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148 Mid-Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans Christopher R. German, Jian Lin, and Lindsay M. Parson (Eds.)

149 Continent-Ocean Interactions Within East Asian Marginal Seas Peter Clift, Wolfgang Kuhnt, Pinxian Wang, and Dennis Hayes (Eds.)

150 The State of the Planet: Frontiers and Challenges in Geophysics Robert Stephen John Sparks, and Christopher John Hawkesworth (Eds.)

151 The Cenozoic Southern Ocean: Tectonics, Sedimentation, and Climate Change Between Australia and Antarctica Neville Exon, James P. Kennett and Mitchell Malone (Eds.)

152 Sea Salt Aerosol Production: Mechanisms, Methods, Measurements, and Models Ernie R. Lewis and Stephen E. Schwartz

153 Ecosystems and Land Use Change Ruth S. DeFries, Gregory P. Anser, and Richard A. Houghton (Eds.)

154 The Rocky Mountain Region—An Evolving Lithosphere: Tectonics, Geochemistry, and Geophysics Karl E. Karlstrom and G. Randy Keller (Eds.)

155 The Inner Magnetosphere: Physics and Modeling Tuija I. Pulkkinen, Nikolai A. Tsyganenko, and Reiner H. W. Friedel (Eds.)

156 Particle Acceleration in Astrophysical Plasmas: Geospace and Beyond Dennis Gallagher, James Horwitz, Joseph Perez, Robert Preece, and John Quenby (Eds.)

157 Seismic Earth: Array Analysis of Broadband Seismograms Alan Levander and Guust Nolet (Eds.)

158 The Nordic Seas: An Integrated Perspective Helge Drange, Trond Dokken, Tore Furevik, Rüdiger Gerdes, and Wolfgang Berger (Eds.)

159 Inner Magnetosphere Interactions: New Perspectives From Imaging James Burch, Michael Schulz, and Harlan Spence (Eds.)

160 Earth’s Deep Mantle: Structure, Composition, and Evolution Robert D. van der Hilst, Jay D. Bass, Jan Matas, and Jeannot Trampert (Eds.)

161 Circulation in the Gulf of Mexico: Observations and Models Wilton Sturges and Alexis Lugo-Fernandez (Eds.)

162 Dynamics of Fluids and Transport Through Fractured Rock Boris Faybishenko, Paul A. Witherspoon, and John Gale (Eds.)

163 Remote Sensing of Northern Hydrology: Measuring Environmental Change Claude R. Duguay and Alain Pietroniro (Eds.)

164 Archean Geodynamics and Environments Keith Benn, Jean-Claude Mareschal, and Kent C. Condie (Eds.)

165 Solar Eruptions and Energetic Particles Natchimuthukonar Gopalswamy, Richard Mewaldt, and Jarmo Torsti (Eds.)

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166 Back-Arc Spreading Systems: Geological, Biological, Chemical, and Physical Interactions David M. Christie, Charles Fisher, Sang-Mook Lee, and Sharon Givens (Eds.)

167 Recurrent Magnetic Storms: Corotating Solar Wind Streams Bruce Tsurutani, Robert McPherron, Walter Gonzalez, Gang Lu, José H. A. Sobral, and Natchimuthukonar Gopalswamy (Eds.)

168 Earth’s Deep Water Cycle Steven D. Jacobsen and Suzan van der Lee (Eds.)

169 Magnetospheric ULF Waves: Synthesis and New Directions Kazue Takahashi, Peter J. Chi, Richard E. Denton, and Robert L. Lysal (Eds.)

170 Earthquakes: Radiated Energy and the Physics of Faulting Rachel Abercrombie, Art McGarr, Hiroo Kanamori, and Giulio Di Toro (Eds.)

171 Subsurface Hydrology: Data Integration for Properties and Processes David W. Hyndman, Frederick D. Day-Lewis, and Kamini Singha (Eds.)

172 Volcanism and Subduction: The Kamchatka Region John Eichelberger, Evgenii Gordeev, Minoru Kasahara, Pavel Izbekov, and Johnathan Lees (Eds.)

173 Ocean Circulation: Mechanisms and Impacts—Past and Future Changes of Meridional Overturning Andreas Schmittner, John C. H. Chiang, and Sidney R. Hemming (Eds.)

174 Post-Perovskite: The Last Mantle Phase Transition Kei Hirose, John Brodholt, Thorne Lay, and David Yuen (Eds.)

175 A Continental Plate Boundary: Tectonics at South Island, New Zealand David Okaya, Tim Stem, and Fred Davey (Eds.)

176 Exploring Venus as a Terrestrial Planet Larry W. Esposito, Ellen R. Stofan, and Thomas E. Cravens (Eds.)

177 Ocean Modeling in an Eddying Regime Matthew Hecht and Hiroyasu Hasumi (Eds.)

178 Magma to Microbe: Modeling Hydrothermal Processes at Oceanic Spreading Centers Robert P. Lowell, Jeffrey S. Seewald, Anna Metaxas, and Michael R. Perfit (Eds.)

179 Active Tectonics and Seismic Potential of Alaska Jeffrey T. Freymueller, Peter J. Haeussler, Robert L. Wesson, and Göran Ekström (Eds.)

180 Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications Eric T. DeWeaver, Cecilia M. Bitz, and L.-Bruno Tremblay (Eds.)

181 Midlatitude Ionospheric Dynamics and Disturbances Paul M. Kintner, Jr., Anthea J. Coster, Tim Fuller-Rowell, Anthony J. Mannucci, Michael Mendillo, and Roderick Heelis (Eds.)

182 The Stromboli Volcano: An Integrated Study of the 2002–2003 Eruption Sonia Calvari, Salvatore Inguaggiato, Giuseppe Puglisi, Maurizio Ripepe, and Mauro Rosi (Eds.)

Geophysical Monograph 183

Carbon Sequestration and Its Role in the

Global Carbon CycleBrian J. McPherson

Eric T. SundquistEditors

American Geophysical UnionWashington, DC

Published under the aegis of the AGU Books Board

Kenneth R. Minschwaner, Chair; Gray E. Bebout, Joseph E. Borovsky, Kenneth H. Brink, Ralf R. Haese, Robert B. Jackson,

W. Berry Lyons, Thomas Nicholson, Andrew Nyblade, Nancy N. Rabalais, A. Surjalal Sharma, Darrell Strobel, Chunzai Wang,

and Paul David Williams, members.

Library of Congress Cataloging-in-Publication Data

Carbon sequestration and its role in the global carbon cycle 1 Brian J. McPherson, Eric T. Sundquist,

editors. p. cm. -- (Geophysical monograph; 183)

Includes bibliographical references.

ISBN 978-0-87590-448-1 1. Carbon sequestration. 2. Carbon cycle (Biogeochemistry) 1. McPherson, Brian J., 1965- II.

Sundquist, E. T. (Eric T.) SD387.C37C3695 2009

577'. I 44--dc22

ISBN: 978-0-87590-448-1

ISSN: 0065-8448

2009044559

Cover Photo: NASA Goddard Space Flight Center Image by Reto St6ckli (land surface, shallow water, clouds). Enhancements by

Robert Simmon (ocean color, compositing, 3D globes, animation). Data and technical support: MODIS Land Group; MODIS Science

Data Support Team; MODIS Atmosphere Group; MODIS Ocean Group. Additional data: USGS EROS Data Center (topography); USGS Terrestrial Remote Sensing, Flagstaff Field Center (Antarctica); Defense Meteorological Satellite Program (city lights). (http://visibleearth.

nasa.gov/viewJec.php?id=2429) Overlying the globe is a graphic of the Keeling Curve, an illustration of the increasing monthly average

CO2 concentrations measured by Charles David Keeling at the Mauna Loa Observatory, Hawaii, over the past 50 years.

Copyright 2009 by the American Geophysical Union

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Figures, tables and short excerpts may be reprinted in scientific books and journals if the source is properly cited.

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The reproduction of multiple copies and the use of full articles or the use of extracts, including figures and tables, for

commercial purposes requires permission from the American Geophysical Union.

Printed in the United States of America.

