Scientists now say last weekend's cataclysmic eruptions by Mount Pinatubo in the Philippines were among the biggest in the past 200 years. They pumped enough ash and gas into the air to color sunsets and lower temperatures slightly throughout the northern hemisphere, perhaps for years.
Experts at the NASA Goddard Space Flight Center in Greenbelt and the National Oceanic and Atmospheric Administration offices in Suitland and Silver Spring don't yet have precise estimates of the global impact of the eruption.
But they have established teams to measure and track the volcano's airborne debris, assess its effect on climate, and put the eruption into historical perspective.
That may take months, but "based on . . . satellite data, it was a couple of times greater than the 1982 eruption of El Chichon in Mexico," said James S. Lynch, meteorologist with NOAA's National Environmental Satellite and Information Service.
High-altitude particles from the El Chichon eruption created colorful sunsets around the world and lowered temperatures globally by up to half a degree Fahrenheit for about three years, said James K. Angell, a research meteorologist at NOAA's Air Resources Laboratory.
Pinatubo's temperature effects may be measurable by scientists, Angell said, but most people won't notice any difference.
Lynch said the blasts from Pinatubo on June 15 and 16 created a cloud of gas and ash that spread over 1.23 million square miles. That is four times the combined size of all the clouds from the 11 major eruptions over six days that preceded it. None of those had exceeded 42,000 square miles.
The debris shot 15 to 19 miles into the stratosphere -- 25 percent higher than any ash plume since the Pinatubo eruptions began June 9.
By Tuesday, U.S. and Japanese weather satellites showed the leading edge of the giant ash cloud had blown west on high-altitude winds across the Indian Ocean and over Saudi Arabia.
Meanwhile, smaller ash clouds from Pinatubo's earlier eruptions had been blown as far as Egypt and were swinging north over Turkey, Lynch said.
Arlin J. Krueger, of NASA's Laboratory for Atmospheres at the Goddard Space Flight Center, has been watching the cloud with another device -- the Total Ozone Mapping Spectrometer, or TOMS. It's the same satellite-borne instrument that produced maps of the ozone "hole" in the atmosphere above Antarctica.
"What we see after a volcanic eruption is a sulfur dioxide [SO2] cloud," Krueger said.
On Sunday, Krueger said, the new cloud was over the South China Sea. "We made an assessment of the amount of SO2 in that cloud and found it was larger than what we saw coming from El Chichon in 1982 . . . roughly a factor of two times larger, and El Chichon was by far the largest one we'd ever seen in the satellite data base," he said.
If that's so, Pinatubo's blast last weekend may eventually be ranked as a strong Category 5 or even a Category 6 eruption on the Volcano Explosivity Index. Only 16 Category 6 eruptions, and one Category 7 have been recorded, Lynch said. Each increase in category represents a 10-fold increase in explosive energy.
The sulfur dioxide content of Pinatubo's cloud is the key to any climate changes or sunset effects.
"The sulfur dioxide is converted to sulfate droplets in the form of sulfuric acid," Krueger said. Once injected into the stratosphere, "the particles stay there, and settle out very slowly over a couple of years."
While it's there, it reflects a portion of the sun's light back into space, and absorbs other wavelengths, preventing it from reaching the ground.
"So . . . the heat budget of the Earth is changed by a small amount," Krueger said.
Scientists aren't yet able to predict accurately how much a given eruption will affect temperatures, but they are working on computer models that may someday provide the answers.
"There are anecdotal stories from the past about the effects of volcanic eruptions," he said. The most famous -- and extreme -- was the "Year Without a Summer," which followed a huge April 1815 explosion of Mount Tambora in Indonesia, the sole Category 7 eruption on record.
The summer of 1816 saw freezing temperatures and crop failures throughout the northern hemisphere, and scientists believe Tambora was the cause.
"The difference now is that we have satellite instruments" capable of measuring the volcano's output and effects on climate, said Krueger.
While it's not easy to sort out the effects of the volcano from the many other factors that might influence surface temperatures, Pinatubo's blast "may be big enough for us to test the volcano-climate models," he said. Better computer models may mean better predictions in the future.
Far easier to predict are the effects of Pinatubo's blast on sunsets around the world. The stratospheric sulfate particles act as a filter, selectively scattering the red wavelengths of sunlight.
"So, if you look in the west . . . after sunset, you will see noticeably rosy skies," Krueger said. They will occur "quite a bit later" than normal sunsets, after all the clouds are in darkness.
Americans should begin to see rosy sunsets by late summer or early fall, Krueger said.