Professor Myles Allen CBE FRS

Abstract:

The Arctic Oscillation index has increased significantly over the past forty years, and such an increase has been simulated in response to greenhouse gas increases in several climate models. However, it has been suggested that an atmospheric model with an upper boundary in the upper stratosphere or mesosphere is required to simulate a realistic response, and that predictions made with standard climate models are hence unreliable. Here we show that a climate model with a 30-km upper boundary shows no increase in its surface Arctic Oscillation response to doubled carbon dioxide when its upper boundary is raised to 80 km. Neither model version shows a significant Arctic Oscillation response to stratospheric ozone depletion.

Abstract:

Using a coupled atmosphere/ocean general circulation model, we have simulated the climatic response to natural and anthropogenic forcings from 1860 to 1997. The model, HadCM3, requires no flux adjustment and has an interactive sulphur cycle, a simple parameterization of the effect of aerosols on cloud albedo (first indirect effect), and a radiation scheme that allows explicit representation of well-mixed greenhouse gases. Simulations were carried out in which the model was forced with changes in natural forcings (solar irradiance and stratospheric aerosol due to explosive volcanic eruptions), well-mixed greenhouse gases alone, tropospheric anthropogenic forcings (tropospheric ozone, well-mixed greenhouse gases, and the direct and first indirect effects of sulphate aerosol), and anthropogenic forcings (tropospheric anthropogenic forcings and stratospheric ozone decline). Using an "optimal detection" methodology to examine temperature changes near the surface and throughout the free atmosphere, we find that we can detect the effects of changes in well-mixed greenhouse gases, other anthropogenic forcings (mainly the effects of sulphate aerosols on cloud albedo), and natural forcings. Thus these have all had a significant impact on temperature. We estimate the linear trend in global mean near-surface temperature from well-mixed greenhouse gases to be 0.9 ± 0.24 K/century, offset by cooling from other anthropogenic forcings of 0.4 ± 0.26 K/century, giving a total anthropogenic warming trend of 0.5 ± 0.15 K/century. Over the entire century, natural forcings give a linear trend close to zero. We found no evidence that simulated changes in near-surface temperature due to anthropogenic forcings were in error. However, the simulated tropospheric response, since the 1960s, is ∼50% too large. Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming. Warming in the troposphere, since the 1960s, is probably mainly due to anthropogenic forcings, with a negligible contribution from natural forcings.

Abstract:

[1] Using a coupled atmosphere/ocean general circulation model, we have simulated the climatic response to natural and anthropogenic forcings from 1860 to 1997. The model, HadCM3, requires no flux adjustment and has an interactive sulphur cycle, a simple parameterization of the effect of aerosols on cloud albedo (first indirect effect), and a radiation scheme that allows explicit representation of well-mixed greenhouse gases. Simulations were carried out in which the model was forced with changes in natural forcings (solar irradiance and stratospheric aerosol due to explosive volcanic eruptions), well-mixed greenhouse gases alone, tropospheric anthropogenic forcings (tropospheric ozone, wellmixed greenhouse gases, and the direct and first indirect effects of sulphate aerosol), and anthropogenic forcings (tropospheric anthropogenic forcings and stratospheric ozone decline). Using an "optimal detection" methodology to examine temperature changes near the surface and throughout the free atmosphere, we find that we can detect the effects of changes in well-mixed greenhouse gases, other anthropogenic forcings (mainly the effects of sulphate aerosols on cloud albedo), and natural forcings. Thus these have all had a significant impact on temperature. We estimate the linear trend in global mean near-surface temperature from well-mixed greenhouse gases to be 0.9 ±0.24 K/century, offset by cooling from other anthropogenic forcings of 0.4 ±0.26 K/century, giving a total anthropogenic warming trend of 0.5 ±0.15 K/century. Over the entire century, natural forcings give a linear trend close to zero. We found no evidence that simulated changes in near-surface temperature due to anthropogenic forcings were in error. However, the simulated tropospheric response, since the 1960s, is ∼50% too large. Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming. Wanning in the troposphere, since the 1960s, is probably mainly due to anthropogenic forcings, with a negligible contribution from natural forcings. INDEX TERMS: 1650 Global Change: Solar variability; 1694 Global Change: Instruments and techniques; 4215 Oceanography: General: Climate and interannual variability (3309);.

Abstract:

We examine the sensitivity of the Arctic Oscillation (AO) index to increases in greenhouse gas concentrations in integrations of five climate models (the Hadley Centre coupled models (HadCM2 and HadCM3), the European Centre/Hamburg models (ECHAM3 and ECHAM4), and the Goddard Institute for Space Studies stratosphere-resolving (GISS-S) model) and in the National Centers for Environmental Prediction reanalysis. With the exception of HadCM2 all the models show a significant positive AO response to greenhouse gas forcing, but in the models lacking a well-resolved stratosphere that response is smaller than observed. In these models the AO index is linearly dependent on the radiative forcing, even up to ∼20 times current CO2 levels. By contrast, the GISS-S stratosphere-resolving model shows an AO response comparable to that observed, but the sensitivity of the model to further increases in forcing is reduced when CO2 levels exceed ∼1.5 times preindustrial values. It has been suggested that greenhouse gas forcing results in the equatorward deflection of planetary waves, which leads to a cooling and strengthening of the polar vortex and hence an increase in the surface Arctic Oscillation. In the observations the number of sudden warmings has reduced dramatically, consistent with this planetary wave effect, leading to a large mean cooling of the vortex. However, neither the GISS-S nor the HadCM3 models are able to reproduce the observed temperature changes, suggesting that this explanation for the impact of the inclusion of a stratosphere in the model may be incomplete.