The present contribution highlights some of the principal issues that greatly complicate the attribution of climate change and climate variations to a specific forcing. One of the reasons for this difficulty is the large difference observed between the places of forcing and response patterns, the latter being mainly determined by feedback processes in the climate system. These feedback processes explain why warming due to both higher TSI or greenhouse gases is largest at high latitudes, in spite of the fact that forcing there is smaller than at low latitudes. There are also clear indications from model studies (e.g., Boer and Yu, 2003) that the climate response patterns are additive in their geographical distribution even if the forcing patterns are different. This stresses the fact that it is the feedback mechanisms which dominate the pattern of response irrespective of the forcing patterns.
Another difficulty is related to the high level of unforced variability in the climate system, resulting from the combination of internal processes. Such processes are chaotic in nature and unpredictable beyond a certain period of time, as predictability is conditioned by the growth of small errors in the initial system state.
The general conclusion of the present discussion is that a reliable attribution of the lower atmospheric response to TSI changes is not achievable on the basis of past temperature records only, as internal modes of variability of the climate system, at least over the last several hundred years, can give rise to variations with amplitudes similar to the ones expected from solar input changes (Bengtsson et al., 2006). We are therefore inclined to conclude that the effect of solar forcing in the troposphere is still an open issue.
The results of this study highlight the need for long term monitoring of both total and spectrally resolved solar irradiance measurements in the upper atmosphere, where the time scale of response to changes in the solar input is significantly shorter than in the troposphere. Furthermore, the stratospheric response to external forcing factors is different depending on the type of considered forcing, thus providing a discrimination criterion among the various forcings of climate.
The following volume chapters, grouped under the title "Detection and attribution of climate change," will underline different aspects of the problem discussed in the present paper. We purposely did not address here the question of how forced stratospheric changes in turn affect the troposphere. This is an area of great interest and importance, which is explored in detail in Section IV of the present volume. A well documented example of such processes is the role of ozone depletion and its influence on the stratospheric circulation over Antarctica (Thompson et al., 2005). Recent studies have demonstrated how this circulation also has affected the troposphere (Thompson et al., ibid), thus contributing to the large warming observed on the Antarctic Peninsula during winter and spring.
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