Scientists find "fingerprint" of human activities in recent tropopause height changes

Scientists from the Lawrence Livermore National Laboratory have determined that human-induced changes in ozone and well-mixed greenhouse gases are the primary drivers of recent changes in the height of the tropopause.

Earlier research has shown that increases in the height of the tropopause over the past two decades are directly linked to stratospheric ozone depletion and increased greenhouse gases.

The new research uses climate model results to provide more quantitative estimates of the relative contributions of natural and human influences to overall tropopause height changes. This work indicates that 80 percent of the roughly 200-meter increase in tropopause height from 1979 to 1999 is directly linked to human activities. Smaller tropopause height increases over the first half of the 20th century are largely caused by natural variations in volcanic aerosols and solar irradiance.

The tropopause is the boundary between the lowest layer of the atmosphere — the turbulently mixed troposphere — and the more stable stratosphere. It lies roughly 10 miles above the Earth´s surface at the equator and five miles above the poles. The location of the tropopause is sensitive to changes in vertical profiles of atmospheric temperature. The Livermore research attempts to understand how different mechanisms affect atmospheric temperatures, and hence tropopause height. It is the first study to show that a model-predicted “fingerprint” of tropopause height changes can be identified in observations.

The paper describing this work, entitled, “Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes,” appears in the July 25 edition of Science. It involves a team of Livermore scientists (Benjamin Santer, Karl Taylor and James Boyle) and researchers from Lawrence Berkeley National Laboratory, the National Center for Atmospheric Research, the Institut für Physik der Atmosphäre in Germany and the University of Birmingham in the United Kingdom.

Using a computer model of the climate system, the Lab scientists and their colleagues examined changes in both man-made forcings (well-mixed greenhouse gases, tropospheric and stratospheric ozone, and the scattering effects of sulfate aerosols) and natural external forcings (solar irradiance and volcanic aerosols). Experiments were performed with a model developed jointly by the National Center for Atmospheric Research and Los Alamos National Laboratory. The innovative aspect of these model runs is that climate forcings were varied both individually and in concert. This allowed the researchers to estimate the contribution of each forcing to overall changes in atmospheric temperature and tropopause height. Completion of this very large ensemble of model runs was made possible by recent developments in high performance computing capabilities at U.S. Department of Energy and National Science Foundation facilities. Output from these and other related climate model runs is available at http://www.nersc.gov/projects/gcm_data/.

The model results reveal the key drivers of recent tropopause height increases — human-caused changes in well-mixed greenhouse gases and stratospheric ozone — act primarily through warming of the troposphere (greenhouse gases) and cooling of the lower stratosphere (ozone). Both of these effects increase tropopause height.

“Tropopause height is an integrated indicator of human-induced climate change,” Santer said. “It reflects global-scale changes in the temperature structure of the atmosphere. Our research shows that the increase in tropopause height over the second half of the 20th century was predominantly due to human activity, and provides independent support for claims of recent tropospheric warming.”

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