Professor Myles Allen CBE FRS

Abstract:

The role of stratospheric ozone changes in determining the climate response to solar forcing is investigated using a version of the Unified Model from the UK Meteorological Office which includes a mixed-layer ocean of constant depth (HadSM3). Two experiments have been performed, both of which include a wavelength-dependent reduction in total solar irradiance (TSI) of 7.5 W m(-2) (0.55%). The second experiment includes, in addition, an estimate of the accompanying stratospheric ozone changes. A large change in TSI is used (approximately a factor of two greater than the 'best-guess' change between present mean levels and the Maunder Minimum) to demonstrate the sensitivity of the climate system to this forcing. Results show that in the annual mean, the temperature response of the model is enhanced by the inclusion of the ozone changes, by approximately 15-20%. We compare results from our TSI and ozone experiment to those of Shindell et al. [Science 294 (2001) 2149] who performed a similar study with the GISS GCM. Temperature changes are greater in our simulation, as expected from the larger magnitude forcing, however the circulation response is very different: our results do not resemble the Arctic Oscillation, whilst those of Shindell et al. [loc. cit.] project strongly onto this leading mode of variability. The lack of a fully resolved stratosphere in our model is a potential reason for this distinction. To test this possibility, we repeated our combined irradiance and ozone experiment using identical model formulations, but with different vertical extents: the first extends to a height of 5 hPa, the second to 0.01 hPa. Both simulations produce a relatively weak surface pressure response to solar forcing that does not strongly resemble the Arctic Oscillation. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

Abstract:

We assess the most probable causes of late twentieth century (1960-1994) tropospheric temperature changes. Optimal detection techniques are used to compare observed spatio-temporal patterns of near-surface and tropospheric temperature change with results from experiments performed with two different versions of the Hadley Centre climate model. We detect anthropogenic forcings, particularly well-mixed greenhouse-gases, with a less certain sulfate aerosol cooling influence. More limited evidence exists for a detectable volcanic influence. Our principal results do not depend upon the choice of model. Both models, but particularly HadCM3, appear to overestimate the simulated climate response to greenhouse gases (especially at the surface) and volcanoes. This result may arise, at least in part, due to errors in the forcings (especially sulfate) and technical details of our approach, which differs from previous studies. We use corrected and uncorrected versions of the radiosonde record to assess sensitivity of our detection results to observational uncertainties. We find that previous corrections applied to the radiosonde temperature record are likely to have been sub-optimal in only taking into account temporal consistency. However, the choice of corrected or uncorrected version has no systematic effect upon our main conclusions. We show that both models are potentially internally consistent explanations of observed tropospheric temperatures.