Cities across the United States are taking action in response to climate change for a number of reasons: recent extreme weather events, available financial resources, motivated leaders, and the goal of achieving co-benefits.113,114,115,116 One strategy being used is to mainstream adaptation and mitigation into land-use, hazard mitigation, development, and capital investment planning.45,115,117 Municipal departments from public works to transportation play roles, as do water and energy utilities, professional societies, school boards, libraries, businesses, emergency responders, museums, healthcare systems, philanthropies, faith-based organizations, nongovernmental organizations, and residents. City governments use a variety of policy mechanisms to achieve adaptation and mitigation goals. They adopt building codes, prioritize green purchasing, enact energy conservation measures, modify zoning, and buy out properties in floodplains. Nongovernmental stakeholders take action through voluntary protocols, rating systems, and public–private partnerships, among other strategies.
U.S. cities are at the forefront of reducing greenhouse gas (GHG) emissions (Ch. 29: Mitigation, KM 1). Urban mitigation actions include acquiring high-performance vehicle fleets and constructing energy efficient buildings. A number of cities are conducting GHG inventories to inform decisions and make commitments to reduce their emissions. Comprehensive urban carbon management involves decisions at many levels of governance.19
Many U.S. cities have also begun adaptation planning. A common approach is to enhance physical protection of urban assets from extreme weather. For example, protection against sea level rise and flooding can involve engineering (such as seawalls and pumps) and ecological solutions (such as wetlands and mangroves) (Ch. 8: Coastal, KM 2).118 Green infrastructure lowers flood risk by reducing impervious surfaces and improving storm water infiltration into the ground.72,119 Green roofs use rooftop vegetation to absorb rainfall. Urban drainage systems can be upgraded to handle increased runoff.72 Climate-resilient building and streetscape design reduces exposure to high temperatures through tree canopy cover and cool roofs with high albedo that reflect sunlight. Ensuring that critical urban infrastructure, such as drinking water systems, continues to provide services through floods or droughts involves a combination of technology, physical protection, and outreach (Ch. 3: Water, KM 3; Ch. 19: Southeast, KM 1).120,121,122
Social and institutional changes are central to urban responses to climate change (Figure 11.7).59,114 Urban development patterns reflect social, economic, and political inequities. As such, decisions about where to prioritize physical protections, install green infrastructure, locate cooling centers, or route public transportation have differential impacts on urban residents.60,123,124,125 If urban responses do not address social inequities and listen to the voices of vulnerable populations, they can inadvertently harm low-income and minority residents.60,123,124
Urban actions can reduce climate change impacts on cities.12,126,127,128,129,130 Urban adaptation plans often begin with small steps, such as improving emergency planning or requiring that development be set back from waterways (Ch. 28: Adaptation).59,131 Not all plans address weightier concerns, tradeoffs, behaviors, and values. For example, coastal cities at risk from sea level rise may be constructing storm surge protections, but not discussing the possibility of eventual relocation or retreat (Ch. 8: Coastal, KM 3).59,131 Increasing tree canopy and planting vegetation to manage storm water and provide cooling can increase water use, which may present difficulties for water-strapped cities.132,133
Urban adaptation and mitigation actions can provide near-term benefits to cities, including co-benefits to the local economy and quality of life (Ch. 29: Mitigation, KM 4).3,19,113,134,135,136,137 Tree canopies and greenways increase thermal comfort and improve storm water management. They also enhance air quality, recreational opportunities, and property values (Figure 11.8). Wetlands serve to buffer flooding and are also a source of biodiversity and ecosystem regulation.
Urban climate change responses are often constrained by funding, technical resources, existing social inequities, authority, and competing priorities.19,114,119,139,140,141 Coordinating among multiple jurisdictions and agencies is a challenge. Using scarce resources to address future risks is often a lower priority than tackling current problem areas. The absence of locally specific climate data and a standard methodology for estimating urban GHG emissions poses additional obstacles to urban responses.19,72,114 Cities are dependent on state and national policies to modify statewide building codes, manage across landscapes and watersheds, incentivize energy efficiency, and discourage development that puts people and property in harm’s way. Strong leadership and political will are central to addressing these challenges.59,131,142 Many U.S. cities participate in networks such as the U.S. Conference of Mayors, ICLEI, the C40 Cities Climate Leadership Group (C40), and 100 Resilient Cities. Others participate in regional coalitions such as the Southeast Florida Regional Climate Change Compact. Multicity networks support development of urban climate policies and peer-to-peer learning (Ch. 28: Adaptation).59,110,113,117,120,143 Effective urban planning to respond to climate change addresses social inequities and quality of life, uses participatory processes and risk management approaches, builds on local knowledge and values, encourages forward-looking investment, and coordinates across sectors and jurisdictions (Ch. 8: Coastal, KM 3).59,60,115,120,124,140,142,144