click to enlarge  A graphic representation of the fingerprints, both manmade and natural, on the vertical structure of the atmosphere. Man-made changes such as the increased production of greenhouse gases causes the stratosphere to cool while the mid- to upper troposphere heats up. A new study shows that natural influences alone would not cause these temperature changes.

Human influences have directly impacted the latitude/altitude pattern of atmospheric temperature. That is the conclusion of a new report by scientists from Lawrence Livermore National Laboratory (LLNL) and six other scientific institutions. The research compares multiple satellite records of atmospheric temperature change with results from a large, multimodel archive of simulations.

“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 online in 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.

“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.”

Source: Lawrence Livermore National Laboratory