Introduction

Due to the direct dependence of all living organisms on their physicochemical environment, the biosphere has always responded to changes in climate, at all scales from local to global. On geological timescales, a quasi-equilibrium between ecosystem (biome) distribution and climate exists, although the slow growth of forests and the slow buildup of organic matter in soils and peat bogs create substantial nonlinear responses at the timescale of centuries. The current interest in global change 'impacts' on the biosphere is due to the novel, potentially devastating nature of 'anthropogenic' climate change in many ecosystems. Human society associates high values with the biosphere, due to its intrinsic value and also as provider of'ecosystem services' such as food, fiber, clean water, as well as spiritual and other values. The concept of 'vulnerability' has been employed to describe this human view on biospheric change. It allows the consistent assessment of biospheric change involving a common metric. It also includes the formulation of future 'risk', expressed by the scenario-based analysis of losses that may arise if trends of global change are extrapolated into the future.

This presentation of impacts focuses not only on changes brought about by anthropogenic perturbations due to global processes such as the emission of greenhouse gases (GHGs) but also the increasing pressure from land-use change including land degradation and deforestation. Global change impacts are, on the one hand, documented from observations and careful attribution to driving forces, and, on the other hand, recognized by consideration of 'expected impacts' due to future trends in drivers, expressed as scenarios. These scenarios are not predictions, because the precise nature of future drivers cannot be known as they depend on human demography, economic development, application of technology, climate protection policy, and other factors. Scenarios are not speculative, however, because they rely on established causal relationships between driving forces and ecosystem response. For example, since the link between GHG emissions, global temperatures, and sea-level rise is well established, and since quick stabilization of GHG concentrations is very unlikely, it can therefore be inferred with high confidence that coastal ecosystems are at risk worldwide, even in places where currently no damage is observed yet. Numerical models for the assessment of future impacts (developed during 25 years by many groups worldwide) are now a mature technique for the consideration ofnonlinear impacts and the combination of multiple forcings - as is amply illustrated by the recent assessments of the Millennium Ecosystem Assessment and the Intergovernmental Panel on Climate Change.

In the following, a summary will first be given about the nature of currently observed changes in the biosphere that can be attributed to global change. Then a scenario-based presentation of expected future impacts will be given.

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