CONTENTS

PrefaceBrian McPherson and Eric T. Sundquist ................................................................................................................vii

An Introduction to Global Carbon Cycle ManagementEric T. Sundquist, Katherine V. Ackerman, Lauren Parker, and Deborah Huntzinger ...............................................1

Section 1: Monitoring the Global Carbon Cycle: A Tribute to David Keeling................................25

The Mauna Loa Carbon Dioxide Record: Lessons for Long-Term Earth ObservationsEric T. Sundquist and Ralph F. Keeling ..................................................................................................................27

The Influence of David Keeling on Oceanic CO2 MeasurementsPeter G. Brewer ....................................................................................................................................................37

Next-Generation Terrestrial Carbon MonitoringSteven W. Running, Ramakrishna R. Nemani, John R. G. Townshend, and Dennis D. Baldocchi ..........................49

Section 2: Assessment of Local and Regional Carbon Sources and Sinks........................................71

Terrestrial Biological Sequestration: Science for Enhancement and ImplementationWilfred M. Post, James E. Amonette, Richard Birdsey, Charles T. Garten Jr., R. Cesar Izaurralde, Philip M. Jardine, Julie Jastrow, Rattan Lal, Gregg Marland, Bruce A. McCarl, Allison M. Thomson, Tristram O. West, Stan D. Wullschleger, and F. Blaine Metting .............................................................................73

Satellite Data Analysis and Ecosystem Modeling for Carbon Sequestration Assessments in the Western United StatesChristopher Potter, Matthew Fladeland, Steven Klooster, Vanessa Genovese, Seth Hiatt, and Peggy Gross ...........89

An Inventory of Carbon Storage in Forest Soil and Down Woody Material of the United StatesCharles H. Perry, Christopher W. Woodall, Michael C. Amacher, and Katherine P. O’Neill ................................101

Quantifying the Spatial Details of Carbon Sequestration Potential and PerformanceShuguang Liu ......................................................................................................................................................117

Soil Inorganic Carbon Sequestration as a Result of Cultivation in the MollisolsElena Mikhailova, Christopher Post, Larry Cihacek, and Michael Ulmer ..............................................................129

Natural Analogs of Geologic CO2 Sequestration: Some General Implications for Engineered SequestrationJulianna E. Fessenden, Philip H. Stauffer, and Hari S. Viswanathan .....................................................................135

Hydrogeochemical Characterization of Leaking, Carbon Dioxide–Charged Fault Zones in East-Central Utah, With Implications for Geological Carbon StorageJason E. Heath, Thomas E. Lachmar, James P. Evans, Peter T. Kolesar, and Anthony P. Williams .........................147

Section 3: Assessing Risks, Benefits, and Impacts of Sequestration.................................................159

Is There an Optimal Timing for Sequestration to Stabilize Future Climate?Vincent Gitz, Philippe Ambrosi, Bertrand Magné, and Philippe Ciais ..................................................................161

Present and Future Changes in Seawater Chemistry due to Ocean AcidificationRichard A. Feely, James Orr, Victoria J. Fabry, Joan A. Kleypas, Christopher L. Sabine, and Christopher Langdon ..........................................................................................................................................175

Erosion of Soil Organic Carbon: Implications for Carbon SequestrationKristof Van Oost, Hendrick Van Hemelryck, and Jennifer W. Harden .................................................................189

Assessing the Potential for CO2 Leakage, Particularly Through Wells, From Geological Storage SitesStefan Bachu and Michael A. Celia .....................................................................................................................203

Scoping Calculations on Leakage of CO2 in Geologic Storage: The Impact of Overburden Permeability, Phase Trapping, and DissolutionChristine Doughty and Larry R. Myer ..................................................................................................................217

Geochemical Impacts of Sequestering Carbon Dioxide in Brine FormationsJohn B. Kaszuba and David R. Janecky ...............................................................................................................239

Quantification of CO2 Trapping and Storage Capacity in the Subsurface: Uncertainty due to Solubility ModelsBiniam Zerai, Beverly Z. Saylor, and Douglas E. Allen .........................................................................................249

Quantification of CO2 Flow and Transport in the Subsurface: Uncertainty due to Equations of State AlgorithmsWeon Shik Han and Brian J. McPherson .............................................................................................................261

Section 4: Evaluation of Carbon Management Requirements...........................................................279

Verification and Accreditation Schemes for Climate Change Activities: A Review of Requirements for Verification of Greenhouse Gas Reductions and Accreditation of Verifiers—Implications for Long-Term Carbon SequestrationTrygve Roed-Larsen and Todd Flach ...................................................................................................................281

Sociopolitical Drivers in the Development of Deliberate Carbon StorageJennie C. Stephens ..............................................................................................................................................293

Considerations for Monitoring, Verification, and Accounting for Geologic Storage of CO2Mike Monea, Ray Knudsen, Kyle Worth, Rick Chalaturnyk, Don White, Malcolm Wilson, Sean Plasynski, Howard G. McIlvried, and Rameshwar D. Srivastava .........................................................................................303

Integrating Terrestrial Sequestration Into a Greenhouse Gas Management PlanJoel R. Brown and Neil Sampson ........................................................................................................................317

A Conceptual Framework for Management of Carbon Sequestration Data and MethodsRobert B. Cook, Wilfred M. Post, Leslie A. Hook, and Raymond A. McCord .......................................................325

Looking Ahead: Research Agenda for the Study of Carbon SequestrationBrian J. McPherson .............................................................................................................................................335

Index.................................................................................................................................................................359

Carbon Sequestration and Its Role in the Global Carbon CycleGeophysical Monograph Series 183Copyright 2009 by the American Geophysical Union.10.1029/2009GM000911

PREFACE

vii

Carbon sequestration has emerged as an important option in policies to mitigate the increasing atmospheric concen-trations of anthropogenic carbon dioxide (CO2). Significant quantities of anthropogenic CO2 are sequestered by natural carbon uptake in plants, soils, and the oceans. These uptake processes are objects of intense study by biogeochemists, ecologists, and other researchers who seek to understand the processes that determine the mass balance (“budget”) among global carbon fluxes. At the same time, many scientists and engineers are examining methods for deliberate carbon se-questration through storage in plants, soils, the oceans, and geological formations.

Studies of natural and deliberate carbon sequestration have much in common. They share many technical require-ments, ranging from measurement techniques to analysis of economic and social trends. Yet the carbon research com-munity has grown so rapidly in recent years that different scientific and technological groups are not taking full advan-tage of each other’s work even though they are working on closely related topics.

This monograph brings together a selection of studies from scientists, engineers, and others who study both natural and deliberate sequestration of carbon from a wide range of perspectives. In the diverse chapters of this volume, readers will find examples of basic and applied research, experimen-tal and theoretical science, and many combinations of these approaches, all described in language that is accessible to a broad range of interested scientists and engineers. Read-ers will also encounter evidence of discontinuities between the bodies of literature describing, on one hand, studies of the global carbon cycle and, on the other hand, research concerning deliberate carbon sequestration. These disconti-nuities are unavoidable, in part because global carbon cycle science has evolved over decades of time, whereas deliberate carbon sequestration has gained scientific traction only dur-

ing this last decade. In this book the editors and authors seek to address these discontinuities by minimizing specialized jargon and providing explanations of basic concepts in a col-lection of chapters that bridges the gap between carbon cycle research and studies of deliberate carbon sequestration.

Perhaps the most significant conclusion the editors drew from the cumulative work of this monograph is that the chal-lenge of carbon cycle management requires layers upon lay-ers of multidisciplinary understanding and collaboration. The science of the carbon cycle has long been a mixing pot for many technical disciplines. Even more challenging is the work of controlling atmospheric carbon dioxide, which hinges not only on fundamental science and technology but also on a complex and interdependent array of social, eco-nomic, and political constraints.

The editors view the graphic of Figure 2 in the chapter of Sundquist and Keeling (this volume) as an appropriate expres-sion of the fierce challenges faced by the scientific community, not only to be able to continue measurements and analysis but also to develop methods to manage the trend in atmospheric CO2 concentrations. This graphic combines the iconic Keel-ing Curve with the patchwork record of support necessary to sustain it, illustrating what Charles David Keeling described as both the “rewards and penalties of monitoring the Earth” [Keeling, 1998]. In our opinion, this juxtaposition—some-times referred to as “beauty and the beast”—embodies the combination of scientific discovery and pragmatic urgency that is the central theme of this volume.

