VERTICAL PATTERNS OF FREE AND FORCED CLIMATE VARIATIONS

Observations of the vertical structure of atmospheric temperature chan ges over the past three decades show that while the global-average low er atmosphere has warmed, the upper troposphere and lower stratosphere have cooled. While these changes may be due to observed anthropogenic increases of greenhouse,eases, decreases of lower stratospheric ozone , and increases of tropospheric aerosols, the changes may also have be en caused by natural unforced internal fluctuations of the climate sys tem. Here we use the results of a 1000-year simulation from a mathemat ical model of the coupled ocean-atmosphere-land system performed witho ut any changes in external forcing, so that we may consider its variat ions as a surrogate for free, internally-generated, natural fluctuatio ns of the climate system. When the global mean surface air temperature is warm in the model, the lower troposphere, upper troposphere and lo wer stratosphere are also warm over most of the Earth, in contrast to the observations of the last three decades and to model simulations of the forced climate response due to increased greenhouse gases. The ob served temperature change of the past three decades is therefore unlik ely to have been caused solely by natural internal variations of the c limate system, thereby strengthening the argument that these changes c an at least partly be attributed to anthropogenic activities.