ACIDIFICATION OF EARTH: AN ASSESSMENT ACROSS MECHANISMS AND SCALES

We examined anthropogenic activities that cause acidification of Earth’s air, waters, and soils, based upon an extensive literature review, compilation of data, and development of maps. We elucidated the underlying biogeochemical reactions and assessed the magnitude of the effects due to acidification resulting from the combustion of fossil fuels and smelting of ores, mining of coal and metal ores, and application of nitrogen fertilizer to soils. These widespread activities have resulted in (1) increased carbon dioxide concentration in the atmosphere that acidifies the oceans; (2) acidic atmospheric deposition that acidifies soils and bodies of freshwater; (3) acid mine drainage that acidifies bodies of freshwater and groundwaters; and (4) nitrification that acidifies soils. The redox-sensitive elements C, Fe, S, and N are intrinsic to human exploitation of energy, mineral, and food resources; and, oxidation of these elements generates acid in the Earth-surface environment. Although natural geochemical reactions of mineral weathering and ion exchange work to buffer acidification, the slow reaction rates or the limited abundance of reactant phases are overwhelmed by the onslaught of anthropogenic acid loading. Relatively recent modifications of resource extraction and usage in some regions of the world have begun to ameliorate local acidification, but expanding use of resources in other regions is causing environmental acidification in previously unnoticed places. We created world maps of coal consumption, copper mining and smelting, and nitrogen fertilizer application that demonstrate the complex spatial heterogeneity of resource consumption as well as the overlap in acidifying potential derived from distinctly different phenomena. Projected population increase by country over the next four decades indicates areas with the highest potential for acidification, so that we might anticipate and plan to offset or mitigate the deleterious environmental effects associated with these global shifts in the consumption of energy, mineral, and food resources.