Brian J. McPhersonUniversity of UtahEric T. Sundquist

U.S. Geological SurveyEditors

�

Carbon Sequestration and Its Role in the Global Carbon CycleGeophysical Monograph Series �83Copyright 2009 by the American Geophysical Union.�0.�029/2009GM0009�4

An Introduction to Global Carbon Cycle Management

Eric T. Sundquist, Katherine V. Ackerman, and Lauren Parker

U.S. Geological Survey, Woods Hole, Massachusetts, USA

Deborah N. Huntzinger

Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan, USA

Past and current human activities have fundamentally altered the global carbon cycle. Potential future efforts to control atmospheric CO2 will also involve significant changes in the global carbon cycle. Carbon cycle scientists and engineers now face not only the difficulties of recording and understanding past and present changes but also the challenge of providing information and tools for new management strategies that are responsive to societal needs. The challenge is nothing less than managing the global carbon cycle.

�. INTRODUCTION

Life on Earth depends on the cycling of carbon through myriad transformations and transfers among the atmosphere, the oceans, plants and animals, soils, rocks, and sediments. The carbon cycle encompasses many cyclic processes, in­cluding, for example, the daily cycle of animal feeding and metabolism, the seasonal cycle of plant growth and decay, and the geologic cycle of sediment burial and weathering. All of these processes are ultimately linked to exchange of CO2 with the atmosphere.

The concentration of atmospheric CO2 is now one third higher than it was during the eighteenth century, and signifi­cantly higher than at any time during the last several hundred thousand years. Because CO2 is an important “greenhouse” gas, there is growing concern that this increase in CO2 con­centrations is causing significant warming and other changes in global climate by altering the heat and water balances of Earth’s surface and atmosphere. Research on the carbon cy­cle has enabled scientists to attribute the rising CO2 concen­trations primarily to human activities, especially the burning

of coal, gas, and oil (“fossil fuels”), with smaller but signifi­cant additional contributions from changing land use, espe­cially deforestation.

As scientists attempt to anticipate future trends in atmo­spheric CO2 and climate, they are challenged by the reali­zation that human activities have fundamentally altered the global carbon cycle. This perception was eloquently ex­pressed more than 50 years ago by Revelle and Suess [�957, pp. �9–20]:

Thus human beings are now carrying out a large­scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. Within a few centuries we are return­ing to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years. This experiment, if adequately documented, may yield a far­reaching in­sight into the processes determining weather and climate. It there­fore becomes of prime importance to attempt to determine the way in which carbon dioxide is partitioned between the atmosphere, the oceans, the biosphere and the lithosphere.

Today, scientists, engineers, and others are working not only to be sure that the experiment is “adequately docu­mented,” but also to provide information and tools that can be used to manage the experiment. The challenge of control­ling atmospheric CO2 levels is a topic of expanding public concern, national policies, and international agreements.

In this brief introduction, we describe how past human ac­tivities have altered the global carbon cycle, and how new

2 INTRODUCTION TO GLOBAL CARBON CyCLE MANAGEMENT

methods of deliberate carbon sequestration may mitigate the future rise in atmospheric CO2. We conclude by summariz­ing some analyses of how effective these methods might be in a context of integrated mitigation strategies that include reductions in CO2 emissions. We seek to avoid specialized jargon so that the information will be accessible to a general technical audience, as well as to specialists in the science and engineering of the global carbon cycle.

2. PAST AND PRESENT EFFECTS OF HUMAN ACTIVITIES ON THE GLOBAL CARBON CyCLE

Figure � illustrates the relationships among the principal carbon fluxes and stocks that compose the natural global

carbon cycle. Atmospheric CO2 is cycled naturally through other forms of carbon over time scales ranging from seconds to millennia and longer. Carbon from atmospheric CO2 is converted by photosynthesis in plants to carbon in leaves, stems, roots, and other organic matter. Some of this carbon is taken up in the bodies of plant­eating animals, but much of the organic carbon produced by plants is respired by both plants and animals to produce the energy they need for sur­vival. The organic carbon consumed during respiration is converted to CO2 and cycled back to the atmosphere. When dead plant matter is buried in soils or sediments, it becomes a food source for microorganisms, yielding not only recycled CO2 but also a supply of nutrients required for continuing plant life. In the oceans, the CO2 needed for photosynthesis

Figure 1. The natural global carbon cycle before the influence of human activities. Sizes of the major reservoirs of stored carbon are shown in billions of metric tons (gigatons of carbon, or GtC), and major carbon fluxes are shown in billions of metric tons per year (GtC/yr). The vertical scale on the left shows the approximate time (in years) necessary for the different reservoirs to affect atmospheric CO2 concentrations. Photosynthesis by land plants is balanced by respiration during plant metabolism and decay of plants and soils. Gas exchange of CO2 between the ocean surface and the atmo­sphere is balanced in a manner that maintains conditions near chemical equilibrium. Weathering and sedimentation fluxes are shown to indicate the relatively small net exchanges that maintain a balance among weathering reactions on land, transport of carbon by rivers to the oceans, and sedimentation of carbon in the oceans. The diagram does not show carbon exchange within the oceans by physical mixing and biological processes, which cycle approximately �0 GtC/yr between surface and deep waters. Similarly, the diagram does not show the methane subcycle, which exchanges 0.�–0.2 GtC/yr with the atmosphere, or smaller fluxes (<0.1 GtC/yr) associated with volcanism and long-term cycling through the Earth’s crust. Detailed documentation can be found in Key et al. [2004], Sabine et al. [2004], and Sundquist and Visser [2004]. The value for vegetation (680 GtC) is adjusted from the value for present­day vegetation given in Sabine et al. [2004], to be consistent with the cumulative effects of human activities as shown in Figure 2.

SUNDqUIST ET AL 3

is supplied from the atmosphere in dissolved form by ex­change with air at the ocean surface. Marine plants are the base of the oceanic food chain, which eventually returns re­spired CO2 to the ocean surface and the atmosphere.

Under certain conditions, carbon buried in soils or in ma­rine sediments may not be cycled back to the atmosphere and oceans for millions of years. These deposits comprise the limestone and organic rock formations (including coal, gas, and oil deposits) that are naturally cycled very slowly through the Earth’s interior by geologic processes. Exposure to weathering ultimately returns rock carbon to the atmos­phere and oceans. Even these very slow modes of carbon cycling play a central role in the long­term sustenance of life on Earth.

Over geologic time scales, natural changes in the bal­ance of fluxes in the global carbon cycle have caused past variations in atmospheric CO2 concentrations. These vari­ations have been associated with past changes in climate [see review by Sundquist and Visser, 2004]. For example, periods of expanded continental glaciation (“ice ages”) dur­ing the last several hundred thousand years were associated with lower atmospheric CO2 levels [Petit et al., �999; Sie-genthaler et al., 2005; Luthi et al., 2008]. Although clearly CO2 is just one of many factors that have affected climate change over the course of Earth history, the geologic record is consistent with current understanding of the radiative con­tribution of CO2 to climate.

Human activities are altering a broad range of carbon cycle processes. Agriculture, forestry, and other forms of land use vastly change Earth’s land cover and redirect close to one­quarter of global net primary productivity (i.e., the net rate of photosynthetic carbon uptake after plants respire the CO2 required for their own metabolism) to the production of food, fuel, clothing, and shelter [Haberl et al., 2007]. Deforesta­tion and soil degradation enhance the release of CO2 from soils and dead plant material. Enhanced rates of erosion and sediment deposition accelerate both the exposure and burial of soil organic matter [see Van Oost et al., this volume]. The burning of fossil fuels produces atmospheric CO2 from or­ganic carbon that has been stored in rocks for periods as long as hundreds of millions of years. Although some human ac­tivities remove carbon from the atmosphere, their overall net effect is to increase the release of CO2 to the atmosphere.

These human effects began thousands of years ago with the conversion of forests and grasslands to agricultural use, releasing CO2 and possibly methane to the atmosphere [Ruddiman, 2005]. The pace of these effects accelerated in recent centuries with growing human population and land use [Houghton et al., �983; DeFries et al., �999; Houghton, 2004]. The burning of fossil fuels caused a more dramatic acceleration of rising CO2 levels in the latter half of the nine­

teenth century [Keeling, �973; Andres et al., �999; Neftel et al., �985]. In recent decades, the rate of increase in atmo­spheric CO2 has accelerated markedly, paralleling the ac­celerating rate of CO2 production by burning fossil fuels and other industrial activities [Keeling et al., �995].

