Indicators to support environmental sustainability of bioenergy systems
Introduction
Indicators to assess the condition of the environment and monitor trends over time are needed to characterize conditions under which resource uses are sustainable. We define environmental indicators as environmental measures (Heink and Kowarik, 2010) that provide information about potential or realized effects of human activities on environmental phenomena of concern. We define environmental sustainability as the capacity of an activity to continue while maintaining options for future generations and considering the environmental systems that support the activity (Bruntland, 1987). Whereas much work has focused on the development of environmental indicators in general, only recently have stakeholders focused attention on developing indicators for sustainable bioenergy systems, and no consensus has yet emerged regarding which indicators should be given the highest priority (Buchholz et al., 2009).
The bioenergy supply chain includes the production or procurement of biomass feedstock, post-production processing and conversion (referred to in this paper as “processing”), and various transportation stages. Beneficial co-products (e.g., distillers grains) and waste by-products (e.g., biorefinery effluent) may be created in different stages of the supply chain. Feedstocks include annual and perennial plants, residues from agriculture, forestry, and related industries, and other organic wastes. The choice of feedstocks is a strong determinant in characterizing a given bioenergy pathway with implications for the applicable set of sustainability indicators.
Bioenergy systems are expected to expand in coming decades for several reasons. First, leaders in many countries view domestic bioenergy systems as more secure and sustainable than imported fossil fuels. Second, economic growth is expected to increase energy demand overall. Third, bioenergy systems are perceived to support rural development and employment. Fourth, technological advances continue to increase the affordability and sustainability of bioenergy. Furthermore, government policies in the United States (U.S.) and Europe call for an expansion of liquid fuel generation and combustion from cellulosic bioenergy feedstock sources, although those feedstocks are not currently in heavy use. The Energy Independence and Security Act of 2007 (EISA) mandates that at least 16 billion gallons (∼60.6 billion liters) of cellulosic biofuel be produced annually in the U.S. by 2022 (EISA, 2007). Member states of the European Union aim for biofuel to comprise 10% of their transportation fuel use by 2020, with incentives to encourage cellulosic and other second-generation biofuels (European Parliament and Council, 2009).
As societies increase use of bioenergy, stakeholders are questioning the environmental benefits of bioenergy compared to other energy options. Currently there is disagreement regarding whether bioenergy systems contribute to or ameliorate environmental problems such as depletion of nutrients in soil, erosion, runoff of nutrients and toxins, consumptive water use, greenhouse gas buildup, biodiversity loss, air pollution, and productivity loss (Jordan et al., 2007, Keeney, 2008, Williams et al., 2009). Differences of opinion often relate to past land use, crop choice, management practices, processing, and prevailing environmental conditions where the feedstock is grown (Jordan et al., 2007, Robertson et al., 2008, Scharlemann and Laurance, 2008, Kline et al., 2009). In the U.S., much of the debate has focused on the historic effects of conventional crop systems in the Midwest, the source of corn (Zea mays) for the majority of current U.S. ethanol production. However, cellulosic bioenergy is often perceived as holding greater opportunity for future sustainability than corn-based ethanol (Robertson et al., 2008, Kline et al., 2009). Because this debate coincides with an expected increase in bioenergy use and because of regulations that require bioenergy to be produced in an environmentally responsible manner, there is a need to characterize conditions under which bioenergy systems can be implemented sustainably (Hecht et al., 2009). This paper presents a set of indicators that can be used to characterize the environmental side of this equation.
The set of environmental indicators selected for assessing the sustainability of different types of bioenergy systems should apply to both large regions and local sites and should be useful to diverse stakeholders. For example, policymakers may focus on sustainability of the entire supply chain, agronomists may recommend sustainable bioenergy feedstock crops and management practices for different locations, and operation managers may seek to improve their feedstock production and processing systems. Indicators may also help in the implementation of certification programs (several are already in development) that can be applied throughout the supply chain or to its components (van Dam et al., 2008).
