destroy ozone, he noted.

The Mount Pinatubo cloud provided a similar surface on sulfuric acid particles for pollutants to react with sunlight and destroy ozone over Temperate Zone regions of the globe, not just over the Antarctic region. "So you get ozone depletion not just over Antarctica but over where we live," noted the researcher.

Ozone depletion by volcanic eruption is a recent phenomenon, said Robock. "Elevated levels of chlorine in the stratosphere only started appearing within the last couple of decades due to human activity," he said.

The scientist added that as the use of chlorofluorocarbons and other human emissions are regulated by International agreement, ozone depletion should disappear in a few decades.

"Researchers are already documenting that the amount of ozone-destroying compounds in the atmosphere has stopped rising, " said Robock.