Because the majority of the U.S. and world population already lives in urban areas, and existing or new urban centers will continue to grow in size and economic importance, research on reducing the climate change and accompanying environmental impacts of urban areas is critical. This includes assessing the differential vulnerability of urban areas and populations to climate change impacts as well as the full range of options for limiting and adapting to climate change. Opportunities for integrated, multidisci-plinary, and use-inspired research abound, but better connections are needed particularly to the applied science, engineering, and planning professions.
Characterizing and quantifying the contributions of urban areas to both local and global changes in climate. The role of large built environments and how they vary in terms of GHG emissions (including per capita emissions), aerosols, ground-level air pollution, and surface reflectivity need to be examined in a systematic and comparative way. Such research should include the extended effect of urban areas on surrounding areas (such as deposition of urban emissions on ocean and rural land surfaces) as well as interactions between urban and regional heat islands and urban vegetation-evapotranspiration feedbacks on climate. Examination of both local and supralocal institutions, markets, and policies will be required to understand the various ways urban centers drive climate change and identify leverage points for intervention.
Understanding the impacts of climate change on cities. Improving assessments of the impacts of extreme events (e.g., heat waves, drought, floods, and storms) and sea level rise on cities will require improved regional climate models, improved monitoring systems, and better understanding of how extreme events will change as climate change progresses. Evaluations of climate change impacts on urban heat islands and local-regional precipitation should extend to the analysis of their combined impacts on urban and periurban ecosystem and landscape function, ecosystem services, and demands on water and energy consumption.
Assessing the vulnerability of cities to climate change. Improved understanding is needed of who and what are threatened by climate change in the urban context, in both developed and developing countries. This includes human cohorts, neighborhoods, infrastructure, and coupled human-environment systems, as well as implications for food and water security. Most of the world's largest cities are in developing nations and have difficulty achieving global standards for clean air and other healthy environmental qualities. At the same time, very few U.S. cities have received concerted attention from climate researchers. As a result, the relative vulnerability of different urban forms (e.g., design, geometry, and infrastructure) and urban configurations relative to other settlement forms is largely unknown and deserves further study. In addition, given the large population adjacent to coastlines, attention to the vulnerability of coastal cities to sea level rise deserves special attention.
Developing and testing methods and approaches for limiting and adapting to climate change in the urban context. Limiting the magnitude of climate change and adapting to its impacts in the urban context raises a wide range of issues, including the relationships among urban land use, heat islands, water and energy use, and air quality. Additional research is needed, for example, on the efficacy and sociological considerations involved in adoption and implementation of white and green roofs, landscape architecture, smart growth, and changing rural-urban socioeconomic and political linkages. Additional questions include the following: What legacy or lock-in effects, including infrastructure and governance, serve as impediments to responses to climate change? What co-benefits can be gained in the reconfiguration of cities? Which adaptation strategies synergistically benefit the goal of limiting climate change, which potentially counteract it, and how can the trade-offs be adjudicated effectively?
Linking air quality and climate change. Research is needed to provide information for decision making about air quality in the face of climate change. This includes measurements, understanding, modeling, and analyses of socioeconomic benefits and trade-offs associated with different GHG emissions-reduction strategies, including those that simultaneously benefit both climate and air quality (see also Chapter 11) and those that could exacerbate one issue while monitoring the other.
Developing effective decision-support tools. What do we know about effective decision making under uncertainty, especially when multiple governance units may be involved? Much research is needed in comparing the results of city action plans for climate change and identifying similarities and differences between and among small and large cities. Questions that need answers include which qualities of these different plans break or create path dependencies (lock-in, e.g., through infrastructure design, tax policies, or other institutions), and which lead to more flexible, adaptive responses to the risks of climate change.
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