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The Global Carbon Cycle
Integrating Humans, Climate, and the Natural World
By Christopher B. Field, Michael R. Raupach, Susan Hill MacKenzie ISLAND PRESS
Copyright © 2004 Scientific Committee on Problems of the Environment (SCOPE)
All rights reserved.
ISBN: 978-1-61091-075-0
CHAPTER 1
The Global Carbon Cycle: Integrating Humans, Climate, and the Natural World
Christopher B. Field, Michael R. Raupach, and Reynaldo Victoria
The Carbon-Climate-Human System
It has been more than a century since Arrhenius (1896) first concluded that continued emissions of carbon dioxide from the combustion of fossil fuels could lead to a warmer climate. In the succeeding decades, Arrhenius's calculations have proved both eerily prescient and woefully incomplete. His fundamental conclusion, linking fossil-fuel combustion, the radiation balance of the Earth system, and global climate, has been solidly confirmed. Both sophisticated climate models (Cubasch et al. 2001) and studies of past climates (Joos and Prentice, Chapter 7, this volume) document the link between atmospheric CO2 and global climate. The basic understanding of this link has led to a massive investment in detailed knowledge, as well as to political action. The 1992 United Nations Framework Convention on Climate Change is a remarkable accomplishment, signifying international recognition of the vulnerability of global climate to human actions (Sanz et al., Chapter 24, this volume).
Since Arrhenius's early discussion of climate change, scientific understanding of the topic has advanced on many fronts. The workings of the climate system, while still uncertain in many respects, are well enough known that general circulation models accurately reproduce many aspects of past and present climate (McAvaney et al. 2001). Greenhouse gas (GHG) emissions by humans are known with reasonable accuracy (Andres et al. 1996), including human contributions to emissions of greenhouse gases other than CO2 (Prinn, Chapter 9, this volume). In addition, a large body of literature characterizes land and ocean processes that release or sequester greenhouse gases in the context of changing climate, atmospheric composition, and human activities. Much of the pioneering work on land and ocean aspects of the carbon cycle was collected in or inspired by three volumes edited by Bert Bolin and colleagues and published by SCOPE (Scientific Committee on Problems of the Environment) in 1979 (Bolin et al. 1979), 1981 (Bolin 1981), and 1989 (Bolin et al. 1989).
The Intergovernmental Panel on Climate Change (IPCC), established by the United Nations as a vehicle for synthesizing scientific information on climate change, has released a number of comprehensive assessments, including recent reports on the scientific basis of climate change (Houghton et al. 2001), impacts of climate change (McCarthy et al. 2001), and potential for mitigating climate change (Metz et al. 2001). These assessments, which reflect input from more than 1,000 scientists, summarize the scientific literature with balance and precision. The disciplinary sweep and broad participation of the IPCC efforts are great strengths.
This volume is intended as a complement to the IPCC reports and as a successor to the SCOPE carbon-cycle books of the 1970s and 1980s. It extends the work of the IPCC in three main ways. First, it provides an update on key scientific discoveries in the past few years. Second, it takes a comprehensive approach to the carbon cycle, treating background and interactions with substantial detail. Managed aspects of the carbon cycle (and aspects subject to potential future management) are discussed within the same framework as the historical and current carbon cycle on the land, in the oceans, and in the atmosphere. Third, this volume makes a real effort at synthesis, not only summarizing disciplinary perspectives, but also characterizing key interactions and uncertainties between and at the frontiers of traditional disciplines.
This volume's centerpiece is the concept that the carbon cycle, climate, and humans work together as a single system (Figure 1.1). This systems-level approach focuses the science on a number of issues that are almost certain to be important in the future and that, in many cases, have not been studied in detail. Some of these issues concern the driving forces of climate change and the ways that carbon-climate-human interactions modulate the sensitivity of climate to greenhouse gas emissions. Others concern opportunities for and constraints on managing greenhouse gas emissions and the carbon cycle.
