Terrestrial Biosphere Dynamics in the Climate System: Past and Future
The terrestrial biosphere is one of the most critical and complex components of the climate system, regulating fluxes of energy, water and aerosols between the earth surface and atmosphere. The terrestrial biosphere also is central to the biogeochemistry
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5.1 Introduction The terrestrial biosphere is one of the most critical and complex components of the climate system, regulating fluxes of energy, water and aerosols between the earth surface and atmosphere. The terrestrial biosphere also is central to the biogeochemistry of our planet, particularly with regard to the global carbon and nitrogen cycles. Prediction or assessment of future earth system change will always be limited, to some degree, by our ability to define how the terrestrial biosphere will respond to altered climatic forcing, and how this response will express itself as biophysical and biogeochemical feedbacks. The terrestrial biosphere plays more central roles in the health of the planet via biodiversity and ecosystem services (i.e. benefits to society). The exact dimensions of these roles are difficult to define, but there is no doubt that a premium must be placed on maintaining biodiversity, as well as the health of ecosystems critical for human sustainability. Given the large magnitude of projected future (e.g. 21" century) climate change, it is simply not possible to make realistic assessments of future regional climate or biogeochemical ecosystem and biodiversity responses, without turning to the record of past climate and ecosystem change. Just as study of the contemporary biosphere is essential for understanding short-term biosphere dynamics, paleoenvironmental research provides the only way to observe and understand how the biosphere responds to climate change on decadal and longer timescales. Moreover, the paleoenvironmental record contains the only data relevant to understanding biosphere dynamics in the face of large climatic change and in the absence of significant anthropogenic influences. Because many aspects of future change are likely to
be without 201h century or even geological analogs (Crowley 1990, Webb 1992), assessments of future conditions will, by necessity, be increasingly based on numerical process models. As with other areas of environmental science (e.g. climate prediction), this means that we must also tum to the paleoenvironmental record to evaluate the realism of predictive models. It is not possible to assess future change without a complete understanding of past change, and how well we can simulate it. The purpose of this chapter is to synthesize the remarkable progress that has recently been made in understanding the role and response of the terrestrial biosphere in the face of climatic change. We first examine the biogeochemical and biophysical roles that the terrestrial biosphere has played in the context of past climate change, and what these roles mean for the future. Realistic biospheric dynamics appear to impart significant positive climate feedbacks, and thus will likely amplify future changes in temperature, precipitation, and other climate variables. The terrestrial biosphere has also played a significant role in modulating atmospheric trace-gas and aerosol concentrations through time. Here, the emphasis is on the Late Quaternary, where we have a rich, well
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