While pre-1972 global vegetation patterns can only be estimated using a variety of historical sources, post-1972 global vegetation patterns can be directly determined using satellite remote sensing. Our ability to assess and evaluate future changes in these patterns has thus changed dramatically through the application of satellite remote sensing. These data sources (satellite remote sensing and in situ or field studies) have been used to make an up-to-date estimate of the impacts of both natural and anthropogenic changes to vegetation cover in selected regions since the 1970s. For example, recent studies of tropical forest and savanna vegetation in Brazil have documented changes in an area of 7,000,000 square kilometers (km2). While estimates of reported vegetation cover change derived using different methodologies vary, there is agreement that a substantial alteration of vegetation cover in Brazil has occurred since 1972.
Changes to global vegetation cover resulting from human activities have profound implications for ecosystem functioning, biogeochemical cycles, and climatic stability. It is generally assumed that human activity has dramatically altered the natural vegetative cover of our planet, especially in the past 300 years (Turner et al. 1990). Upon closer analysis, one discovers not only a considerable ignorance of the present global distribution of terrestrial land cover types, but no systematic, reliable, and comprehensive compendium of human-caused changes to the natural vegetation cover on a global scale (Townshend et al., 1991). In an effort to improve this state of knowledge, the Aspen Global Change Institute devoted its first of three 1995 summer sessions to understanding changes to global vegetation patterns and their relationship to human activity.
A review of existing estimates of global land cover reveals a high degree of disagreement (Townshend et al., 1991). This is also true for global estimates of agricultural land and forested land, two categories of land cover which can be easily mapped. Substantial disagreements in extent of various land cover types globally result from the fact that estimates rely on reconciling numerous separate sources employing widely-varying criteria. It is thus not surprising that such a high degree of disagreement is present. Furthermore, not only does the total area of the various classes vary substantially among authorities, but the specific spatial distributions of ten vary widely even when the total global estimates of a cover type are similar.
Major efforts have been made to synthesize current global land cover knowledge to generate global digital data bases (Matthews, 1983 and Henderson-Sellers et al., 1986). Although these represent improvements on previous knowledge, they suffer from unavoidable errors inherent in the primary data upon which they are based.
Not only are accurate estimates of global land cover of the utmost importance for under standing the coupled Earth-climate system, the extent of land cover type fragmentation is also important. For example, if Earth's climate does become warmer, the ability for terrestrial ecosystems to adapt by "ecosystem migration" could be compromised if natural ecosystems are broken up into non-contiguous pieces. In addition, biological diversity concerns are directly related to habitat destruction and fragmentation.
The various tools for satellite-based assessment were discussed at this AGCI session. These tools include optical remote sensing, and passive and active (radar) microwave remote sensing, all coupled with "ground truth" validation and numerical model simulations. The model simulations are necessary to evaluate and assess the impact of global vegetation changes on environmental parameters and processes, as ecological, bio-geochemical, and climate models are all needed to completely evaluate the potential impacts of these vegetation changes on the global environment.
The extension of these techniques to other regions of the tropics (i.e., non-Brazilian South America, Southeast Asia, and tropical Africa) was discussed at this Aspen Global Change Institute (AGCI) session, and regional/historical differences between these areas and the Brazilian Amazon were identified.
Workshop Topic (s):
- Human Contributions & Responses
- Land-Use/Land-Cover Change