The burning of fossil fuels has increased the concentration of carbon dioxide (CO2) in the atmosphere from 280 ppmv (volume parts per million) to 385 ppmv over the last 200 years. This increase is larger than has occurred over the past 800,000 years. Equilibration of increasing amounts of CO2 with surface waters will decrease the pH of the oceans (called ocean acidification) from a current value of 8.1 to values as low as 7.4 over the next 200 years. Decreasing the pH affects the production of solid CaCO3 by microorganisms in surface waters and its subsequent dissolution. CO2 dissolution in the ocean can also affect acid–base equilibria, metal complex formation, solid–liquid equilibria, and the adsorption of ions to charged surfaces. Thermodynamic principles can be used to understand these processes in natural waters.