In the grand cycles, atoms of carbon, nitrogen, sulfur, and phosphorus cycle back and forth from living plants and animals to the soil, atmosphere, and oceans. The winds and water currents of earth move these atoms global distances.

As the cycling of these four elements has developed over billions of years, biology has become deeply intertwined with geology. Biochemical processes, driven by sunlight and involving specialized molecules and bacteria, extract these elements from storage in inorganic reservoirs and make them available as nutrients for plants. Closely related biochemical and geophysical processes return these elements to storage and replenish the inorganic reservoirs. Energy in the form of high-temperature heat from the interior of the earth drives much slower cycling, which retrieves, in the form of volcanic emissions, the small fractions of nutrients that are not rapidly cycled.

All four grand cycles are now strongly influenced by human activity. Although the interactions are similar for sulfur and phosphorus, the chapters in this section consider only nitrogen and carbon.

The carbon cycle has been perturbed by fossil fuel use, deforestation, and changes in agriculture and animal husbandry. During the past two centuries these practices have led to more than a 25% increase in the concentration of carbon dioxide and more than a doubling of the concentration of methane in the atmosphere. The principal direct consequence, just at the edge of detectability today, is likely to be a modification of patterns of surface temperature and rainfall, as well as a rise in sea level.