The volume is a result of a rapid assessment project (RAP) orchestrated by SCOPE (http://www.icsu-scope.org) and the Global Carbon Project (GCP, http://www.globalcarbonproject.org). Both are projects of the International Council for Science (ICSU, http://www.icsu.org), the umbrella organization for the world's professional scientific societies. The GCP has additional sponsorship from the World Meteorological Organization (http://www.wmo.ch) and the Intergovernmental Oceanographic Commission (http://ioc.unesco.org/iocweb/). The RAP process assembles a group of leading scientists and challenges them to extend the frontiers of knowledge. The process includes mutual education through a series of background papers and an intensive effort to develop cross-disciplinary perspectives in a series of collectively written synthesis papers. To provide timely synthesis on rapidly changing issues, the timeline is aggressive. All of the authors worked with the editors and the publisher to produce a finished book within nine months of the synthesis meeting.
The book is organized into seven parts. Part I contains the crosscutting chapters, which address the current status of the carbon cycle (Sabine et al., Chapter 2), the future carbon cycle of the oceans and land (Gruber et al., Chapter 3), possible trajectories of carbon emissions from human actions (Edmonds et al., Chapter 4), approaches to reducing emissions or sequestering additional carbon (Caldeira et al., Chapter 5), and the integration of carbon management in the broader framework of human and Earth-system activities (Raupach et al., Chapter 6). Part II surveys the carbon cycle, including historical patterns (Joos and Prentice, Chapter 7), recent spatial and temporal patterns (Heimann et al., Chapter 8), greenhouse gases other than CO2 (Prinn, Chapter 9), two-way interactions between the climate and the carbon cycle (Friedlingstein, Chapter 10), and the socioeconomic trends that drive carbon emissions (Nakicenovic, Chapter 11). Parts III through VII provide background and a summary of recent findings on the carbon cycle of the oceans (Le Quéré and Metzl, Chapter 12; Greenblatt and Sarmiento, Chapter 13), the land (Foley and Ramankutty, Chapter 14; Baldocchi and Valentini, Chapter 15; Nabuurs, Chapter 16), land-ocean margins (Richey, Chapter 17; Chen, Chapter 18), humans and the carbon cycle (Romero Lankao, Chapter 19; Lebel, Chapter 20; Tschirley and Servin, Chapter 21), and purposeful carbon management (Sathaye, Chapter 22; Edmonds, Chapter 23; Sanz et al., Chapter 24; Manne and Richels, Chapter 25; Bakker, Chapter 26; Brewer, Chapter 27; Smith, Chapter 28; and Robertson, Chapter 29).
The key messages from this assessment focus on five main themes that cut across all aspects of the carbon-climate-human system. The overarching theme of the book is that all parts of the carbon cycle are interrelated. Understanding will not be complete, and management will not be successful, without a framework that considers the full set of feedbacks, a set that almost always transcends both human actions and unmanaged systems. This systems perspective presents many challenges, because the interactions among very different components of the carbon cycle tend to be poorly recognized and understood. Still, the field must address these challenges. To do that, we must start with four specific themes that link the ideas discussed throughout the book. These four themes are (1) inertia and the consequence of entrained processes in the carbon, climate, and human systems, (2) unaccounted-for vulnerabilities, especially the prospects for large releases of carbon in a warming climate, (3) gaps between reasonable expectations for future approaches to managing carbon and the requirements for stabilizing atmospheric CO2, and (4) the need for a common framework for assessing natural and managed aspects of the carbon cycle. Each of these themes is previewed here and discussed extensively in the following chapters.
Inertia
Many aspects of the carbon-climate-human system change slowly, with a strong tendency to remain on established trajectories. As a consequence, serious problems may be effectively entrained before they are generally recognized (Figure 1.2). Effective management may depend on early and consistent action, including actions with financial costs. The political will to support these costs will require the strongest possible evidence on the nature of the problems and the efficiency of the solutions.
The carbon-climate-human system includes processes that operate on a wide range of timescales, including many that extend over decades to centuries. The slow components have added tremendously to the challenge of quantifying human impacts on ocean carbon (Sabine et al., Chapter 2) and ocean heat content (Levitus et al. 2000). They also prevent the ocean from quickly absorbing large amounts of anthropogenic carbon (Sabine et al., Chapter 2) and underlie the very long lifetime of atmospheric CO2.