A primary objective of carbon cycle research is to account for the complete mass balance or “budget” of the CO2 pro­duced by human activities (sometimes termed “anthropo­genic” CO2). A full accounting of the CO2 budget includes its sources, the processes that remove it from the atmosphere (“carbon sinks”), and the places and forms in which the carbon from CO2 is stored (“carbon reservoirs”). Figure 2 shows the estimated magnitudes of recent anthropogenic CO2 fluxes, including rates of uptake by the natural carbon cycle, and their estimated cumulative historical influence on carbon reservoirs. The production of CO2 by burning fossil fuels is clearly the dominant anthropogenic source. Land­use change (primarily deforestation and forest degradation in tropical regions) also releases significant quantities of CO2. yet the annual increase in atmospheric CO2 accounts for only about 40–45% of the CO2 produced annually by deforestation, soil degradation, and fossil fuel consump­tion [Canadell et al., 2007]. Most of the anthropogenic CO2 is being removed from the atmosphere by the response of the natural carbon cycle. Anthropogenic CO2 is being dis­solved in the oceans, and some is apparently being taken up by forest growth and other changes on land. The overall anthropogenic CO2 budget leaves a residual term that can only be explained by a significant counter balancing terres­trial carbon sink. Whereas some findings point toward the likelihood of large forest carbon sinks in northern temperate latitudes [Tans et al., �990; Caspersen et al., 2000], other recent studies suggest the possibility of significant terrestrial uptake in the tropics [Jacobson et al., 2007; Stephens et al., 2007].

One of the most significant uncertainties in projecting future change in the carbon cycle is its potential response to climate change. Recent studies have emphasized the po­tential importance of climate­carbon feedback mechanisms that may be critical to understanding both the climate sys­tem’s sensitivity to CO2 and the carbon cycle’s sensitivity to climate [Fung et al., 2005; Friedlingstein et al., 2006; Cox and Jones, 2008]. The changing carbon cycle is viewed not only as a primary driver of climate change but also as a primary source of uncertainty in projecting future climate trends. For example, boreal forests and northern peatlands, which account for hundreds of billions of tons of global ter­restrial carbon storage, are already experiencing significant warming, resulting in large­scale thawing of permafrost and dramatic changes in aquatic and forest ecosystems [Oechel et al., �993; Ise et al., 2008; Schuur et al., 2008]. Significant

4 INTRODUCTION TO GLOBAL CARBON CyCLE MANAGEMENT

quantities of carbon in these environments may be vulnera­ble to fire and decomposition under warming conditions, po­tentially releasing large quantities of CO2 to the atmosphere. Temperate forests and woodlands may also be susceptible to carbon loss caused by climate change [Breshears et al., 2005; van Mantgem et al., 2009]. Research directed toward improving projections of climate change is increasingly in­tertwined with research directed toward improving projec­tions of the carbon cycle [Friedlingstein et al., 2006; Field et al., 2007].

In response to public concern about the potential future ef­fects of changes in atmospheric CO2 and climate, scientists and engineers are studying possible ways to mitigate these changes through reductions in CO2 emissions, augmenta­tion of carbon sinks, or both. As described in the chapters written for this volume, many investigators are exploring the feasibility of new methods of deliberate carbon storage or sequestration. As with the effects of past and current hu­man activities, potential future efforts to control atmospheric CO2 will involve significant changes in the global carbon

Figure 2. Effects of human activities on the global carbon cycle. Units and left­hand vertical scale are as depicted in Figure 1. The black arrows and numbers represent the natural carbon cycle before the influence of human activities over the last two centuries (from Figure 1; relatively small long-term fluxes are not shown). The grey arrows and the grey and white numbers represent approximate annual fluxes due to human activities in the year 2007. Straight arrows represent anthropogenic sources; curved arrows represent natural carbon fluxes and their response to anthropogenic sources. The dashed lines illustrate potential pathways for deliberate carbon sequestration. Estimated cumulative effects through the year 2007 are shown as added to and subtracted from the values for natural carbon cycle reservoirs, which are given in parentheses. Data sources for the natural carbon cycle are as described for Figure �. Sources for annual anthropo­genic fluxes are from the Global Carbon Project [2008] Carbon budget and trends 2007 [www.globalcarbonproject.org, 26 September 2008], and references cited therein. Cumulative fluxes are from Boden et al. [2009] for fossil fuels (340 GtC); Defries et al. [�999] and Houghton et al. [2008] for land use change (2�0 GtC through 2005); and Field et al. [2007] for oceanic CO2 uptake (�50 GtC, including extrapolation from 2003 through 2007 assuming 2.3 GtC/yr from Global Carbon Project [2008]). The 2007 atmospheric reservoir (8�0 GtC) and annual ocean­atmosphere gas exchange fluxes (25 and 27 GtC/yr) are adjusted to be consistent with the 2007 mean atmospheric CO2 concentration of 383 parts per million. The cumulative uptake by vegetation and soils (�80 GtC) is calculated by difference among the other cumu­lative anthropogenic sources and sinks (340 + 2�0 – 220 – �50 = �80 GtC).

SUNDqUIST ET AL 5

cycle (dashed arrows in Figure 2). Carbon cycle scientists and engineers now face not only the difficulties of record­ing and understanding past and present changes but also the challenge of providing information and tools for new man­agement strategies that are responsive to societal needs. The challenge is nothing less than managing the global carbon cycle.

3. THE SCIENCE AND TECHNOLOGy OF CARBON SEqUESTRATION

The term “carbon sequestration” has been applied in the scientific literature to both natural and deliberate processes that remove CO2 from the atmosphere or from emissions sources. The natural mechanisms of CO2 uptake by the car­bon cycle (the carbon sinks shown in Figures � and 2) are clearly not sufficient to offset the accelerating pace of human CO2 emissions. The need for more direct mitigation strate­gies is a central issue in consideration of energy and environ­mental policies. Options under discussion now encompass atmospheric CO2 reductions through all possible mecha­nisms, including deliberate carbon sequestration; use of re­newable nonfossil fuels and power sources; and increased energy conservation and efficiency. Of these options, only deliberate carbon sequestration offers the possibility of di­rect removal of atmospheric CO2, through enhancement of plant growth and enrichment of soil carbon storage. Like­wise, CO2 emissions captured from power plants and other point sources may be injected into the deep ocean and into subsurface rock formations (Figure 2).

In the paragraphs below, we describe the most widely stud­ied modes of deliberate carbon sequestration. Because the goal of these activities is to mitigate the rise in atmospheric CO2, we examine them in the context of the global carbon cycle fluxes and reservoirs described above. We summarize estimates of potential global sequestration capacities, and we describe important uncertainties and limitations. This information is critical to determining whether deliberate car­bon sequestration can effectively control atmospheric CO2 levels.

3.1. Deliberate Terrestrial Sequestration

Terrestrial carbon sequestration (sometimes termed “bi­ological” or “ecological” sequestration) can be enhanced by the deliberate augmentation of natural carbon uptake in plants and soils. This is accomplished through forest and soil management practices that enhance the storage of car­bon or reduce CO2 emissions. (Please note: The use of for­est and agricultural products to mitigate CO2 emissions by displacement of fossil fuels is beyond the scope of this vol­

ume.) Forest carbon storage can be increased by planting of new forests on previously unforested lands (afforestation) or on previously forested lands (reforestation, for example, on lands that were previously cleared for agriculture). Car­bon storage in managed forests can be enhanced by timber harvest and replanting practices that increase carbon den­sity (i.e., carbon per unit area) and reduce forest degrada­tion. Carbon sequestration can also be enhanced through management of forest stands on farms (agroforestry) and in urban and suburban settings (urban forestry). On agricultural lands, soil carbon storage can be increased by a variety of practices, including improved crop residue management, extended crop rotations, cover crops, erosion control, im­proved water and nutrient management, and increased uti­lization of cultivation systems that require minimal tillage (reduced tillage) or no tillage (no­till). Carbon storage on grazing lands can be increased by practices such as improved fire management and optimized intensity and timing of grazing.

