RESEARCHERS IDENTIFY CAUSES OF 1979-2003
UNUSUAL STEP-LIKE COOLING OF GLOBAL LOWER STRATOSPHERE

Findings from satellite observations combined with a new NOAA computer model show that a significant cooling of the global lower stratosphere between 1979 and 2003 occurred in steps. Research published this week finds that while human influences led to the overall cooling during the period, natural factors helped modulate the evolution of the cooling.

“This research advances our knowledge of fundamental influences, such as the role of greenhouse gases and volcanic eruptions, that force changes in the Earth's climate,” said Venkatachalam Ramaswamy, senior scientist at NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J., and lead author of the paper. “The findings, derived from combining observations and numerical modeling studies, improves our understanding of how human and natural forcings cause changes in the Earth's climate system.”

The lower stratosphere is the region of the Earth's atmosphere from about 8 to 14 miles above the surface, where ozone plays a critical role in absorbing harmful ultraviolet radiation from the sun. Microwave Sounding Unit data from NOAA's polar orbiting satellites have shown that the two major volcanic eruptions, El Chichon in 1982 and Pinatubo in 1991, were initially followed by brief warming periods, then later by prolonged cooling periods in the global lower stratosphere.

“The unusual manner in which the cooling of the lower stratosphere evolved between 1979 and 2003 is very likely unique and unprecedented, and has not previously been well understood,” Ramaswamy said. “Climate model simulations show that human influences, namely stratospheric ozone depletion and greenhouse gas increases, and natural factors, namely volcanic aerosols and variations in the sun's energy output, combined to produce two step-like decreases in the lower stratospheric temperatures, one in the 1980s and the other in the 1990s.”

The simulations successfully replicated the complex observed stages of this temperature change, including the two step-like features marking the transitions of the lower stratosphere to a progressively colder state.

The simulations used a state-of-the-art coupled atmosphere-ocean model, developed at GFDL, one of NOAA's Office of Oceanic and Atmospheric Research laboratories. This laboratory has performed world-leading research in climate modeling for more than 50 years. The numerical calculations were performed on the NOAA supercomputer located at GFDL. The results of this research were carried out by a team that included scientists from GFDL, the National Center for Atmospheric Research, Lawrence Livermore National Laboratory, University of Miami and Rutgers University.

The researchers performed a variety of simulations employing combinations of the known changes in various factors, including human-made factors such as ozone depletion and long-lived greenhouse gas increases, and natural mechanisms of climate variability to see how these factors affected the cooling trend. The research indicates that the evolution of the cooling during 1979-2003 results from unusual juxtaposition of human-induced factors, volcanic events and the variations in the sun's output.

If ozone depletion continues, it would be an even more dominant factor in further lowering the stratospheric temperatures in the future. If there is complete ozone recovery because of the phasing out of the ozone-depleting substances, the stratospheric cooling trend would be governed by the long-lived greenhouse gases (e.g., CO2). Variations in the sun's output and any potential volcanic activity could affect the pattern. The findings are in a paper titled “Anthropogenic and Natural Influences in the Evolution of Lower Stratospheric Cooling” which was published in this week’s Science magazine.

The National Oceanic and Atmospheric Administration, an agency of the U.S. Commerce Department, is dedicated to enhancing economic security and national safety through the prediction and research of weather and climate-related events and providing environmental stewardship of our nation's coastal and marine resources. Through the emerging Global Earth Observation System of Systems (GEOSS), NOAA is working with its federal partners and 60 countries to develop a global monitoring network that is as integrated as the planet it observes.