BOX 10.6

In the 1995 IPCC Summary for Policy Makers: The Science of Climate Change, the concluding section says that “there are still many uncertainties.” This section observes that “[m]any factors currently limit our ability to project and detect future climate change. In particular, to reduce uncertainties further work is needed on the following priority topics:

  • Estimation of future emissions and biogeochemical cycling (including sources and sinks) of greenhouse gases, aerosols, and aerosol precursors and projections of future concentrations and radiative properties.

  • Representations of climatic processes in models, especially feedbacks associated with clouds, oceans, sea ice, and vegetation, in order to improve projections of rates and regional patterns of climate change.

  • Systematic collection of long-term instrumental and proxy observations of climate system variables (e.g., solar output, atmospheric energy components, hydrological cycles, ocean characteristics, and ecosystem changes) for the purposes of model testing, assessment of temporal and regional variability, and for detection and attribution studies. ”

The IPCC further notes that “[f]uture unexpected, large and rapid climate system changes (as have occurred in the past) are, by their nature, difficult to predict. This implies that future climate changes may also involve “surprises. ” In particular, these arise from the non-linear nature of the climate system. When rapidly forced, non-linear systems are especially subject to unexpected behaviour. Progress can be made by investigating non-linear processes and sub-components of the climatic system.”110

Dynamic vegetation models in which land use and land cover changes are interrelated in terms of processes and feedbacks offer an advanced approach to coupling the human driving variables with ecosystem response functions, hydrological dynamics, atmospheric conditions, and edaphic (fire-related) factors. Such models account for the role of transient states of secondary succession following disturbance. These developing models can simulate ecosystem responses with particular emphasis on vegetation dynamics on timescales from decades to centuries and provide a means of investigating responses to disturbances such as deforestation. However, a fundamental problem in assessing the results of terrestrial ecosystem models is a lack of good validation data.

Finally, since agricultural and forestry production provides the essential food, fuel, and economic resources for the world, monitoring and modeling of biospheric primary production are important to support global economic and political policy making. Fortunately, during the past decade of the USGCRP, it has become possible to investigate the magnitude and geographical distribution of



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