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In the current climate debate the increase of anthropogenic greenhouse gases and its effects on global climate change has been widely investigated for atmospheric layers close to the ground. However, the global mean of observed temperature increase is rather small with about 0.85 K for the time period from1880 to 2012 which means a linear trend of 0.06K per decade.
In the atmosphere above ~8 km there is a reverse in the temperature trend: an increase in level of greenhouse gases leads to an additional cooling of the middle atmosphere which is induced by enhanced infrared emission to space. The global change of the atmosphere is particularly present in the atmospheric height range from 50 to 75 km. Any investigation about the physical causes of this rapid change in the upper atmosphere might be only answered by numerical climate models (e.g. LIMA). From climate modeling we conclude that the balance of infrared radiation is mainly determined by carbon dioxide and ozone.
There exist only a few long-term time series of observations, e.g. lidar temperature measurements in France since 1979, which document large temperature trends of about 3-4 Kelvin per decade at heights near 65-70 km at midlatitudes in summer (see Fig 1). These measurements demonstrate beyond doubt that the cooling of the upper atmosphere is about a factor 10 larger than the greenhouse warming near the surface. Hence, the height region of 50-75 km can be taken as an powerful indicator which should allow to detect most efficiently any climate signal.
The figure shows temperature trends from different observations (July, 45°N) and calculated trends from LIMA, adapting different reanalyses in the lower altitude range (0-45 km, "20th century": 0-28 km).
- F.-J. Lübken, U. Berger, and G. Baumgarten, Temperature trends in the midlatitude summer mesosphere, J. Geophys. Res., 118, 13347-13360, doi:10.1002/2013JD020576, 2013.
- U. Berger and F.-J. Lübken, Mesospheric temperature trends at mid-latitudes in summer, Geophys. Res. Lett., L22804, doi:10.1029/2011GL049528, 2011.
- F.-J. Lübken, U. Berger, and G. Baumgarten, Stratospheric and solar cycle effects on long-term variability of mesospheric ice clouds, J. Geophys. Res., D00106, doi:10.1029/2009JD012377, 2009.