The Carbon ClimateHuman 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 ( and the Global Carbon Project (GCP, Both are projects of the International Council for Science (ICSU,, the umbrella organization for the world's professional scientific societies. The GCP has additional sponsorship from the World Meteorological Organization ( and the Intergovernmental Oceanographic Commission ( 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 aggres-

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