Effects of city expansion on heat stress under climate change conditions.

We examine the joint contribution of urban expansion and climate change on heat stress over the Sydney region. A Regional Climate Model was used to downscale present (1990–2009) and future (2040–2059) simulations from a Global Climate Model. The effects of urban surfaces on local temperature and vapor pressure were included. The role of urban expansion in modulating the climate change signal at local scales was investigated using a human heat-stress index combining temperature and vapor pressure. Urban expansion and climate change leads to increased risk of heat-stress conditions in the Sydney region, with substantially more frequent adverse conditions in urban areas. Impacts are particularly obvi- ous in extreme values; daytime heat-stress impacts are more noticeable in the higher per- centiles than in the mean values and the impact at night is more obvious in the lower percentiles than in the mean. Urban expansion enhances heat-stress increases due to cli- mate change at night, but partly compensates its effects during the day. These differences are due to a stronger contribution from vapor pressure deficit during the day and from tem- perature increases during the night induced by urban surfaces. Our results highlight the in- appropriateness of assessing human comfort determined using temperature changes alone and point to the likelihood that impacts of climate change assessed using models that lack urban surfaces probably underestimate future changes in terms of human comfort.

Figure 7. Present and future frequency of adverse conditions caused by heat stress. Percentage of days when daily maximum (a) and minimum (b) W exceed different thresholds in the surrounding rural (green), urban (black) and new urban (red) areas as defined in the top right map. Probability for each of the areas is calculated according to CTL (plain), CC (horizontal striped) and CC_LU (diagonal striped) runs.