Climate change projections for sustainable and healthy cities

The ambition to develop sustainable and healthy cities requires city-specific policy and practice founded on a multidisciplinary evidence base, including projections of human-induced climate change. A cascade of climate models of increasing complexity and resolution is reviewed, which provides the basis for constructing climate projections—from global climate models with a typical horizontal resolution of a few hundred kilometres, through regional climate models at 12–50 km to convection-permitting models at 1 km resolution that permit the representation of urban induced climates. Different approaches to modelling the urban heat island (UHI) are also reviewed—focusing on how climate model outputs can be adjusted and coupled with urban canopy models to better represent UHI intensity, its impacts and variability. The latter can be due to changes induced by urbanisation or to climate change itself. City interventions such as greater use of green infrastructure also have an effect on the UHI and can help to reduce adverse health impacts such as heat stress and the mortality associated with increasing heat. Examples for the Complex Urban Systems for Sustainability and Health (CUSSH) partner cities of London, Rennes, Kisumu, Nairobi, Beijing and Ningbo illustrate how cities could potentially make use of more detailed models and projections to develop and evaluate policies and practices targeted at their specific environmental and health priorities. Practice Relevance Large-scale climate projections for the coming decades show robust trends in rising air temperatures, including more warm days and nights, and longer/more intense warm spells and heatwaves. This paper describes how more complex and higher resolution regional climate and urban canopy models can be combined with the aim of better understanding and quantifying how these larger scale patterns of change may be modified at the city or finer scale. These modifications may arise due to urbanisation and effects such as the UHI, as well as city interventions such as the greater use of grey and green infrastructures. There is potential danger in generalising from one city to another—under certain conditions some cities may experience an urban cool island, or little future intensification of the UHI, for example. City-specific, tailored climate projections combined with tailored health impact models contribute to an evidence base that supports built environment professionals, urban planners and policymakers to ensure designs for buildings and urban areas are fit for future climates.


The urban canopy and boundary layers
The UHI can be split into two vertical layers (Oke 1976;Oke 1978;Oke et al. 2017).
The lower layer is the urban canopy layer, the layer at building level caused by the roughness of the urban surface. This is dominated by the urban surface and will be different depending on the material and form of the surrounding environment, for example an area dense with buildings versus a park. This layer is produced by microscale processes ( Figure  1).
The upper layer is the urban boundary layer, a local to mesoscale concept ( Figure 1). This is the area of the planetary boundary layer where the presence of the city has modified the climatic characteristics. The increased roughness of the city can cause local slowing of wind over the city. This causes convergence of air and can cause the boundary layer to 'dome' up over the city. For potentially tens of kilometres downwind of the city, an urban plume of rising air can occur.

Contributory causes of the UHI
Six factors which affect the surface energy balance of a city and thus are contributory causes of the UHI have been identified (Howard 1833; Oke 1981). These are listed below and discussed briefly in terms of their influence on the surface energy balance. This energy balance can be expressed as: where Q* is the net surface radiant flux density, QF the anthropogenic heat flux density, QE the latent heat flux density, QH the sensible heat flux density and QS is the heat stored (Oke 1982).

• Anthropogenic heat (QF)
o Heat is generated within the city from three main sources: vehicle emissions, buildings, and metabolic heat of people (Allen et al. 2011) • Impervious surfaces o Moisture availability is required for latent heat loss (Oke 1982) and tends to be lower over impervious city surfaces such as asphalt, concrete and metal and where drainage systems remove water, than over soil and vegetation such as in city parks. In cities the main energy release tends to come from sensible heating rather than latent energy (Oke et al. 1992). • Thermal properties of the city fabric o City materials tend to have higher thermal conductivity and heat capacity than surrounding areas. Thus QS is generally larger in cities. The properties of these materials also mean that they release stored heat relatively easily, thus playing a major role in the nocturnal UHI (Oke 1982). • Surface geometry o Urban street canyons prevent the radiation of heat back to the sky in all directions and the building walls also tend to absorb heat, then radiate it back to the surface (Oke et al. 1991). Thus dense cities with tall buildings cool at night much slower than surrounding rural areas (Oke 1981).

• Urban roughness
o Tall inflexible buildings are associated with higher roughness parameters than the more flexible and softer vegetation in surrounding rural areas. Air decelerates as it flows over higher roughness areas, limiting the dispersal of heat and pollution generated by the city (Barlag & Kuttler 1990). A roughness sublayer can form, up to several times the average building height in extent and consisting of interactive waves and plumes of heat, humidity and pollutants. • Air pollution o Increased levels of aerosols from air pollution in urban areas are expected to absorb, scatter and reflect incoming short-wave radiation and emit it as longwave radiation. Thus polluted urban areas should receive less solar radiation (Li et al. 2018;Oke 1978). In practice, these differences tend to be partially offset by the typically lower albedo of urban areas.