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.