evidence: Scientists estimate that surface ocean pH has fallen by about 0.1 pH unit from preindustrial times to today. Because pH is a measure of hydrogen ion concentration and the the pH scale is logarithmic — for every drop of 1 pH unit, hydrogen ion levels increase by a factor of 10 — a 0.1-unit pH drop is equivalent to about a 26% increase in the ocean hydrogen ion concentration. If we continue on the expected trajectory for fossil-fuel use and rising atmospheric CO2, pH is likely to drop by 0.3-0.4 units by the end of the 21st century and increase ocean hydrogen ion concentration (or acidity) by 100-150% above what it was in preindustrial times. — Scott Doney, Senior Scientist, Woods Hole Oceanographic Institution, USA