Several new results highlight the critical role of inertia for the carbon cycle on land. It is increasingly clear that a substantial fraction of the current terrestrial sink, perhaps the majority, is a consequence of ecosystem recovery following past disturbances. Across much of the temperate Northern Hemisphere, changes in forestry practices, agriculture, and fire management have allowed forests to increase in biomass or area (Nabuurs, Chapter 16). Evidence that much of the recent sink on land is a result of land management has important implications for the future trajectory of the carbon cycle. Beginning with Bacastow and Keeling (1973), most estimates of future carbon sinks have assumed that recent sinks were a consequence of CO2 fertilization of plant growth and that past responses could be projected into the future with a CO2-sensitivity coefficient or beta factor (Friedlingstein et al. 1995). To the extent that recent sinks are caused by management rather than CO2 fertilization, past estimates of future sinks from CO2 fertilization are likely to be too optimistic (Gruber et al., Chapter 3). Eventual saturation in sinks from management (Schimel et al. 2001) gives them a very different trajectory from that of sinks from CO2fertilization, especially those calculated by models without nutrient limitation (Prentice 2001).
In the human system, inertia plays a number of critical roles. The dynamics of development tend to concentrate future growth in carbon emissions in countries with developing economies (Romero Lankao, Chapter 19). This historical inertia, combined with potentially limited resources for carbon-efficient energy systems (Sathaye, Chapter 22), creates pressure for massive future emissions growth. Slowly changing institutions and incentive mechanisms in all countries (Lebel, Chapter 20) tend to entrain emissions trajectories further.
Inertia is profoundly important in the energy system, especially in the slow pace for introducing new technologies. The slow pace reflects not only the long time horizon for research and development, but also the long period required to retire existing capital stocks (Caldeira et al., Chapter 5). The long time horizon for bringing technologies to maturity and retiring capital stocks is only part of the timeline for the non-emitting energy system of the future, which also depends on the development of fundamentally new technologies (Hoffert et al. 2002). The search for fundamentally new energy sources cannot, however, constitute the entire strategy for action, because the entrained damage may be unacceptably large before new technologies are ready (Figure 1.2). A diverse portfolio of energy efficiency, new technologies, and carbon sequestration offers the strongest prospects for stabilizing atmospheric CO2 (Caldeira et al., Chapter 5).
Vulnerability
A fundamental goal of the science of the carbon-climate-human system is to understand and eventually reduce the Earth's vulnerability to dangerous changes in climate. This agenda requires that we understand the mechanisms that drive climate change, develop strategies for minimizing the magnitude of the climate change that does occur, and create approaches for coping with the climate change that cannot be avoided. Successful pursuit of this agenda is simpler when the carbon-climate-human system generates negative feedbacks (that tend to suppress further climate change), and it is more complicated when the system generates positive feedbacks (Figure 1.1). Positive feedbacks are especially challenging if they occur suddenly, as threshold phenomena, or if they involve coupled responses of the atmosphere, land, oceans, and human activities.
We are entering an era when we need not—and in fact must not—view the question of vulnerability from any single perspective. The carbon-climate-human system generates climate change as an integrated system. Attempts to understand the integrated system must take an integrated perspective. Mechanistic process models, the principal tools for exploring the behavior of climate and the carbon cycle on land and in the oceans, are increasingly competent to address questions about interactions among major components of the system (Gruber et al., Chapter 3). Still, many of the key interactions are only beginning to appear in models or are not yet represented. For these interactions, we need a combination of dedicated research and other tools for taking advantage of the available knowledge. In assessing the vulnerability of the carbon cycle to the possibility of large releases in the future, we combine results from mechanistic simulations with a broad range of other kinds of information.
Several new lines of information suggest that past assessments have underestimated the vulnerability of key aspects of the carbon-climate-human system. Several of these concern climate-carbon feedbacks. Simulations with coupled climate-carbon models demonstrate a previously undocumented positive feedback between warming and the terrestrial carbon cycle, in which CO2 releases that are stimulated by warming accelerate warming and further CO2 releases (Friedlingstein, Chapter 10, this volume). The experiments to date are too limited to support an accurate quantification of this positive feedback, but the range of results highlights the importance of further research. The behavior of two models of comparable sophistication is so different that, with similar forcing, they differ in atmospheric CO2 in 2100 by more than 200 parts per million (ppm).
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Excerpted from The Global Carbon Cycle by Christopher B. Field, Michael R. Raupach, Susan Hill MacKenzie. Copyright © 2004 Scientific Committee on Problems of the Environment (SCOPE). Excerpted by permission of ISLAND PRESS.
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