Terrestrial carbon sequestration cannot be separated from broader issues of land use and management. Enhanced se­questration often occurs as an ancillary result of varied land­use objectives and circumstances. As shown in Figure 2, significant quantities of anthropogenic CO2 are taken up by vegetation and soils, but only a small fraction of this uptake results from activities undertaken specifically to sequester carbon. Figure 2 also shows the global importance of CO2 sources attributable to changes in land use, primarily defor­estation and forest degradation. Management changes that reduce these sources of CO2 are widely regarded as an ef­fective forest management strategy for near­term carbon se­questration [Nabuurs et al., 2007]. A measure of the potential for maintaining present carbon sequestration by reducing deforestation and degradation would be the current global rate of approximately �.5 metric gigatons of carbon (GtC)/yr released to the atmosphere by land­use change (Figure 2). However, this potential rate does not take into account the need to replace the lumber and other products that are acquired as a result of the activities that release CO2. Simi­larly, the potential global sequestration capacity of restored land might be the cumulative amount lost due to land­use change (2�0 GtC in Figure 2), but this technical limit would require abandonment of extensive croplands and settlements [Kauppi et al., 200�].

Terrestrial carbon sequestration is inherently limited by (�) rates of plant growth and accumulation of plant­derived carbon and (2) land areas that can be used for sequestration. Estimates of potential terrestrial sequestration therefore de­pend on estimates of potential plant carbon accumulation and land availability. Estimates of carbon accumulation can be based on measurements in forests and soils that are managed

6 INTRODUCTION TO GLOBAL CARBON CyCLE MANAGEMENT

using the sequestration­enhancing practices described above. Long­term regional trends can be based on data accumu­lated over time through forest and soil inventories. Forest inventories are usually focused on timber volume, so conver­sion calculations are necessary to estimate carbon mass and to account for nontimber carbon storage (understory, dead wood, litter, roots, and soil carbon) [see Perry et al., this vol­ume; Kurz and Apps, �999; Smith, 2002]. Similarly, because estimates from soil inventories are frequently limited to the relatively shallow layers (<1 m in depth), additional infor­mation and calculations are needed to estimate the effects of management on more deeply buried soil carbon [Batjes, �996; Davidson and Trumbore, �995; Jobbagy and Jackson, 2000]. Potential future carbon accumulation can be esti­mated by using empirical response curves, which represent idealized rates of carbon accumulation as a function of time following a change in land use or management [Houghton et al., �983; Kurz and Apps, �994; West et al., 2004]. More sophisticated dynamic ecosystem models utilizing data from remote sensing [see chapters by Running et al., Potter et al., and Liu et al., this volume; Schimel et al., �994] are required to estimate carbon accumulation under conditions of chang­ing climate and other disturbances.

Estimates of potential terrestrial carbon accumulation rates are inherently uncertain. Spatial heterogeneities of to­pography, climate, ecosystems, and soil substrates combine to make it very difficult to “scale up” from site-specific mea-surements to regional trends. Plants and soils are vulnerable to fires, pests, and other difficult-to-predict disturbances. Land management decisions are often made on the basis of changing short­term priorities, such as year­to­year weather and market conditions [Brown and Sampson, this volume]. The contributions of some fundamental carbon fluxes are still not well quantified. For example, there has been con­siderable debate about the manner in which carbon fluxes associated with soil erosion and sediment deposition should be included in estimates of terrestrial carbon accumulation [Van Oost et al., this volume; Stallard, �998; Smith et al., 200�; McCarty and Ritchie, 2002; Lal, 2003]. Likewise, the effects of soil inorganic carbon fluxes are a subject of ongo­ing discussion [Mikhailova et al., this volume; Lal and Kim-ble, 2000; Monger and Martinez-Rioz, 200�]. All of these sources of uncertainty are compounded under conditions of changes in climate and land use.

The availability of land is a major source of uncertainty in estimating potential future terrestrial carbon sequestration. The importance of land area to global carbon cycle man­agement can be shown by plotting historical and predicted changes in terrestrial carbon storage as a function of the areas of affected land. Plate � shows historical and poten­tial future forest carbon trends plotted as vectors defined by

changes in carbon storage and land area. Present­day (year 2005) forests occupy about one­third (~4000 Mha) of the global vegetated land surface and account for more than 40% (~�600 GtC) of the carbon stored in terrestrial vege­tation and soils [Saugier et al., 200�; Sabine et al., 2004]. Historical deforestation has reduced global forest area by more than 20% and forest carbon storage by more than �0%. As shown in Plate �a, the range of historical deforestation vectors can be extrapolated to indicate a potential range of trends for future deforestation. These trends also provide an important context for consideration of published estimates of potential future forest carbon sequestration. Plates �b–d show vector representations of potential sequestration re­ported for forestry activities over the next 50–�00 years. Plate �b shows a range of early estimates that did not in­clude estimates of potential effects of deforestation. Plates �c and d show estimates reported by assessments of the Intergovernmental Panel on Climate Change (IPCC) in �996 and 2007, respectively, including estimates of potential fu­ture deforestation [Brown et al., �996; Nabuurs et al., 2007]. The inclusion of deforestation radically alters the projected net effect of global forestry activities. Given the magnitude of anticipated future deforestation reported by the IPCC in 1996 and 2007, the only projections that yield significant net gains in future forest carbon storage are those based on scenarios of very high economic incentives for sequestra­tion activities [Sathaye et al., 2006]. For this reason, the international scientific community has focused increasing attention on the importance of reducing deforestation and forest degradation [Schlamadinger et al., 2005; IPCC, 2007].

Much of the historical deforestation shown in Plate �a is due to the spread of agriculture. Cultivation is known to de­plete soil carbon storage, and the extent of historical carbon depletion in agricultural soils is a benchmark for evaluat­ing potential sequestration by improvements in cropland soil management [Davidson and Ackerman, �993; Lal, 200�, 2004a]. Plate 2 shows a vector plot of estimated historical cropland soil carbon depletion versus area. The bar graph in the figure compares estimates of historical cropland car­bon loss with estimates of potential future cropland carbon sequestration. Although published estimates of historical cropland areas are roughly consistent with independent es­timates of modern cropland areas, the estimates of cropland soil carbon depletion vary by more than a factor of two. Not surprisingly, published estimates of potential cropland soil carbon sequestration capacity vary widely despite generally consistent assumptions about the total cropland land area. Reflecting these uncertainties, published estimates of poten­tial global cropland sequestration rates over the next 20 to 50 years span a range of more than a factor of two (Figure 3). The most recent assessment of the IPCC reported a potential

SUNDqUIST ET AL 7

Plate 1. Historical and projected future trends in global forest area and carbon storage (vegetation plus soils to a �­m depth). (a) Historical and extrapolated potential future deforestation, with insets showing the fields represented by panels (b)­(d). (b) Early estimates of potential future forest carbon sequestration, which did not include estimates of future de­forestation. (c) Estimates of potential future deforestation and forest carbon sequestration reported in Brown et al. [�996]. (d) Estimates of potential future deforestation and forest carbon sequestration reported in IPCC [2007]. Modern values were estimated from data given by Jobbagy and Jackson [2000] and Sabine et al. [2004], adjusted to account for defor­estation to the year 2005. Predisturbance values were estimated from DeFries et al. [�999] and Houghton [2008], with extrapolations forward to 2005 and backward to �700 based on consistent carbon/area ratios. For details of vectors in (b)–(d), see the references cited. Vectors in (d) correspond to scenarios 3 and 6 of Sathaye et al. [2006].

8 INTRODUCTION TO GLOBAL CARBON CyCLE MANAGEMENT

Plate 2. Historical trends in global cropland area and soil carbon storage (vectors), and historical cropland soil carbon loss and potential future sequestration (bar graph relative to scale at left). Modern values are from Ramankutty and Foley [�999] and Jobbagy and Jackson [2000]. To the extent possible, based on documentation in the references cited, the potential sequestration values shown represent only effects of soil carbon management on existing croplands, without estimates of offsets for biofuels. For further details see the references cited.

Plate 3. Types of geologic carbon sequestration. Adapted from Sundquist et al. [2008], drafted by Eric A. Morrissey and Sean Brennan.

SUNDqUIST ET AL 9

rate near the low end of previous published estimates [Smith et al., 2007, 2008], but higher estimates based on earlier studies continue to have support [e.g., Lal, 2008]. Additional soil carbon sequestration may be possible through improved management of grazing lands, restoration of wetlands, and restoration of lands that have been degraded by erosion, sa­linization, desertification, and other disturbances [Lal, 200�, 2008; Smith et al., 2007]. New technologies may also en­hance potential soil carbon sequestration [Post et al., this volume].

