New greenhouse gas (GHG) standards for cars and light trucks are taking effect for model year 2017, progressing towards an anticipated sales-weighted average level of 173 g/mile C0 2 for model year 2025, and fuel economy standards increasing each year to the Corporate Average Fuel Economy (CAFE) target of 51.4 mpg fleet-wide by 2025 (for a projected vehicle sales mix). As a result, vehicle manufacturers are looking for solutions that can meet these goals without sacrificing marketable vehicle attributes (Nehuis et al., 2014;U.S. EPA, 2012aU.S. EPA, , 2014. Reducing mass enables vehicles to operate more efficiently during the use phase because energy demands (e.g., acceleration, rolling friction) on the powertrain are reduced. This reduction in mass can have major benefits on the total life-cycle impacts of vehicles because the current use phase accounts for 84-88% of the total life-cycle energy consumption and GFIG emissions for conventional light-duty vehicles. Comparatively, the manufacturing contributes approximately 4-7% of the energy consumption over the life of a light-duty vehicle (Keoleian and Sullivan, 2012;Mcauley, 2003; Sullivan and Cobas-Flores, 2001; Sullivan et al., 1998). Because of this dominant contribution of impacts from the use phase, mass reduction efforts and other use-phase efficiency measures provide an effective means to reduce the total life-cycle impacts. Flowever, the share of life-cycle impacts between the production and use phase for vehicles is likely to shift away from the use phase with increasing efficiency and with reduced light-duty vehicle GFIG emissions standards, as shown in the example comparison in Fig. 1

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Thermoelectric power production comprised 41% of total freshwater withdrawals in the U.S., surpassing even agriculture. This review highlights scenarios of the electric sector's future demands for water, including scenarios that limit both CO 2 and water availability. A number of studies show withdrawals decreasing with retirement of existing electricity generating units. Consumption, the evaporative losses, also decreases in many scenarios. However, climate mitigation scenarios relying heavily on nuclear and carbon capture technologies may induce increases in water consumption. These increases in consumption represent a potential tradeoff between climate mitigation and adaptation of the electric sector to climate-related changes in water resources. It also points to the need for both analyses and technological solutions from the chemical engineering community.

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Electric sector water use, in particular for thermoelectric operations, is a critical component of the water-energy nexus. On a life cycle basis per unit of electricity generated, operational (e.g., cooling system) water use is substantially higher than water demands for the fuel cycle (e.g., natural gas and coal) and power plant manufacturing (e.g., equipment and construction). However, could shifting toward low carbon and low water electric power operations create trade-offs across the electricity life cycle? We compare business-as-usual with scenarios of carbon reductions and water constraints using the MARKet ALlocation (MARKAL) energy system model. Our scenarios show that, for water withdrawals, the trade-offs are minimal: operational water use accounts for over 95% of life cycle withdrawals. For water consumption, however, this analysis identifies potential trade-offs under some scenarios. Nationally, water use for the fuel cycle and power plant manufacturing can reach up to 26% of the total life cycle consumption. In the western United States, nonoperational consumption can even exceed operational demands. In particular, water use for biomass feedstock irrigation and manufacturing/construction of solar power facilities could increase with high deployment. As the United States moves toward lower carbon electric power operations, consideration of shifting water demands can help avoid unintended consequences.

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The environment and its interactions with human systems, whether economic, social, or political, are complex. Relevant drivers may disrupt system dynamics in unforeseen ways, making it difficult to predict future conditions. This kind of "deep uncertainty" presents a challenge to organizations faced with making decisions about the future, including those involved in air quality management. Scenario Planning is a structured process that involves the development of narratives describing alternative future states of the world, designed to differ with respect to the most critical and uncertain drivers. The resulting scenarios are then used to understand the consequences of those futures and to prepare for them with robust management strategies. We demonstrate a novel air quality management application of Scenario Planning. Through a series of workshops, important air quality drivers were identified. The most critical and uncertain drivers were found to be "technological development" and "change in societal paradigms." These drivers were used as a basis to develop four distinct scenario storylines. The energy and emissions implications of each storyline were then modeled using the MARKAL energy system model. NO x emissions were found to decrease for all scenarios, largely a response to existing air quality regulations, whereas SO 2 emissions ranged from 12% greater to 7% lower than 2015 emissions levels. Future-year emissions differed considerably from one scenario to another, however, with key differentiating factors being transition to cleaner fuels and energy demand reductions.Implications: Application of scenarios in air quality management provides a structured means of sifting through and understanding the dynamics of the many complex driving forces affecting future air quality. Further, scenarios provide a means to identify opportunities and challenges for future air quality management, as well as a platform for testing the efficacy and robustness of particular management options across wide-ranging conditions. IntroductionIn the United States, air quality managers at the federal and state levels develop emission reduction strategies to meet air quality standards. Emission inventories are projected into the future, both with and without each candidate strategy. Air quality models are then used to evaluate the resulting changes in air quality. At the federal level, illustrative emission reduction strategies are a key component of Regulatory Impact Analyses (RIAs) (e.g., U.S. Environmental Protection Agency [EPA], 2009[EPA], , 2012a[EPA], 2012b[EPA], , 2014). These strategies demonstrate how an air quality standard can be met and are used in estimating the resulting costs and health and environmental benefits. For states, emission reduction strategies often support State Implementation Plans (SIPs), which specify how the state will comply with air quality standards into the future (e.g., California Air Resources Board [CARB], 2007; Texas Commission on Environmental Quality [TCEQ], 2012). In RIA and SIP applicatio...