Professor Myles Allen CBE FRS

Abstract:

The Antarctic polar night jet has intensified during spring in recent decades due to stratospheric ozone depletion and rising greenhouse gas (GHG) concentrations and this has had substantial effects on the region's climate. GHG concentrations will rise over the 21st century whereas stratospheric ozone is expected to recover and there is uncertainty in future southern hemisphere (SH) circulation trends. We examine sensitivity to the physics parameterisation of the 21st century SH circulation projection of a coupled atmosphere-ocean General Circulation Model and the sensitivity of the contribution from stratospheric ozone recovery. Different model parameterizations give a greater range of future trends in the position of the tropospheric jet than has been found in previous multi-model comparisons. Ozone recovery causes a weakening and northward shift of the DJF tropospheric jet. Varying the physics parameterization affects the zonal wind response to ozone recovery of the SON stratosphere by ∼10% and that of the DJF troposphere by ∼25%. The projected future SAM index changes with and without ozone recovery and the SAM index response to ozone recovery alone are found to be strongly positively correlated with projected 21st century global warming. © 2012. American Geophysical Union. All Rights Reserved.

Abstract:

Climate policies address emissions of many greenhouse gases including carbon dioxide, methane, nitrous oxide and various halogen-containing compounds. These are aggregated and traded on a CO2-equivalent basis using the 100-year global warming potential (GWP100); however, the GWP 100 has received scientific and economic criticism as a tool for policy1-4. In particular, given international agreement to limit global average warming to 2°C, the GWP100 does not measure temperature and does not clearly signal the need to limit cumulative CO 2 emissions5-7. Here, we show that future peak temperature is constrained by cumulative emissions of several long-lived gases (including CO2 and N2O) and emission rates of a separate basket of shorter-lived species (including CH 4). For each basket we develop an emissions-equivalence metric allowing peak temperature to be estimated directly for any emissions scenario. Today's emissions of shorter-lived species have a lesser impact on ultimate peak temperature than those nearer the time of peaking. The 2°C limit could therefore be met by setting a limit to cumulative long-lived CO2-equivalent emissions while setting a maximum future rate for shorter-lived emissions. © 2012 Macmillan Publishers Limited. All rights reserved.

Abstract:

Incomplete understanding of three aspects of the climate system-equilibrium climate sensitivity, rate of ocean heat uptake and historical aerosol forcing-and the physical processes underlying them lead to uncertainties in our assessment of the global-mean temperature evolution in the twenty-first century 1,2. Explorations of these uncertainties have so far relied on scaling approaches 3,4, large ensembles of simplified climate models 1,2, or small ensembles of complex coupled atmosphere-ocean general circulation models 5,6 which under-represent uncertainties in key climate system properties derived from independent sources 7,9. Here we present results from a multi-thousand-member perturbed-physics ensemble of transient coupled atmosphere-ocean general circulation model simulations. We find that model versions that reproduce observed surface temperature changes over the past 50 years show global-mean temperature increases of 1.4-3 K by 2050, relative to 1961-1990, under a mid-range forcing scenario. This range of warming is broadly consistent with the expert assessment provided by the Intergovernmental Panel on Climate Change Fourth Assessment Report, but extends towards larger warming than observed in ensembles-of-opportunity 5 typically used for climate impact assessments. From our simulations, we conclude that warming by the middle of the twenty-first century that is stronger than earlier estimates is consistent with recent observed temperature changes and a mid-range 'no mitigation' scenario for greenhouse-gas emissions. © 2012 Macmillan Publishers Limited. All rights reserved.