Allowing for solar forcing in the detection of human influence on tropospheric temperatures

Previous workers have used the radiosonde record of atmospheric vertical temperature structure to detect anthropogenic influence on climate [e.g. Santer et al., 1996a]. However, none of these studies explicitly considered the influence of natural forcing factors. Whilst decadal changes of radiative solar activity are small, they could be amplified by various mechanisms (e.g. Haigh, 1996), potentially accounting for a significant fraction of the observed signal. Gillett et al.[2000] show that the diagnostics used to detect anthropogenic influence on atmospheric temperature structure are strongly influenced by solar forcing. Here we repeat the analysis of Alien and Tett, [1999] (AT99) including a model-simulated solar signal, to assess whether the observed tropospheric warming might be explained by solar forcing and the observed stratospheric cooling by the effects of ozone depletion. It transpires that previously reported results are robust to the inclusion of solar forcing since the greenhouse gas response is detectable even in the presence of solar forcing and stratospheric ozone depletion, allowing for an arbitrary amplitude of both solar and ozone signals.