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 remain.

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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