The current global carbon cycle is in a state of transition. Human activities over the past few centuries have had a profound impact on many aspects of the system. As we begin to assess ways to monitor and potentially manage the global carbon cycle, it is imperative that we better understand how the system, including human dynamics and the bio-

geochemical processes controlling CO2 and other carbon gases in the atmosphere, has operated in the past and how it is operating today. How will the land-ocean-atmosphere processes respond to human activities, and what are the societal dynamics that will determine how humans will respond to changes in the land, ocean, and atmosphere systems? We have made tremendous progress over the past few decades at reducing the uncertainties in Table 2.1 and Colorplate 1, but until we can confidently explain and model the contemporary carbon cycle, our ability to predict future changes in atmospheric CO2 concentrations will be limited.

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