Peter Sherman, Alexander Archibald

Rising global mean temperatures and an increase in the fraction of the population living in urban areas is leading to a number of grand challenges. Contrasting area-weighted and population-weighted trends in an ensemble of CMIP5 climate model simulations under a range of climate and population change scenarios (RCPs and SSPs) shows that population-weighted changes in future surface temperature are almost double the area-weighted changes. Furthermore, we show that the Urban Heat Island (UHI) effect can have a large bearing on these calculations. By incorporating and comparing two parsimonious parameterizations of the UHI we calculate it contributes up-to 56% of the total uncertainty in population-weighted temperature exposure by the year 2050. However, in the long-term uncertainty in population-weighted temperature is dominated by model and scenario uncertainties rather than the UHI. We suggest that more efforts are required to develop improved parameterizations of the UHI for similar studies in CMIP6.