Global Biogeochemical Feedbacks in the
Earth System
Martin Heimann
Max-Planck-Institute for Biogeochemistry, Jena, Germany
Martin.heimann@bgc-jena.mpg.de
Climate change induced feedbacks in global biogeochemistry have
been postulated since a long time ago. Such biogeochemical feedbacks
can arise primarily on two levels: (1) In a global context, changes in
biological and chemical processes on land and in the ocean may
induce changes in atmospheric greenhouse gas abundances as well as
aerosols, and thus impact the radiative balance of the atmosphere. (2)
On a more local scale, changes in surface properties, such as land
vegetation cover or ocean color affect the local surface-atmosphere
exchanges of energy, moisture and momentum. A quantitative
assessment of these effects necessitates the development of global
climate models that include the representation of biogeochemical
processes in an interactive, coupled way. Only recently have such
models been developed, primarily by coupling the global carbon cycle
into climate models. A recent intercomparison of such model
simulations, performed in the Coupled Carbon Cycle Climate Model
Project (C4MIP), shows that (1) carbon cycle – climate feedbacks can
significantly enhance global warming caused by the emissions of
fossil fuel CO2, but (2) the magnitude of the feedback varies
substantially among the various model simulations. The models differ
significantly in how they represent vegetation and soil interactions in
the boreal and arctic areas of the northern hemisphere as well as in the
tropics, illustrating on the one hand inadequate process understanding
and on the other hand the importance of these key regions in the
global climate system.
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There exist in the Earth System several additional vulnerable carbon
pools such as wetlands and permafrost, which, if destabilized, may
further amplify climate change, but which are not yet included in
current Earth System models. Additional scope for climate –
biogeochemical feedbacks can be expected in interactions with the
global cycles of methane and nitrous oxide.
Can climate driven biogeochemical feedback processes already be
observed on a regional or global scale? Up to now, only scarce
observational evidence exists but this is expected to change in the near
future.
Finally, the assessment of biogeochemical feedbacks in the Earth
System cannot be restricted to changes in atmospheric composition
alone but needs also to consider changes in land surface properties.
At present, however, these are dwarfed by the large impacts from
direct human driven changes in land use and land management.
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