Human additions of CO2 are in the margin of error of current measurements and the gradual increase in CO2 is mainly from oceans degassing as the planet slowly emerges from the last ice age.
Carbon dioxide combines with water to form carbonic acid, which then dissociates to form bicarbonate ions and hydrogen ions (H+), so that increasing concentrations of CO2 in the atmosphere have been decreasing the pH (acidifying) of the surface ocean (NRC, 2010c). Since the preindustrial period, pH has declined by approximately 0.1 pH unit corresponding to a 30 percent increase in acidity. By 2100, the acidity is projected to increase by 100-150 percent compared to preindustrial values. Geologic records indicate that when the increase in atmospheric CO2 is gradual, oceanic pH and carbonate levels remain relatively stable due to processes that occur in equilibrium, such as dissolution of CaCO3 shells, weathering of terrestrial rock, and tectonic processes. However, the current rate of increase of atmospheric CO2 exceeds the rate at which natural processes can buffer these pH changes.