We examine changes in tropopause height, a variable that has hitherto been neglected in climate change detection and attribution studies. The pressure of the lapse rate tropopause, p(LRT), is diagnosed from reanalyses and from integrations performed with coupled and uncoupled climate models. In the National Centers for Environmental Prediction (NCEP) reanalysis, global-mean p(LRT) decreases by 2.16 hPa/decade over 1979-2000, indicating an increase in the height of the tropopause. The shorter European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis has a global-mean p(LRT) trend of -1.13 hPa/decade over 1979-1993. Simulated p(LRT) trends over the past several decades are consistent with reanalysis results. Superimposed on the overall increase in tropopause height in models and reanalyses are pronounced height decreases following the eruptions of El Chichon and Pinatubo. Interpreting these p(LRT) results requires knowledge of both T(z), the initial atmospheric temperature profile, and DeltaT(z), the change in this profile in response to external forcing. T( z) has a strong latitudinal dependence, as does DeltaT( z) for forcing by well-mixed greenhouse gases and stratospheric ozone depletion. These dependencies help explain why overall tropopause height increases in reanalyses and observations are amplified toward the poles. The pronounced increases in tropopause height in the climate change integrations considered here indicate that even AGCMs with coarse vertical resolution can resolve relatively small externally forced changes in tropopause height. The simulated decadal-scale changes in p(LRT) are primarily thermally driven and are an integrated measure of the anthropogenically forced warming of the troposphere and cooling of the stratosphere. Our algorithm for estimating p(LRT) (based on a thermal definition of tropopause height) is sufficiently sensitive to resolve these large-scale changes in atmospheric thermal structure. Our results indicate that the simulated increase in tropopause height over 1979-1997 is a robust, zero-order response of the climate system to forcing by well-mixed greenhouse gases and stratospheric ozone depletion. At the global-mean level, we find agreement between the simulated decadal-scale p(LRT) changes and those estimated from reanalyses. While the agreement between simulated p(LRT) changes and those in NCEP is partly fortuitous (due to excessive stratospheric cooling in NCEP), it is also driven by real pattern similarities. Our work illustrates that changes in tropopause height may be a useful "fingerprint'' of human effects on climate and are deserving of further attention.
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