To the Editor —

Jones et al.1 lay out the strong case for the many virtues of using 'ecoysytem-based approaches to adaptation' (EbA) to address current and future climate change. However, their article did not mention the burgeoning use of this approach within cities to address climate-related impacts and other environmental ills, where it is commonly being called 'urban green infrastructure' (UGI). Because most of the world's population will increasingly reside in urban areas, UGI ecological practices have the potential to benefit the most people as a buffer against climate change.

Some UGI approaches are familiar and old, such as urban park systems and urban forestry, including street trees. Others are comparatively new but increasingly well proven, such as green roofs2. The performance, costs and benefits of other possible approaches are still being assessed. Examples include bio-swales, innovative 'green street' techniques, permeable pavements, green walls or rain gardens. Research is needed to assess quantitatively how effective UGI approaches are compared with alternative engineered systems such as, for example, using higher albedo pavements and roofs for mitigation of urban heat islands. We also need to evaluate their overall benefits when implemented at large scales, which will be necessary to make an impact on urban climate, hydrology and ecology.

But the case for UGI as compared with 'hard infrastructure' adaptation solutions — termed 'grey infrastructure' solutions among urban practitioners — has the same basic rationale and merit as for EbA: multiple co-benefits. As a prime example, consider storm-water management in combined sewage overflow watersheds, where even moderate rainfalls lead to untreated sewage being shunted into local waterways. This problem will be exacerbated in cities that experience increases in precipitation from global warming combined with an increasing use of water by the growing urban populations. In this case, grey infrastructure solutions such as large-storage detention tanks can be pursued at great cost, running to billions of dollars3,4. However, storage systems that send water to treatment facilities will only deliver the one benefit of temporary runoff detention.

Green infrastructure solutions to this problem will deliver not only detention benefits but also retention benefits, which keeps precipitation from entering the system and being processed as wastewater. In this way, the processing energy and burden on treatment facilites are reduced. But there are also other benefits. For example, natural hydrologic recharge to currently impervious areas can be restored. Also, through evapotranspiration cooling, searing urban surface temperatures and the urban heat-island effect can be reduced. This in turn reduces building energy needs and greenhouse-gas-related emissions5. Some projects have successfully restored endangered ecosystems on rooftops6.

The large potential prospective funding flows for adaptation measures that Jones et al. highlight should recognize UGI as an important category of EbA strategies that are specifically appropriate within cities worldwide.