Although much work is still needed to identify, test, and implement a small set of environmental indicators that is useful to the diverse stakeholders involved in bioenergy systems, progress has been made. Sustainability attributes of agricultural practices in general have been discussed and defined by the Millennium Ecosystem Assessment (MEA, 2005), the National Sustainable Agriculture Information Service (Sullivan, 2003, Earles and Williams, 2005), and Dale and Polasky (2007). In addition, several national and international efforts are underway to select sustainability indicators for bioenergy, including the Roundtable on Sustainable Biofuels (RSB, 2010), U.S. Biomass Research and Development Board, Global Bioenergy Partnership (GBEP, 2010), and Council on Sustainable Biomass Production (CSBP, 2010). The preliminary suites of indicators arising from these efforts are diverse, and the differences among them are important, but here we note two broad characteristics. First, these suites tend to include numerous, broadly defined indicators. Second, many of the indicators in these suites tend to focus on assessments of management practices and their predicted environmental effects rather than on measurements that relate to realized environmental effects. These approaches have advantages. Large numbers of broad indicators can in principle capture a wide range of environmental effects. Also, assessing management practices may often be less expensive than making empirical measurements; indeed, simple measurements of some effects, such as tropospheric ozone formation, may not be feasible with respect to particular bioenergy systems. On the other hand, measuring large numbers of indicators can be prohibitively expensive (NRC, 2008a). Furthermore, current understanding of the effects of bioenergy management practices on the environment is limited, especially for systems not yet in wide use, such as cellulosic bioenergy. Therefore a need remains for a small set of concrete indicators that focus on realized environmental effects of bioenergy systems.
This paper identifies a suite of 19 indicators selected to collectively characterize important effects that many bioenergy systems have or are likely to have on environmental sustainability. The suite is organized according to six categories: soil quality, water quality and quantity, greenhouse gases, biodiversity, air quality, and productivity. These categories were selected to reflect the major areas of environmental concern surrounding bioenergy systems. They are also similar to categories used by national and international efforts working to establish suites of sustainability indicators for bioenergy. For each category, we discuss the relationship of proposed indicators to ecosystem properties and address measurement considerations. After presenting indicators in each category, we discuss future research directions, applications of these indicators to specific bioenergy systems, and interpretation of these indicators. This paper provides a basis for other researchers and investigators to move forward to evaluate and implement environmental indicators for bioenergy systems.
Section snippets
Approach
Where feasible, indicators were selected to empirically measure environmental effects rather than to infer such effects through assessment of management practices. In some cases, however, models based on management practices are the only feasible way to estimate the environmental effects of bioenergy systems (e.g., greenhouse gas fluxes or secondary particulate formation, discussed in Sections 3.3 Indicator of greenhouse gas flux, 3.5 Indicators of air quality, respectively).
Our selection of
Indicators of soil quality
Among the environmental systems for which indicators have been chosen, soils are especially important because soil quality affects the broader ecosystem, the immediate productivity of bioenergy crops, and the maintenance of productive capacity for future generations. Our selection of soil indicators was influenced by prior research on soil indicators in general (Doran and Parkin, 1996, Garten et al., 2003, Karlen et al., 2003, Pattison et al., 2008, Adair et al., 2009) as well as on agronomy
Developing and testing suite of indicators
These 19 indicators collectively represent how bioenergy systems may affect environmental sustainability with respect to soil quality, water quality and quantity, greenhouse gas concentrations, biodiversity, air quality, and productivity. Transitions from fossil-fuel based energy systems to bioenergy systems can affect environmental sustainability because of increases or decreases in various anthropogenic stresses, including resource exploitation; changes in land use, water use, and disturbance
Conclusion
We identify a suite of 19 indicators in six categories to measure the environmental sustainability of bioenergy systems. The suite is intended to be a practical toolset for capturing key environmental effects of bioenergy across a range of bioenergy systems, including different pathways, locations, and management practices. To evaluate the hypothesis that the suite meets this goal, and also to help measure variability and establish appropriate targets, the suite should be field tested in
Acknowledgments
Robin Graham, Gbadebo Oladosu, Andy Aden, and two anonymous referees provided helpful comments on earlier versions of this paper. Tristram West provided advice on greenhouse gas accounting. Jennifer Smith helped organize references. This research was supported by the U.S. Department of Energy (DOE) under the Office of the Biomass Program. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for DOE under contract DE-AC05-00OR22725.
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