The history of human carbon cycle management shows that terrestrial carbon sequestration is closely linked to land­ management decisions that involve multiple resources. Deci­sions about future terrestrial sequestration will require careful consideration of priorities and tradeoffs. For example, con­verting farmlands to forests or wetlands may increase carbon sequestration and provide other environmental benefits, but the loss of farmlands may lead to further decisions to convert land to agriculture in other areas. Similarly, forest manage­ment decisions will reflect not only carbon sequestration, but broader optimization of forest resources under circum­stances that involve much broader factors. As scientists and engineers work to determine the complex effects of clim­ate and land­use change on potential future terrestrial carbon sequestration, they are constantly reminded that the carbon cycle can never be managed in isolation from a wide range of natural resources and human needs.

3.2. Geologic Sequestration

Unlike terrestrial carbon sequestration, geologic CO2 (carbon) sequestration does not rely on the enhancement of natural car­bon cycle processes. It is a more distinctly technological ap­proach that begins with the capture and compression of CO2 from the exhaust of fossil fuel power plants and other major sources. The captured CO2 is then piped to injection sites, where it is pumped � to 3 km below the land surface and injected into porous rock formations (Plate 3). The effective­ness of geologic CO2 sequestration depends on how much CO2 can be injected into subsurface rocks, how long it will stay trapped there, and whether the process of subsurface injection and storage will have any negative environmental consequences. All of these factors will involve both techni­cal and socioeconomic constraints that must be evaluated re­alistically before implementation. (Geological sequestration involves the storage of carbon as CO2; to be consistent with this chapter’s focus on global carbon cycle management, we use the term “geologic carbon sequestration” and express rates and quantities in units of carbon mass.)

Technologies for geologic sequestration build on the long experience of the oil and gas industry in the pumping and managing of subsurface fluids, including deep injection of CO2 to enhance recovery of oil. Compared with the rates of terrestrial carbon uptake shown in Figures � and 2, cur­rent rates of geologic sequestration are very small (on the

Figure 3. Published estimates of potential carbon sequestration rates in agricultural soils. Bars represent range of esti­mates. Short bars represent single values when no range was reported. To the extent possible, based on documentation in the references cited, the values shown represent only effects of soil carbon management on existing croplands, without estimates of offsets for biofuels. For further details, see the references cited.

�0 INTRODUCTION TO GLOBAL CARBON CyCLE MANAGEMENT

order of a few million metric tons of carbon per year [NETL, 2008]. However, much larger rates are envisioned to take advantage of the potential permanence and capacity of geo­logic storage [IPCC, 2005].

The capture and transport of CO2 pose some of the prin­cipal challenges in the implementation of geologic carbon sequestration. For example, power plants that utilize carbon capture and storage (CCS) technologies are expected to re­quire �0–40% more energy than equivalent plants without CCS. CO2 capture technologies for power plants include postcombustion separation from flue gas, separation from syngas produced by the precombustion reaction of coal with high-pressure steam and oxygen (coal gasification), and pro­duction of high­CO2 exhaust gases by using oxygen rather than air in combustion (oxy­combustion). Postcombustion extraction requires the use of liquid solvents to remove the CO2 from the flue gas, followed by extraction of the CO2 from the solvents. Coal gasification and oxy-combustion both produce gases that are more enriched in CO2, thus sim­plifying or avoiding the need for liquid solvent extraction but requiring more extensive investment in precombustion equipment [Thambimuthu et al., 2005; Rubin, 2008].

After capture at power plants or other sources, CO2 is com­pressed for transport and injection. The chemical and physical properties of compressed CO2 play an important role in the feasibility of geologic sequestration. CO2 becomes a liquid when compressed to high pressures for transport. As shown in Plate 4, when CO2 is subjected to the combination of higher pressures and temperatures that characterize geologic injection sites, it becomes what is known as a supercritical fluid. The permanence of geologic sequestration depends on the combined effectiveness of several physical and chemical mechanisms that combine to trap this supercritical CO2. The CO2 is less dense than the saline groundwater it displaces in the pore space of the rock formations where it is injected. It will rise buoyantly until trapped beneath an impermeable barrier, or seal, formed by a stratigraphic or other structural discontinuity. This physical trapping mechanism is compa­rable to the natural geologic trapping of oil and gas and can theoretically retain fluids for thousands to millions of years. CO2 that is not physically trapped in this manner may es­cape through leakage pathways (see below), or may migrate slowly through the rock pore space and become trapped as a residual fluid held in place by molecular surface tension [Ide et al., 2007]. Some of the injected CO2 will eventually dis­solve in groundwater, and some may be trapped in the form of carbonate minerals formed by chemical reactions with the surrounding rock [Kharaka et al., 2006]. All of these proc­esses are susceptible to change over time after CO2 injec­tion [Hovorka et al., 2006]. In general, the physical trapping mechanisms are viewed as more important over short time

scales (a few decades); the relative importance of chemical dissolution and mineral reactions increases over time scales of centuries to thousands of years [Benson et al., 2005; Ben-son and Cole, 2008].

Because the permanence of geologic sequestration is one of its principal benefits, a critical need is to understand the potential for leakage of injected CO2 back to the atmosphere. Faults, fractures, and stratigraphic discontinuities may offer pathways for leakage. Many potential structural traps are known in areas where oil and gas have been extracted, and CO2 injection can enhance oil recovery, so these areas tend to be preferred sequestration injection sites. Unfortunately, these areas are also perforated by existing or abandoned wells that may act as conduits for leakage [Bachu and Celia, this volume; Gasda et al., 2004]. The injection process itself may affect the geomechanical integrity of trapping structures [Hawkes et al., 2005].

Valuable information about potential leakage can be gath­ered in studies of natural geologic analogs of CO2 storage and venting [see Fessenden et al. and Heath et al., this volume; Evans et al., 200�]. However, many aspects of geologic se­questration have no natural analogs. Much has been learned from ongoing commercial sequestration operations such as those in Norway and Algeria. Sequestration field tests also have become a primary venue for learning how to monitor and anticipate the fate of injected CO2 [McPherson, this vol­ume; DOE, 2008]. Numerical models are an essential tool for understanding the complex interactions among the many factors that control fluid chemistry and transport. Models of geochemical interactions, multiphase fluid transport, and the combined effects of geochemistry and transport (“re­active transport”) are widely utilized [Gunter et al., 2000; McPherson and Cole, 2000; White et al., 2005; Kaszuba and Janecky, this volume]. Uncertainties in model simulations must be quantified [Zerai et al. and Han and McPherson, this volume] to provide meaningful assessments of risk. A very wide range of scientific and engineering expertise is being mobilized to understand the permanence of geologic CO2 trapping mechanisms over the full range of potential storage time scales [see Doughty and Myer and McPherson chapters, this volume].

Just as the potential global amount of terrestrial carbon se­questration is limited by available land area, the global capac­ity for geologic carbon sequestration is constrained by the pore volume and distribution of potential storage sites. One frame of reference for geologic storage of injected CO2 is to com­pare the volume of extracted fuels to the volume of injected CO2 produced by combustion of equivalent fuel volumes.

As Figure 4 shows, the combustion of oil and bituminous coal produces volumes of compressed supercritical CO2 that exceed the volume of extracted fuel by factors of three and

SUNDqUIST ET AL ��

Plate 4. Pressure and temperature relationships of carbon dioxide–phase stability fields, with fields representing ocean and terrestrial hydrostatic pressure and temperature gradients. Pressures and temperatures of injection sites are taken from Sasaki et al. [2008].

�2 INTRODUCTION TO GLOBAL CARBON CyCLE MANAGEMENT

Plate 5. Published estimates of global geologic carbon sequestration capacity. Bars indicate range of estimates; short bars indicate single value with no range stated. Where available, separate estimates are shown for deep saline formations, depleted oil and gas reservoirs, and unmineable coal beds.

SUNDqUIST ET AL �3

four, respectively. The combustion of natural gas produces a nearly equal volume of compressed CO2, but the storage of CO2 in dissolved form requires saline groundwater vol­umes many times larger than the volumes of the extracted fuels. The importance of this comparison is not that the vol­ume of injected CO2 will exceed the geologic “void” left by extracted fuels, but that the infrastructure and subsurface impacts of large­scale CO2 sequestration are potentially comparable to and perhaps greater than the vast existing op­erations of the fossil fuel industries.

