At the global level, the IPCC Working Group I identified four major aspects of climate change relevant to cities (IPCC, 2007a). First, heat waves are very likely to increase in frequency over most land areas. Second, heavy precipitation events are very likely to increase in frequency over most areas; available data suggest that a significant current increase in heavy rainfall events is already occurring in many urbanized regions. The resulting risk poses challenges to urban society, physical infrastructure, and water quantity and quality. Third, the area affected by drought is likely to increase. There is high confidence that many semi-arid areas will suffer a decrease in water resources due to climate change. Drought-affected areas are projected to increase in extent, with the potential for adverse impacts on multiple sectors, including food production, water supply, energy supply, and health. Fourth, it is likely that intense tropical cyclone activity will increase. It is also likely that there will be increased incidence of extreme high sea levels (excluding tsunamis).
The review identifies four key findings with very high or high confidence. First, climate change effects can amplify the risks that cities face from non-climate stresses. These non-climate stresses include: large slum populations that live in low-quality housing lacking access to basic social services; city-wide lack of access to effective and efficient physical infrastructure; often poor quality of urban air, water, and waste disposal systems; lack of land use planning and other urban governance systems among others. Further, the climate change associated risks for cities stem primarily from extreme events – implying that cities need to assess risk for droughts, floods, storms, and heat waves, in order to plan and implement adaptation strategies. However, gradual changes such as a rise in mean temperature do affect cities in at least two significant ways: by increasing the frequency and intensity of extreme events and burdening existing infrastructure.