Soil acidification is a very important process in the functioning of earth's ecosystems. A major source of soil acidity is CO2, derived from the respiration of plant roots and microbes, which forms carbonic acid in soil waters. Because elevated atmospheric CO2 often stimulates respiration of soil biota in experiments that test ecosystem effects of elevated atmospheric CO2, we hypothesize that rising atmospheric CO2 (which has increased from ∼200 ppm since the interglacial and may exceed 550 ppm by the end of the 21st century) is significantly increasing acid inputs to soils. Here, using column-leaching experiments with contrasting soils, we demonstrate that soil CO2 is a much more potent agent of soil acidification than is generally appreciated, capable of displacing almost all exchangeable base cations in soils, and even elevating Al(III) concentrations in H2CO3-acidified soil waters. The potent soil acidifying potential of soil H2CO3 is attributed to the low pKa,1 of molecular H2CO3 (3.76 at 25°C), which contrasts greatly with that of (a convention that combines CO2 (aq) and molecular H2CO3, the pKa,1 of which is 6.36 at 25°C). This distinction is significant for soil systems because of soil's greatly elevated CO2, their variety of sinks for H+, and the wide range of contact times between soil solids, water, and gas. Modelling suggests that a doubling of atmospheric CO2 may increase acid inputs from carbonic acid leaching by up to 50%. Combined with the results of CO2 studies in whole ecosystems, this implies that increases in atmospheric CO2 since the interglacial have gradually acidified soils, especially poorly buffered soils, throughout the world.