Probabilistic climate change projections for UK cities

Long integration climate projections such as those provided for the Intergovernmental Panel on Climate Change (IPCC) generally do not account for the presence of urban areas and the role of the urban micro-climate in future climate scenarios, except through their contribution to greenhouse gas emission scenarios. This is partly a consequence of the small size of urban areas compared to the resolution of most Global Climate Models (GCMs). In 2009 a state of the art approach to generating high resolution probabilistic climate projections for the UK was published utilising evidence from a large ensemble of perturbed physics climate models and a combination of dynamical and statistical downscaling methods. While providing 25km resolution projections the method did not explicitly consider the potential influence of the urban land surface.

Making use of a simple urban surface exchange scheme, both stand alone, and coupled to the 25km resolution regional climate model, we demonstrate a method for revising the published projections to account for the urban landscape. In the case of the UK we are able to show that warming is dominated by greenhouse gas effects, and the inclusion of a fixed urban surface has minimal impact on rates of future warming near the surface, or on the associated probabilistic projections. Observational evidence from the 20th century also suggests that central London has warmed at the same rate as its surroundings through recent climate change. However, the Met Office regional and global climate models show that this is not the case for other regions of the world. Significant differences in the warming rates of urban and non-urban surfaces are simulated in response to radiatively forced climate change, as a result of important regional climate feedbacks that influence the surface radiation balance. Furthermore, future urban growth or changes in energy consumption would contribute to differential rates of urban warming. A simple inexpensive urban surface exchange scheme nested within a climate model can provide an integrated tool for investigating potential city and regional-scale impacts of climate change from both global and local forcings.