Observing a Vulnerable Carbon Cycle

The carbon cycle and indeed the entire earth system are now inextricably linked with human activities (Global Carbon Project 2003; Steffen et al. 2004; Field and Raupach 2004), so that the ‘carbon—climate—human system’ constitutes a single, coupled entity in which interacting processes link all of its major components. Linking processes of primary significance include

• 1. The human drivers of energy consumption and land-use change, through increases in both population and per capita consumption

• 2. The role of human energy systems as sources of CO₂ and other greenhouse gases (GHGs)

• 3. Land-use change (deforestation, increases in agricultural and urban land use) and its consequences for both GHG emissions and resource (water, land, ecosystem) condition

• 4. Climate forcing by CO₂ and other GHGs, following from drivers 1, 2 and 3

• 5. The changing roles of the ocean and the terrestrial biosphere as sinks and sources of CO₂ and other GHGs, driven by the disequilibrium of the earth system through human activities

• 6. Impacts of climate change through declines in resource condition and human well-being

• 7. Attempts by human societies to reduce their impact on the global environment, for example, through reductions in GHG emissions to avoid ‘dangerous climate change’ (Schellnhuber et al. 2006)

Through the first six of these processes humankind is unintentionally influencing the earth system, while the seventh is an effort to manage global-scale human impacts on the earth system by mitigating their causes.