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The Fourth Assessment Report (AR4) released by the Intergovernmental Panel on Climate Change (IPCC) in 2007 was unequivocal in its message that warming of the global climate system is now occurring, and found, with "very high confidence" that it was "very likely" 1 that the observed warming was due to anthropogenic emissions of GHGs [1].2 To address the problem of climate change, the IPCC also reported an outline of approaches to reduce GHG emissions to desired levels [2]. These approaches focused on energy production and use, and included improved energy end use and production efficiency, use of low-carbon fuels and renewable energy sources, development of policies such as land use planning that encourage system-wide energy efficiency, control of non-CO2 GHG emissions, and development and application of carbon capture and storage (CCS) technologies.

These expected changes in technologies and practices will lead to changes in the impacts to the environment, some of which are beneficial and others of which are not. Energy conservation, for instance, will generally have beneficial environmental impacts. However, most of the available means of GHG mitigation result in some degree of adverse environmental impact. This is most clearly seen in the recent scientific (and increasingly public) debate about whether biofuels have a positive or negative influence on the environment [146, 147, 148, 149].

This chapter attempts to identify some of the adverse environmental consequences associated with GHG mitigation for a wide range of mitigation approaches, but will not attempt to quantify those impacts or their costs. This discussion will not evaluate specific GHG mitigation strategies, but is intended to identify the potential environmental impact (other than the intended mitigation of climate change) of implementing those strategies. The purpose is to provide a starting point for further and more in-depth evaluations of the environmental impacts with the aim of avoiding as many adverse impacts as possible as GHG mitigation approaches are implemented. A comprehensive discussion of the adverse environmental impacts associated with all possible, or even likely, mitigation options cannot be covered in a single chapter. Rather, our intent is to discuss selected options across key sectors, as a means to highlight that there can be adverse impacts associated with potential strategies to mitigate climate change.

We will also note that we do not address here the range of mitigation strategies and their associated impacts for the highly diverse industrial sector. Some of the

1 The IPCC defined terms of confidence and probability for use in their reports. "Very high confidence" was defined as having at least a 90% chance of being correct. "Very likely" was defined as having a likelihood of the stated outcome of greater than 90%.

2 The widely accepted list of greenhouse gases (and that used by the IPCC) are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).

material below has relevance to the industrial sector. For instance, the discussions of carbon capture and sequestration and use of alternative fuels in the power generation sector are largely relevant to industrial sector fuel combustion. The environmental impacts associated with process changes, use of more efficient motors and other equipment, and tradeoffs between changing components versus changing systems are not discussed here. We have also omitted discussion of the agricultural sector outside the section on biomass as an energy source. Although we recognize the importance of the industrial sector relative to GHG mitigation, we have chosen to focus on power generation, transportation, and residential/commercial energy end use in the available space.

It has been well recognized for some time that many of these approaches are likely to have multiple environmental benefits, and that accounting for improvements in air quality and similar environmental indicators will further increase the benefits associated with many of the GHG mitigation approaches [3]. Energy efficiency, for instance, will reduce the need for fossil fuel combustion in general, which will reduce emissions of the full suite of pollutants generated by fossil-fuel electric generating stations, motor vehicles, and industries. Similarly, use of natural gas rather than coal will result in lower emissions of sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter (PM).

The recent Energy Technology Perspectives report published by the International Energy Agency (IEA) stated that achieving the desired objectives of reducing GHG emissions and minimizing the impacts of increased GHG concentrations in the atmosphere "will require a transformation in how we generate power; how we build and use homes and communities, offices and factories; and how technologies are developed and deployed in the transport sector" [4]. The report amplified the basic conclusions of numerous other studies that have pointed out the need for significant changes in how energy is produced and used. The range and extent of technological change needed to reduce GHG emissions are tremendous, if the reductions that are anticipated to be necessary are to be achieved [5].

Some caveats are in order before we begin to discuss the consequences themselves. First and most importantly, identification of the consequences in this discussion should be considered as a preparation for avoiding or mitigating them, rather than as an argument to delay or avoid adoption of serious GHG mitigation approaches themselves. Although actions to mitigate climate change are being developed and proposed to avoid long term and potentially catastrophic consequences of global warming, the specific adverse environmental impacts of the proposed changes in consumer behavior, technology, and energy use must also be considered in the development of GHG mitigation strategies. The adverse impacts of the mitigation approaches are likely to be experienced more quickly and more immediately than the impacts on warming due to a major climate change mitigation program, and even though we may be working to minimize the impacts of climate change, we must also continue to protect human and environmental health.

Second, we must emphasize that the purpose of this discussion is to identify some of the potential adverse environmental impacts that have largely been overlooked during the broader discussions of how to mitigate GHG emissions.

Although many of the GHG mitigation approaches can provide significant environmental benefits beyond GHG reductions, it is well beyond the scope of this effort to attempt to quantify or even identify the full range of possible beneficial or adverse consequences.

The types of mitigation approaches and the extent to which they are deployed, and thus the extent of any adverse environmental impacts, will depend upon the total level of required GHG reduction. Because the level of GHG emission reduction will depend upon economic growth, population growth, and the carbon intensity of energy production over several decades, the approach typically taken to evaluate possible emission reduction requirements has focused on different scenarios of future changes.3 The IPCC, for instance, has developed a series of emissions scenarios [6] that describe possible future conditions and these have provided a widely used set of consistent scenarios that researchers and policy makers can use for evaluating different mitigation strategies. This evaluation will focus on the key technologies identified in the IPCC scenarios.

Finally, it must be emphasized that adoption of mitigation technologies does not take place in a vacuum. Changes in the mix of technologies for energy production and use are made in the context of the existing energy system, and the impacts of such changes are properly evaluated in comparison to the range of options that are available at the time those changes are made. Most commonly, the comparison is to a business-as-usual (BAU) option. Although the BAU option is often considered to be a "no action" approach, it is more accurately characterized as a "continuing action" approach, and the environmental impacts of mitigation approaches are most appropriately compared against the impacts of continuing current practices. As noted above, however, the current discussion is not meant to be such a comprehensive evaluation of the broader advantages and disadvantages of mitigation technologies, but rather to point out that most, if not all, mitigation approaches have their own environmental impacts that cannot be ignored, even when the net environmental benefit is positive (and often strongly so).

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