Although all of Earth's major biogeochemical cycles have been impacted by human activities, the calcium cycle has been one of the first to display significant changes. Harvesting of crops and the presence of acids and increased levels of carbon dioxide in rainwater from the burning of fossil fuels have altered the weathering rates of minerals and stripped calcium ions from soils, altering the primary sources of microbe- and plant-available calcium and upsetting community structures. The carbon dioxide is also depressing the saturation state of seawater with respect to calcium biominerals like aragonite and calcite, affecting the growth of organisms such as corals, coccolithophores, and pteropods. The ensuing shifts in limiting nutrients and competitive advantages within terrestrial communities, alteration of food webs, shifts in the balance between calcareous and noncalcareous plankton in the ocean, and diminishment of the reefbuilding ability of corals will in turn alter the delivery and cycling of nutrients and other elements in the terrestrial and marine biospheres and provide further perturbations to atmospheric concentrations of CO2. As a result, anthropogenic impacts on the calcium cycle have ecological consequences that reach far beyond those ecosystems which are proximally impacted.