the settlement of flood plains, the use of vulnerable water supplies is already linked to a form of exclusion. An objection could be raised that resolving land issues in exposed cities will attract people to these cities. Moreover, if the measures are funded centrally rather than locally, the cost differentials that might otherwise help to drive people and enterprises away from hazardous cities will be attenuated (see Kahn, 2010 for a simple treatment of this sort of effect). It is true that, like many measures that help cities in hazardous locations adapt to climate change, resolving land issues will also make these cities more attractive to live in. The effect on migration is difficult to predict, however. In any case, with respect to land issues, the obvious response is to fund the measures locally, or, better still, to resolve land issues in less hazardous locations as well, rather than to let them continue to fester in exposed cities.
Climate Regimes, Urban Density and Finding a Safe Place in the City The density and size of urban settlement provide the proximities that are key to the growth of industry, trade, and services. They also provide the basis for many of the environmental and social advantages that well-managed cities can provide (relative to dispersed rural settlements at comparable income levels). Environmentally, density can help people both to reduce carbon emissions and to protect themselves from the effects of climate change, provided it is part of a broader low-carbon strategy (Dodman, 2009). Residential density also reduces land costs and is often central to the struggles of low-income groups to gain a foothold in the city. The effects of settlement size and density are contingent on other factors, however, and there can be conflicts as well as complementarities between economic, environmental, and social improvements. Density lowers carbon emissions from transport if it lowers trip distances and facilitates efficient public transport, but not if it simply concentrates unrelated people and activities. It protects people from climate change if it diverts settlement from unsafe locations and makes use of returns to agglomeration in adaptation, but not if it concentrates people and activities in unsafe locations. Density helps to provide a viable foothold for the urban poor if it creates the basis for healthy and friendly neighbourhoods where people can ply their trades, but not if it forces people into overcrowded and under-serviced squatter settlements or into high-rise apartments.
Advocating compact cities Declining urban density is a longstanding phenomenon in North America, and it has been documented at lower levels in European and Japanese cities. Recent evidence suggests that urban density is declining in most of the world. A study of a global sample of cities found that densities in the built-up urban areas declined at about 2 per cent a year, though with a small but significant decline in urban fragmentation; densities within larger urban agglomerations declined somewhat more slowly (Angel et al. 2011). While cities outside of North America are higher in density (see also Schneider and Woodcock, Fai r A n d E ffe ct i ve R e s po n s e s to U rban i zat i o n A n d C li m at e Ch a nge : Tap pi ng Sy n e rgi e s an d Avo i d i ng E xc lus i o nary P o li c i e s
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