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Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes
B. D. Santer,1*M. F. Wehner,2T. M. L. Wigley,3R. Sausen,4G. A. Meehl,3K. E. Taylor,1C. Ammann,3J. Arblaster,3W. M. Washington,3J. S. Boyle,1W. Brüggemann5
Observations indicate that the height of the tropopause—theboundary between the stratosphere and troposphere—hasincreased by several hundred meters since 1979. Comparable increasesare evident in climate model experiments. The latter show thathuman-induced changes in ozone and well-mixed greenhouse gasesaccount for 80% of the simulated rise in tropopause height over1979–1999. Their primary contributions are through coolingof the stratosphere (caused by ozone) and warming of the troposphere(caused by well-mixed greenhouse gases). A model-predicted fingerprintof tropopause height changes is statistically detectable intwo different observational ("reanalysis") data sets. This positivedetection result allows us to attribute overall tropopause heightchanges to a combination of anthropogenic and natural externalforcings, with the anthropogenic component predominating.
1 Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA. 2 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. 3 National Center for Atmospheric Research, Boulder, CO 80303, USA. 4 Deutsches Zentrum für Luftund Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, D-82234 Wessling, Germany. 5 University of Birmingham, Edgbaston, Birmingham B15 2T T, UK.
* To whom correspondence should be addressed. E-mail: santer1{at}llnl.gov
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80% of the simulated rise in tropopause height over 1979–1999. Their primary contributions are through cooling of the stratosphere (caused by ozone) and warming of the troposphere (caused by well-mixed greenhouse gases). A model-predicted fingerprint of tropopause height changes is statistically detectable in two different observational ("reanalysis") data sets. This positive detection result allows us to attribute overall tropopause height changes to a combination of anthropogenic and natural external forcings, with the anthropogenic component predominating. 
