"We want the global-warming community to know that we've identified a possible explanation for why satellite atmospheric temperature and surface temperature trends can disagree," said Charles "Chick" Keller, director of Los Alamos' Institute for Geophysics and Planetary Physics. "The truth is that the temperature trends probably do agree when you consider the effect that massive ozone depletion caused by large volcanic eruptions has on the stratosphere and upper troposphere."
Keller and his colleagues - Manvendra Dubey and Howard Hanson of Los Alamos' Atmospheric and Climate Sciences Group, and Tracy Light of Los Alamos' Space and Atmospheric Sciences Group - presented their findings today at the American Geophysical Union's fall meeting in San Francisco.
The researchers set out to explain why scientists have seen less warming in the troposphere, the lowest layer of the atmosphere, than at the surface. If global warming were actually occurring, some scientists have said, then observers should be able to document warming trends in the atmosphere as well as on the surface. This doesn't always happen, however, and critics of global-warming theory use the trend disparity to discount the idea that Earth is slowly heating due to a buildup of greenhouse gases such as carbon dioxide and other environmental factors.
Keller and his colleagues on the Los Alamos team looked at existing temperature data gathered by high-altitude balloons and satellites for clues about atmospheric warming trends.
"When you look at the first 13 years of satellite temperature records, you see pretty good agreement with the surface records. But from 1992 to 1997 there is disagreement. During that time the stratosphere - the atmospheric layer above the troposphere that contains the ozone layer - cooled dramatically," Keller said. "We wondered if we could see some factor that would cause this, and that's when we started looking at the June 15, 1991, eruption of Mount Pinatubo in the Philippines."
The massive eruption spewed huge quantities of ash and aerosols into the stratosphere. Through a complicated mechanism, volcanic dust enhances destruction of ozone by chlorofluorocarbons already present. Consequently, the eruption led to a wholesale depletion of Earth's protective ozone layer in the stratosphere, which has been observed. Because ozone absorbs the sun's ultraviolet rays, the layer normally heats the stratosphere; but with depletion of ozone from the volcanic blast, the stratosphere cooled.
"We surmise that this very cold stratosphere might have had a cooling effect on the troposphere, the lower atmospheric layer where the weather is," Keller said.
Keller and his colleagues noticed that after the Pinatubo eruption and subsequent stratospheric cooling, surface temperatures and atmospheric temperatures first cooled, then began to rise at comparable rates, but they maintained a temperature differential greater than what had been seen before the volcano.
The observation also led Keller and his colleagues to look at how El Niño and La Niña phenomena affected the atmosphere during the entire satellite record period from 1979 to present. As a general rule, during El Niño years, the upper and lower troposphere see greater warming than the surface, and tropospheric layers see greater cooling than the surface during La Niña years. However, this was not the case after Mount Pinatubo erupted. Keller and his team looked at temperatures during the 1992 El Niño season. The upper troposphere was cooler than expected during that year, indicating that a cold stratosphere nestled directly above may have affected the troposphere.
Moreover, Keller and colleagues noticed that researchers at the Max Planck Institute for Meteorology had used a computer simulation with a crude approximation of ozone depletion to look at atmospheric temperatures and found that the upper troposphere did cool during the aftermath of Pinatubo. The Los Alamos researchers see this as another indication that their tropospheric-cooling hypothesis has merit.
Keller said he and his team next will use Los Alamos computer models of oceans and sea ice coupled with computer models of the atmosphere and land surface developed by the National Center for Atmospheric Research to tackle the problem. His research team will plug in 20 years of correct ozone concentrations to see whether the models will arrive at the level of atmospheric cooling measured during the 1992-1997 time frame.
"Critics of global warming hold the hypothesis that global warming doesn't exist because it isn't seen in the atmosphere by satellites," Keller said. "This observation by our team potentially eliminates one of the prime objections to global-warming theory. I think further study could show that ozone depletion keeps the whole system from warming up, which is what you see in the satellite temperature data."
Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, a team composed of Bechtel National, the University of California, The Babcock & Wilcox Company, and the Washington Division of URS for the Department of Energy's National Nuclear Security Administration.
Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.