THE VULNERABILITY OF THE CARBON CYCLE IN THE 21st CENTURY: AN ASSESSMENT OF CARBON-CLIMATE-HUMAN INTERACTIONS

Reference

Gruber, N., P. Friedlingstein, C. B. Field, R. Valentini, M. Heimann, J. E. Richey, P. Romero-Lankao, D. Schulze, and C.-T.A. Chen, The vulnerability of the carbon cycle in the 21st century: An assessment of carbon-climate-human interactions, In: Toward CO2 Stabilization: Issues, Strategies, and Consequences, Eds: C. B. Field and M. R. Raupach, Island Press, 2003.

Abstract
In most scenario calculations to date, emissions from fossil-fuel burning are prescribed, and a carbon cycle model computes the time evolution of atmospheric CO2 as the residual between emissions and uptake by land and ocean, typically without considering feedbacks of climate on the carbon cycle (see, e.g., Schimel et al. 1996). The global carbon cycle is, however, intimately embedded in the physical climate system and tightly interconnected with human activities. As a consequence, climate, the carbon cycle, and humans are linked in a network of feedbacks, of which only those between the physical climate system and the carbon cycle have been explored so far (Friedlingstein, Chapter 10, this volume). One example of a carbon-climate feedback begins with the modification of climate through increasing atmospheric CO2 concentration. This modification affects ocean circulation and consequently ocean CO2 uptake (e.g.,Sarmiento et al. 1998; Joos et al. 1999; Matear and Hirst 1999). Similar feedbacks occur on land. For example, rising temperatures lead to higher soil respiration rates, which lead to greater releases of carbon to the atmosphere (e.g., Cox et al. 2000; Friedlingstein et al. 2003). Human actions can also lead to feedbacks on climate. If climate change intensifies pressure to convert forests into pastures and cropland, then the climate change may be amplified by the human response (Raupach et al., Chapter 6, this volume). These positive feedbacks increase the fraction of the emitted CO2 that stays in the atmosphere, increasing the growth rate of atmospheric CO2 and accelerating climate change. Negative feedbacks are also possible. For example, a northward extension of forest or increased rates of plant growth in a warmer climate could increase rates of carbon storage, constraining further climate change.

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