Introduction

The global carbon (C) cycle can be characterized in multiple ways. In Chapter 2, C pools, fluxes, and mechanisms controlling the size and rates of pools and fluxes of C are reviewed as a whole. In this chapter, the primary concern is with annual net exchanges of C between the atmosphere and three other major C pools – land, oceans, and fossil fuels – with an emphasis on the role that land use and land-use change have in adding C to or removing it from the atmosphere. The net exchanges are important because they determine the rate at which carbon dioxide (CO2) accumulates in the atmosphere and, hence, the rate and extent of climate change.

The dominant feature of the annual net global emissions of C from land use and land-use change is the gradually increasing trend over the past 155 years (1850 to 2005; Figure 3.1) and probably much longer (Kaplan, Krumhardt, and Zimmermann 2009). This trend, however, is misleading for at least two reasons. First, the trend is not representative of any specific geographic region; because the trend is global, it obscures regional trends. The gradually increasing trend is the result of recently decreasing net emissions and increasing sinks in developed countries (in temperate and boreal zones) offsetting more rapidly increasing net emissions from developing countries in the tropics (Figure 3.2). Second, the net emissions hide the much larger gross sources and sinks of C from land use and land-use change (Figure 3.3). More C is emitted to and removed from the atmosphere each year as a direct result of human activity than is revealed by estimates of the net flux.