In the context of global climate change, the ‘extra’ heat experienced in cities will become increasingly problematic. Global climate models predict an enhanced frequency and intensity of heat waves, and the large-scale atmospheric conditions typically giving rise to UHI development (e.g., low wind speed, clear skies), are also expected to increase. Indeed, the last IPCC assessment report (AR5) mentions urban heating as one of the 8 key risks due to global change.
Moreover, since the physical causes of the UHI phenomenon are strongly linked to the presence of non-natural surfaces (buildings, roads, parking lots, etc...), increasing urbanization by itself also acts to increase the UHI effect.
Urban policy makers are faced with large uncertainties about the impact of climate change on the local urban conditions. Starting from recognized global climatic projections of the Intergovernmental Panel on Climate Change (IPCC) last assessment report (AR5), we calculate future urban heat island intensities with the UrbClim model. Possible time frames are 2026-2045 and 2081-2100.
The UrbClim model also allows to assess the combined effect of climate change and continuous urbanisation.
Within the European 7th Framework Projects RAMSES and NACLIM, the VITO Urban Climate team demonstrates its ability to downscale global climate model predictions towards the urban scale using an ensemble approach in which a representative selection of global climate models is used. Using the output of only one GCM is not reliable. Case studies are worked out for the cities of Antwerp, Berlin, Bilbao, Hyderabad, Lisbon, London, New York, Rio, Skopje.
The figure on the left shows an ensemble of climate projections using an ensemble of 10 IPCC global climate models in which our UrbClim model was nested. The different dash lines show the results for the calculation using the different models. The thick line shows the ensemble average and the error band gives an indication for the uncertainty. These results were obtained for the IPCC RCP8.5 scenario.
The figure above shows a comparison in the number of heatwave days between the current situation (1986 - 2005) and the future one (RCP8.5, 2081 - 2100). Again we show ensemble means and a heatwave day is defined as a day in minimum 3 consecutive days for which both the 3-day running daytime maximum and nighttime minimum air temperatures surpass their respecive 98 percentile values. It is clear that for some cities, current climate projection predict a dramatic increase in number of heatwave days.