Geologic carbon sequestration as a global strategy to mitigate CO2 emissions: Sustainability and environmental risk
Fossil fuels are abundant, inexpensive to produce, and are easily converted to usable energy by combustion as demonstrated by mankind's dependence on fossil fuels for over 80% of its primary energy supply (13). This reliance on fossil fuels comes with the cost of carbon dioxide (CO{sub 2}) emissions that exceed the rate at which CO{sub 2} can be absorbed by terrestrial and oceanic systems worldwide resulting in increases in atmospheric CO{sub 2} concentration as recorded by direct measurements over more than five decades (14). Carbon dioxide is the main greenhouse gas linked to global warming and associated climate change, the impacts of which are currently being observed around the world, and projections of which include alarming consequences such as water and food shortages, sea level rise, and social disruptions associated with resource scarcity (15). The current situation of a world that derives the bulk of its energy from fossil fuel in a manner that directly causes climate change equates to an energy-climate crisis. Although governments around the world have only recently begun to consider policies to avoid the direst projections of climate change and its impacts, sustainable approaches to addressing the crisis are available. The common thread of feasible strategies to the energy climate crisis is the simultaneous use of multiple approaches based on available technologies (e.g., 16). Efficiency improvements (e.g., in building energy use), increased use of natural gas relative to coal, and increased development of renewables such as solar, wind, and geothermal, along with nuclear energy, are all available options that will reduce net CO{sub 2} emissions. While improvements in efficiency can be made rapidly and will pay for themselves, the slower pace of change and greater monetary costs associated with increased use of renewables and nuclear energy suggests an additional approach is needed to help bridge the time period between the present and a future when low-carbon energy is considered cheap enough to replace fossil fuels. Carbon dioxide capture and storage (CCS) is one such bridging technology (1). CCS has been the focus of an increasing amount of research over the last 15-20 years and is the subject of a comprehensive IPCC report that thoroughly covers the subject (1). CCS is currently being carried out in several countries around the world in conjunction with natural gas extraction (e.g., 2, 3) and enhanced oil recovery (17). Despite this progress, widespread deployment of CCS remains the subject of research and future plans rather than present action on the scale needed to mitigate emissions from the perspective of climate change. The reasons for delay in deploying CCS more widely are concerns about cost (18), regulatory and legal uncertainty (19), and potential environmental impacts (21). This chapter discusses the long-term (decadal) sustainability and environmental hazards associated with the geologic CO{sub 2} storage (GCS) component of large-scale CCS (e.g., 20). Discussion here barely touches on capture and transport of CO{sub 2} which will occur above ground and which are similar to existing engineering, chemical processing, and pipeline transport activities and are therefore easier to evaluate with respect to risk assessment and feasibility. The focus of this chapter is on the more uncertain part of CCS, namely geologic storage. The primary concern for sustainability of GCS is whether there is sufficient capacity in sedimentary basins worldwide to contain the large of amounts of CO{sub 2} needed to address climate change. But there is also a link between sustainability and environmental impacts. Specifically, if GCS is found to cause unacceptable impacts that are considered worse than its climate-change mitigation benefits, the approach will not be widely adopted. Hence, GCS has elements of sustainability insofar as capacity of the subsurface for CO{sub 2} is concerned, and also in terms of whether the associated environmental risks are acceptable or not to the public.