The emergence of life on earth has led to the conversion of carbon dioxide in the atmosphere and carbon dissolved in the oceans into innumerable inorganic and organic compounds on land and in the sea. The development of different ecosystems over millions of years has established patterns of carbon flows through the global environmental system. Natural exchanges of carbon between the atmosphere, the oceans, and terrestrial ecosystems are now being modified by human activities, primarily as a result of fossil fuel burning and changing land use. This activity has led to a steady addition of carbon dioxide to the atmosphere and enhancement of the atmospheric concentration by more than 28 percent over the past 150 years.
We need to understand the global environmental system and in particular the circulation of carbon in nature, as well as how human activities have modified it, to assess how we may do so increasingly in the future. In addition to reducing emissions from fossil fuel use, we may also have an opportunity to reduce the rate of build-up of carbon dioxide in the atmosphere by taking advantage of the fact that carbon can accumulate in vegetation and soils in terrestrial systems-an opportunity that was brought into focus at the Third Conference of the Parties (COP) to the Framework Convention on Climate Change (FCCC) in Kyoto. This introductory chapter provides an overview of our present understanding of the fundamental natural processes at work, which is essential for an analysis of the opportunities, limitations, and implications of actions related to land use and land-use change.
Natural flows of carbon between the atmosphere, the oceans, and the terrestrial and freshwater systems vary from one part of the globe to another and in time (i.e., between seasons, from one year to the next, and over decades and centuries). It is often difficult to separate changes resulting from human interventions from these natural variations.
Some of the measures specified in the Kyoto Protocol are ambiguous because the terminology being used is not always adequately defined. Clarifications in this regard are required, and we need to analyze the implications of alternative interpretations as a basis for political agreements on how to proceed. The measures specified in the Protocol might also induce secondary changes that need to be evaluated. Knowledge about the functioning of global biogeochemical cycles, particularly the global carbon cycle, is essential in this context.
Information about human-induced disturbances of sources and sinks of other greenhouse gases is also provided. Methane emissions may be changed unintentionally when actions are taken to enhance carbon dioxide sinks, and sources and sinks of nitrous oxide will be modified if the cycle of nitrogen is disturbed. Changes in the nitrogen cycle will in turn influence terrestrial ecosystems and thereby the exchange of carbon dioxide between the terrestrial system and the atmosphere.
To judge the long-term consequences of the ways in which human activities disturb the circulation of carbon in nature and change its distribution between natural reservoirs, we need to analyze the carbon cycle in detail, particularly with respect to the terrestrial ecosystems. This analysis will also shed light on the implications of the measures specified in the Kyoto Protocol.
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