Current Status and Past Trends of the Global Carbon Cycle

Christopher L.Sabine, Martin Heimann, Paulo Artaxo, Dorothee C. E. Bakker, Chen-Tung Arthur Chen, Christopher B. Field, Nicolas Gruber, Corinne Le Quere, Ronald G. Prinn, Jeffrey E. Richey, Patricia Romero Lankao,Jayant A. Sathaye, and Riccardo Valentini

In a global, long-term perspective, the record of atmospheric CO2 content documents the magnitude and speed of climate-driven variations, such as the glacial-interglacial cycles (which drove CO2 variations of ~100 parts per million [ppm] over 420,000 years). These observations also, however, document a remarkable stability, with variations in atmospheric CO2 of <20 ppm during at least the last 11,000 years before the Industrial Era (Joos and Prentice, Chapter 7, this volume). In this longer-term context, the anthropogenic increase of ~100 ppm during the past 200 years is a dramatic alteration of the global carbon cycle. This atmospheric increase is also a graphic documentation of profound changes in human activity. The atmospheric record documents the Earth system's response to fossil-fuel releases that increased by more than 1,200 percent between 1900 and 1999 (Nakicenovic, Chapter 11, this volume).

To understand and predict future changes in the global carbon cycle, we must first understand how the system is operating today. In many cases the current fluxes of carbon are a direct result of past processes affecting these fluxes (Nabuurs, Chapter 16, this volume). Thus, it is important to understand the current carbon cycle in the context of how the system has evolved over time. The Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC) recently compiled a global carbon budget (Prentice et al. 2001). While that budget reflected the state of the art at that time, this chapter presents a revised budget based on new information from model studies and oceanographic observations. The IPCC-TAR budget focused on the overall carbon bal ance between the major active reservoirs of land, atmosphere, and ocean. In this chapter we present a somewhat more comprehensive representation of the connections between the reservoirs, together with our current understanding of the biogeochemical processes and human driving forces controlling these exchanges. We also introduce the key processes involved in controlling atmospheric concentrations of CO2 and relevant non-CO2 gases (e.g., CH4, N2O) that may be susceptible to changes in the future, either through deliberate management or as direct and indirect consequences of global change.

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