"Human activity has very different effects on the temperature of the upper and lower atmosphere, and a very different fingerprint from purely natural influences," said Benjamin Santer, the lead researcher in the paper appearing in the Sept.16 online edition of the Proceedings of the U.S. National Academy of Sciences . "Our results provide clear evidence for a discernible human influence on the thermal structure of the atmosphere."
Observational satellite data and the computer model-predicted response to human influence have a common latitude/altitude pattern of atmospheric temperature change. The key features of this pattern are global-scale tropospheric warming and stratospheric cooling over the 34-year satellite temperature record. (The troposphere is the lowest portion of Earth's atmosphere. The stratosphere lies above the troposphere.)
"Current climate models are highly unlikely to produce this distinctive signal pattern by internal variability alone, or in response to naturally forced changes in solar output and volcanic aerosol loadings," Santer said.
Natural internal fluctuations in climate are generated by complex interactions of the coupled atmosphere-ocean system, such as the well-known El Nino/Southern Oscillation. External influences include human-caused changes in well-mixed greenhouse gases, stratospheric ozone and other radiative forcing agents, as well as purely natural fluctuations in solar irradiance and volcanic aerosols. Each of these external influences has a unique "fingerprint" in the detailed latitude/altitude pattern of atmospheric temperature change.
Fingerprint information has proved particularly useful in separating human, solar and volcanic influences on climate.
"The pattern of temperature change that has been observed vertically in the atmosphere, from ground level to the stratosphere, fits with what is expected from human-caused increases in greenhouse gases. The observed pattern conflicts with what would be expected from an alternative explanation, such as fluctuations in the sun's output," Santer said.
Another LLNL co-author of the paper, Celine Bonfils, noted that major volcanic eruptions also can profoundly disturb the vertical structure of atmospheric temperature. "During the recovery from such eruptions, tropospheric warming and stratospheric cooling also occur" Bonfils said. "But in contrast to volcanic influences, human-caused atmospheric temperature changes affect all latitudes and last longer. This suggests that the recent changes in temperature are not simply a recovery from past volcanic events."
Other Livermore scientists include Jeff Painter, Peter Gleckler, Charles Doutriaux and Karl Taylor. The research team included scientists from Remote Sensing Systems (Carl Mears and Frank Wentz), the Massachusetts Institute of Technology (Susan Solomon), the University of Adelaide, Australia (Tom Wigley), the NASA/Goddard Institute of Space Studies (Gavin Schmidt), the Canadian Centre for Climate Modelling and Analysis (Nathan Gillett) and the Nansen Environmental and Remote Sensing Center, Norway (Peter Thorne).