The large difference between the volume requirements of compressed and dissolved CO2 is one source of uncertainty in estimates of global geologic sequestration capacity. A compilation of published estimates is shown in Plate 5. A recent analysis [Dooley et al., 2006] suggests that the global storage capacity of physical traps associated with depleted oil and gas reservoirs is about 220 GtC. The potential global

storage capacity of deep porous rock formations that contain saline groundwater is much larger (estimated by Dooley et al. to be approximately 3,000 GtC), but these formations are not as well­characterized as oil and gas reservoirs, so less is known about the effectiveness of trapping mechanisms at these sites. Unmineable coal beds have also been proposed for potential CO2 storage, particularly in conjunction with coalbed methane recovery [Gunter et al., �997]. Comparison of the global capacity estimates shown in Plate 4 is difficult because of the many differences in assumptions and calcula­tion methods. Standardization and transparency of capacity assessment methodologies will greatly improve estimates of the potential effectiveness of geologic sequestration [Bachu et al., 2007; CSLF, 2007; DOE, 2008; Burruss et al., 2009; McPherson, this volume].

To fully assess the potential for geologic carbon se­questration, economic costs and environmental risks must

Figure 4. Volumes of compressed CO2 (density 700 kg/m3) and CO2­saturated brine (�9.8 times free­phase volume; DOE Atlas [2008]) produced by combustion of unit volumes of bituminous coal, oil, and natural gas. Conversion factors are shown in Table �.

�4 INTRODUCTION TO GLOBAL CARBON CyCLE MANAGEMENT

be taken into account. Many of these factors will depend on local conditions, and will vary according to the type of storage formation. Depleted oil and gas reservoirs are well characterized and thus less prone to unknown risks, but they are limited in capacity and geographic distribution and may require greater proportional investment in infrastructure. The potential capacity of formations containing saline water is larger and more widely distributed, but few of these for­mations are well­characterized, leading to large uncertainty in capacity estimates. Unmineable coal beds may have the advantages of proximity to large power plants and methane recovery, but their storage characteristics may be poorly characterized, and potential coal resources may be rendered unusable. Environmental risks of CO2 injection may include induced seismic disturbances, deformation of the land sur­face, contamination of potable water supplies, and adverse effects on ecosystems and human health. Numerous regula­tory issues will also affect the implementation of geologic sequestration, including determination of rules affecting in­jection wells, postinjection ownership, and liability across multiple jurisdictions. As with terrestrial sequestration, geo­logic sequestration cannot be accomplished in isolation from a broad range of environmental and societal concerns [Sund-quist et al., 2008].

3.3. Oceanic Sequestration

As shown in Figure 2, the world’s oceans currently ac­count for a global net uptake of about 2 GtC/yr. This up­take is not a result of deliberate sequestration, but occurs naturally through chemical reactions between dissolved in­organic carbon in the ocean surface and the increasing CO2 concentration in the atmosphere. The natural ocean uptake of CO2 was a primary interest of Roger Revelle when he described the “large­scale geophysical experiment” more than 50 years ago. He hired C.D. Keeling to perform the atmospheric and oceanic measurements needed to document the experiment [discussed further in Sundquist and Keeling and Brewer chapters, this volume]. In fact, the oceans will be the primary long­term sink for anthropogenic CO2 that is not sequestered by other means. The chemical reactions between atmospheric CO2 and the ocean surface occur rap­idly, and over time any absorbed CO2 is mixed downward throughout the oceans. Over time scales of thousands of years, any increase in atmospheric CO2 will be attenuated in this way by ocean­surface equilibration and mixing with the entire global ocean volume. The capacity of this natural sequestration mechanism is large; for example, the oceans could absorb approximately �,000 GtC while maintain­

Table 1. Conversion Factors and Sources Used in Volume Calculations for Figure 4Conversion Factors Value Unit Data source

Density of CO2 700 kg/m3 IPCC, 2005Density of crude oil 860 kg/m3 Ecofys, 2002Density of natural gas at injection conditions �30–300 kg/m3 Oldenburg, 2007Density of natural gas at STP 0.7 kg/m3 Braker and Mossman, �980Density of bituminous coal �320 kg/m3 Wood et al., �983Heat content of natural gas �028 Btu/ft3 EIA. 2008Heating value of bituminous coal �9–30 MMBtu/metric ton EIA. �995 Carbon coefficientsNatural gas ��7.� lbs CO2/MMBtu EIA. 2006Bituminous coal 205.3 lbs CO2/MMBtu EIA. 2006Liquefied petroleum gases 537.8 lbs CO2/barrel EIA. 2006Motor gasoline 822.9 lbs CO2/barrel EIA. 2006Distillate fuel oil 940.� lbs CO2/barrel EIA. 2006Residual fuel oil �093.4 lbs CO2/barrel EIA. 2006Jet fuel (kerosene jet fuel + aviation gasoline) 904.6 lbs CO2/barrel EIA. 2006Petroleum coke �356.5 lbs CO2/barrel EIA. 2006

Major refined products of crude oil (%) Liquefied petroluem gases 4.3 percent EIA. 2009Motor gasoline 46.� percent EIA. 2009Distillate fuel oil 23.5 percent EIA. 2009Residual fuel oil 4.6 percent EIA. 2009Jet fuel (kerosene jet fuel + aviation gasoline) 9.9 percent EIA. 2009Petroleum coke 4.8 percent EIA. 2009

SUNDqUIST ET AL �5

ing equilibrium with present­day atmospheric CO2 levels [Kheshgi et al., 2005; Caldeira et al., 2005]. This capacity could be enhanced by chemical reactions between dissolved CO2 and marine carbonate sediments. However, the rate of ocean mixing limits natural ocean CO2 absorption to a pace that cannot match the rate of human production of industrial CO2. Moreover, the same long­term mixing and equilibra­tion would assure that any deliberate injection of CO2 into the deep ocean, although potentially sequestering the CO2 for a period of perhaps centuries, would eventually equili­brate with higher levels of CO2 in the atmosphere.

Injection of CO2 into the oceans will require capture tech­nologies and infrastructure similar to those described above for geologic sequestration. In typical ocean temperatures, injected CO2 will be in liquid form at depths greater than 400–500 m (Plate 4). At high concentrations, the liquid CO2 may form a solid hydrate phase. At depths greater than 3,000 m, the liquid CO2 is compressed to a density greater than that of the surrounding seawater. These properties have led to various suggestions for pumping CO2 into the oceans at rates sufficient to mitigate rising atmospheric CO2 levels and to reduce the peak concentrations expected without mitiga­tion in the next century [Caldeira et al., 2005; Adams and Caldeira, 2008]. For example, it has been proposed that liq­uid CO2 could be sequestered as a CO2 “lake” on the sea floor [Ohsumi, �995; Haugan and Alendal, 2005) or perhaps injected into sediments beneath the deep sea floor [House et al., 2006].

A significant deterrent to oceanic sequestration is the grow­ing evidence for negative impacts of acidification caused by the chemical reactions that occur when CO2 is dissolved in seawater [Feely et al., this volume]. Many marine organ­isms and ecosystems depend on the formation of carbonate skeletons and sediments that are vulnerable to dissolution in acidic waters. Laboratory and field measurements indicate that CO2-induced acidification may eventually cause the rate of dissolution of carbonate to exceed its rate of formation in these ecosystems. The impacts of ocean acidification on coastal and marine food webs and other resources are poorly understood [Kleypas et al., 2006]. Similarly, many environ­mental uncertainties have deterred proposals to enhance the ocean CO2 uptake by fertilizing marine ecosystems [Bues-seler et al., 2008; Adams and Caldeira, 2008]. As with other sequestration methods, the implementation of oceanic sequestration requires consideration of wide­ranging eco­nomic, environmental, and sociopolitical constraints.

