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Vulnerability of the carbon cycle: nonlinear dynamics, thresholds, and regime shifts

Dynamics of the carbon-climate-human system are likely to contain unknown surprises and thresholds induced by nonlinear feedbacks and interactions among major compartments and processes of the system. These include:

  • The stability of ocean circulation (e.g., through possible slowdown or shutoff of the thermohaline circulation).
  • The ability of terrestrial ecosystems to sequester carbon in the future as mechanisms responsible for current sinks saturate (CO2 fertilisation; forest regrowth after abandonment).
  • The uncertain permanence of current terrestrial carbon stocks due to changes on control processes such as switches due to phenology, soil respiration, changes in seasonal freeze-thaw dynamics, thawing of permafrost, changes in water table, drought, absence/presence of snow, fire, insect infestation.
  • Feedbacks between terrestrial and marine systems such as ocean NPP enhancement due to dust deposition from land.
  • The societal and policy drivers for the changes in carbon systems and for carbon management (linked with the perceptions of risk, due to changing climate and consequent development of new institutional regimes to control greenhouse gas accumulation in the atmosphere).

Most of these processes are the result of interactions between the changing climate, human systems and the global carbon cycle, with the potential for accelerating or decelerating the build up of atmospheric CO2. The implications of these interactions are that, to achieve stabilisation, an enormous reduction in fossil fuel emission and increase in carbon sequestration is urgently needed.

Although such interactions are likely to occur, their quantification is difficult and is restricted to subsets of their components for the Earth system because fully coupled carbon-climate-human models do not exist.

Coupled carbon-climate models, however, have provided major insights on the types of possible nonlinear responses. These models show a slow down of the terrestrial sink strength by the middle of this century, with it becoming a source by the end of the century. Coupling biophysical and decision-making models have also produced unpredicted results.