Human processes are critically linked to the climate system as contributing
causes of global change, as determinants of impacts, and through responses.
Representing these linkages poses perhaps the greatest challenge to modelling
the total Earth system. But understanding them is essential to understanding
the behaviour of the whole system and to providing useful advice to inform policy
and response. Significant progress has been made, but formidable challenges
Human activities have altered the Earth system, and many such influences are
accelerating with population growth and technological development. The use of
fossil fuels and chemical fertilisers are major influences, as is the human
transformation of much of the Earth's surface in the past 300 years.
Land-use change illustrates the potential complexity of linkages between human
activity and major non-human components of the Earth system. The terrestrial
biosphere is fundamentally modified by land clearing for agriculture, industrialisation,
urbanisation, and by forest and rangeland management practices. These changes
affect the atmosphere through an altered energy balance over the more intensively
managed parts of the land surface, as well as through changed fluxes of water
vapour, CO2, CH4 and other trace gases between soils, vegetation, and the atmosphere.
Changed land use also greatly alters the fluxes of carbon, nutrients, and inorganic
sediments into river systems, and consequently into oceanic coastal zones.
The response of the total Earth system to these changes in anthropogenic forcing
is currently not known. Sensitivity studies with altered land cover distributions
in general circulation models have shown that drastic changes, such as total
deforestation of all tropical or boreal forests, may lead to feedbacks in atmospheric
circulation and a changed climate that would not support the original vegetation
(e.g., Claussen, 1996). Regional climate simulations, on the other hand, have
shown that at the continental scale, important teleconnections may exist through
which more modest tropical forest clearing may cause a change in climate in
undisturbed areas. Coupling the global to the local is a key challenge; regional
studies may prove to be uniquely valuable.
Human land-use change will continue and probably accelerate due to increasing
demands for food and fibre, changes in forest and water management practices,
and possibly large-scale projects to sequester carbon in forests or to produce
biomass fuels. In addition, anthropogenic changes in material and energy fluxes,
resulting from such activities as fossil fuel combustion and chemical fertiliser
use, are expected to increase in the coming decades. Predictions of changes
in the carbon and nitrogen cycles are sensitive to estimates of human activity
and predictions of the impacts of these global changes must take into account
human vulnerability, adaptation, and response. Predicting the future response
of the Earth system to changes in climate and in parallel to changes in land
use and land cover will require projections of trends in the human contributions
to these global changes; this sort of modelling presents difficult challenges
because of the multiple factors operating at local, regional, continental, and
global levels to influence local land-use decisions.
In sum, the human element probably represents the most important aspect both of the causes and effects of climate change and environmental impacts. Any policy intervention will have human activities as its immediate target.
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