3.4. Geochemical Sequestration

Because of the urgency of mitigating the effects of indus­trial CO2 production, and because of the many constraints

on implementation of the carbon sequestration methods de­scribed above, prominent scientists have pressed for broad consideration of novel approaches to global carbon cycle management [National Research Council, 2003; Cicerone, 2006; Broecker, 2007]. Inevitably, these approaches include geochemical methods that mimic the long­term cycling of CO2 through the Earth’s crust [Lackner, 2002]. When CO2 is dissolved in water, it forms carbonic acid, which reacts on land with carbonate and silicate minerals during chemi­cal weathering to yield dissolved bicarbonate and carbonate ions. When these ions are transported by rivers and streams to the oceans, they combine with dissolved calcium and magnesium to form carbonate minerals that accumulate in sediments and thus are returned to the Earth’s crust. Geo­chemical sequestration occurs when these reactions lead to net accumulation of carbon in dissolved or mineral form. Their natural geologic effectiveness is conspicuous in the large amounts of carbon dissolved in the oceans and retained in limestones and other carbonate sediments.

Deliberate geochemical sequestration involves the accel­eration of natural weathering and burial processes. Methods have been suggested for increasing the rates of weathering of silicate minerals [Lackner et al., �997] and carbonate minerals [Rau and Caldeira, �999]. Some proposals focus on the accelerated formation of carbonate minerals (mineral carbonation), which are generally more stable than the com­bination of silicate and dissolved CO2 reactants [McGrail et al., 2006; Oelkers et al., 2008]. A particularly creative idea is the addition of alkalinity to ocean surface waters, which would enhance natural ocean uptake of atmospheric CO2 while buffering ocean acidification [Kheshgi, �995]. Although high costs and slow rates of reaction make geo­chemical sequestration less attractive now than many alter­natives, this option certainly merits further investigation and discovery during the current period of nascent carbon cycle management [Stephens and Keith, 2008].

4. CAN CARBON SEqUESTRATION CONTROL ATMOSPHERIC CARBON DIOXIDE?

Each of the sequestration methods examined above is sub­ject to practical constraints that extend well beyond its par­ticular technical limitations and uncertainties. To determine whether carbon sequestration can effectively control atmos­pheric CO2, a broad convergence of expertise in science, technology, economics, and policy is necessary. One ap­proach to this requirement is the development of integrated assessment models that combine simulations of carbon cycle interactions with scenarios that represent potential future so­cial and economic trends [Gitz et al., this volume]. Recently, the U.S. Climate Change Science Program (CCSP) published

�6 INTRODUCTION TO GLOBAL CARBON CyCLE MANAGEMENT

an analysis of results from three such models that were con­strained by a common set of prescribed trends for stabili­zation of atmospheric greenhouse gases (mainly CO2) over the next century [CCSP, 2007]. These simulations indicate that a reduction of more than 75% in projected annual global emissions would be necessary to stabilize atmospheric CO2 at about 550 parts per million during the next century. This concentration would be about twice the level of CO2 in the preindustrial atmosphere and about 45% higher than the at­mospheric CO2 concentration in 2007.

The CCSP model scenarios include incentives for ag­gressive implementation of geologic carbon sequestration, as well as other mitigation measures. As shown in Plate 6a, the estimated amount of global geologic sequestration over the next century is projected in all three models to be sub­stantially smaller than the cumulative emission reductions needed from changes by all other methods. In these models, the needed amount of geologic sequestration is also smaller than the estimated global capacity of depleted oil and gas reservoirs, which implies a limited global utilization of car­bon storage in relatively unknown deep formations that con­tain saline water (Plate 6b). However, individual countries may not have adequate depleted oil and gas reservoirs to meet their project geologic sequestration needs.

The model simulations shown in Plate 6 do not reflect many of the uncertainties in costs and environmental risks of geologic carbon sequestration. The models did not compare scenarios for future deliberate oceanic, terrestrial, or geo­chemical sequestration. Future disturbances of vegetation and soils may add to future CO2 emissions and increase the amount of mitigation required to stabilize atmospheric CO2. Nevertheless, the CCSP simulations illustrate the important perspective that sequestration is necessary but insufficient to control atmospheric CO2. Stabilizing atmospheric CO2 is likely to require substantial changes in energy sources and use as well as carbon management. Many of these changes will probably have significant, long-lasting impacts on land, water, and ecosystem resources. The CCSP [2007, p. 3] integrated modeling report concludes that stabilizing at­mospheric CO2 will “require a transformation of the global energy system, including reductions in the demand for en­ergy . . . and changes in the mix of energy technology and fuels.” The task of managing the global carbon cycle has become large indeed!

5. CONCLUSION: BEGINNERS WELCOME

We are witnessing the unintended consequences of our human “experiment” in global carbon cycle management. Past and present events demonstrate the profound extent of human influence on the carbon cycle, yet we are humbled

by the task of deliberately redirecting this influence. The challenge of controlling atmospheric CO2 levels is twofold: (�) maximizing existing mechanisms of carbon storage in terrestrial and oceanic components of the carbon cycle, and (2) minimizing CO2 emissions through deliberate car­bon sequestration and changing energy technologies. Each of these approaches requires improved understanding that extends beyond simply accounting for current and potential CO2 sources and sinks. Each has distinct geological, hydro­logical, and ecological impacts that must be assessed on a wide range of spatial and temporal scales. Each approach is dependent on the other: Trends in existing CO2 sinks will depend on the nature and methods of changes in CO2 emissions, and CO2 reduction strategies will be framed by anticipated changes in existing CO2 sinks. Rational policy discussion and implementation will require an integrated perspective that includes both maximization of existing car­bon storage and reduction of CO2 emissions. The separation between carbon cycle research and carbon cycle manage­ment is not sustainable.

The need for integration extends beyond the traditional boundaries of science and engineering to encompass the “hu­man dimensions” of factors that affect fossil fuel consump­tion, energy and agriculture utilization, land management, and policy at scales ranging from local to global. Scientists and engineers must provide information that can be under­stood and applied outside their own communities. The poten­tial effectiveness of carbon sequestration must be estimated by methods that quantify effects on atmospheric CO2 and the global carbon cycle. National inventories of CO2 sources and sinks are conducted annually using rigorous international guidelines developed by the IPCC under the United Nations Framework Convention on Climate Change (UNFCCC). The IPCC inventory guidelines are widely cited in the for­mulation of accounting standards for carbon sequestration activities, such as those evaluated under the Kyoto Protocol. Although not ratified by some countries with very large CO2 emissions, the Protocol has initiated emissions trading and other greenhouse gas emissions mitigation mechanisms that have required the elaboration of rigorous standards for car­bon sequestration accounting [see Roed-Larsen and Flach, this volume]. Estimates of potential carbon sequestration extend into the future well beyond the limits of current agreements, but the credibility of these estimates will de­pend on monitoring and verification standards that are being developed from the IPCC inventory guidelines, the Kyoto Protocol accounting methodologies, and other emerging in­ternational protocols and standards.

International greenhouse gas accounting standards are converging around principles of relevance, completeness, consistency, transparency, and accuracy. These principles

SUNDqUIST ET AL �7

Plate 6. Estimated global atmospheric CO2 (as carbon) mitigation needs and potential sequestration capacities. (a) Modeled cumulative global CO2 emissions reduction and sequestration needed by the year 2�00 to stabilize atmo­spheric CO2 at 550 parts per million. Models used by CCSP [2007] were IGSM [Sokolov et al., 2005], MERGE [Manne and Richels, 2005], and MINICAM [Brenkert et al., 2003]. (b) Estimated global sequestration capacities. Maximum terres­trial sequestration capacity corresponds to the total historical depletion of vegetation and soil carbon as given in Figure 2. Potential geologic sequestration capacities are from Dooley et al. [2006].

�8 INTRODUCTION TO GLOBAL CARBON CyCLE MANAGEMENT

are quite familiar to scientists and engineers, whose work depends entirely on the acquisition and communication of high­quality information. The accounting principles are consistent with widely accepted guidelines for sequestra­tion project-level monitoring, verification, and accounting [Monea et al., this volume] and with recommendations on the documentation and availability of sequestration data [Cook et al., this volume]. Most important, standards of in­formation quality and transparency are essential to effective communication with nonscientists concerning the wide array of decisions that are inherent in carbon cycle management. Conversely, scientists and engineers are well­advised to be aware of the sociopolitical drivers that may be important factors in these decisions [see Stephens, this volume].

The nature of carbon cycle management demands com­munication of information across many groups and interests. We must be prepared to talk to “newcomers” from outside our particular disciplines and communities of practice. Be­ginners are welcome!

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