Effect of tropopause height changes on the calculation of ozone trends and their radiative forcing

Three separate estimates of ozone trends since 1970 are obtained from ozonesonde analyses in the Northern Hemisphere, at up to 11 point locations. Our analysis shows that there has been an increase in the annually averaged tropopause height of between 330-520 m at all stations, accompanied by an overall upward movement of the ozone profile. This movement of the ozone profile has led to an extra 2-15% decade(-1) ozone depletion between roughly 8 and 22 km. We argue that as the part of the ozone trend associated with "dynamical" movement of the ozone profile is not a direct result of anthropogenic chemical ozone depletion, it ought not to be included in anthropogenic radiative forcing calculations. This leads to a stratospheric ozone radiative forcing which is roughly 50% of the original estimate. Combining stratospheric and tropospheric ozone changes to calculate the total ozone radiative forcing in the Northern Hemisphere for the 1970-1997 period gives a radiative forcing of -0.09 W m(-2), excluding the effects of dynamical movement of the ozone profile, which were calculated to have led to a further "ozone feedback" of -0.1 W m(-2). These results suggest estimating ozone radiative forcing from observational ozone trend data, without accounting for the movement of the ozone profile could be fundamentally flawed, due to the unquantified feedbacks in the climate system, making it difficult to separate out the part of the ozone trend associated directly with the anthropogenic forcing.