Feedbacks and thresholds within human systems and human-managed systems, and between the climate system and human systems, are a closely related research need that spans both this research theme and several of the other research themes described in this chapter. For example, crops respond to multiple and interacting changes in temperature, moisture, CO2, ozone, and other factors, such as pests, diseases, and weeds. Experimental studies that evaluate the interactions of multiple factors are needed, especially in ecosystem-scale experiments and in environments where temperature is already close to optimal for crops. Of particular concern are water resources for agriculture, which are influenced at regional scales by competition from other uses as well as by changing frequency and intensity of rainfall. Assessments that evaluate crop response to climate-related variables should explicitly include interactions with other resources that are also affected by climate change. Designing effective agricultural strategies for limiting and adapting to climate change will require models and analyses that reflect these complicated interactions and that also incorporate the response of farmers and markets not only to production and prices but to policies and institutions (see Themes 3, 4, and 7 below).
In fisheries, sustainable yields require matching catch limits with the growth of the fishery. Climate variability already makes forecasting the growth of fish populations difficult, and future climate change will increase this uncertainty. There is considerable uncertainty about—and considerable risk associated with—the sensitivity of fish species to ocean acidification. Further studies of connections between climate and marine population dynamics are needed to enhance model frameworks for effective fisheries management. Most fisheries are also subject to other stressors, such as increasing levels of pollution, and the interactions of these other stresses should be analyzed and incorporated into models. Finally, all of these efforts should be linked to the analysis of effective institutions and policies for managing fisheries. (See Chapter 9 for additional details of links between climate change and agriculture and fisheries.)
The role of large built environments (including the transportation and energy systems associated with them) in shaping GHG emissions, aerosol levels, ground-level air pollution, and surface reflectivity need to be examined in a systematic and comparative way to develop a better understanding of their role in climate forcing. This should include attention to the extended effect of urban areas on other 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 to 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 to identify leverage points for intervention. (See Chapter 10 and Theme 4 later in this chapter for additional details.)
Finally, the identification and evaluation of unintended consequences of proposed or already-initiated strategies to limit the magnitude of climate change or adapt to its impacts will need to be evaluated as part of the overall evaluation of the efficacy of such approaches. This topic is explored in more detail later in the chapter, but it depends on a robust Earth system research enterprise.
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