evidence: Seawater has a pH of 8.2 on average because it contains naturally occurring alkaline ions that come primarily from weathering of continental rocks. When seawater absorbs carbon dioxide from the atmosphere, carbonic acid is produced (see Box 1), reducing the water’s pH. Since the dawn of industrialization, average surface ocean pH has decreased to about 8.1. [...] Ocean acidification is occurring at a rate 30 to100 times faster than at any time during the last several million years driven by the rapid growth rate atmospheric CO2 that is almost unprecedented over geologic history. According to the Intergovernmental Panel on Climate Change (IPCC), economic and population scenarios predict that atmospheric CO2 levels could reach 500 ppm by 2050 and 800 ppm or more by the end of the century. This will not only lead to significant temperature increases in the atmosphere and ocean, but will further acidify ocean water, reducing the pH an estimated 0.3 to 0.4 units by 2100, a 150 percent increase in acidity over preindustrial times. Assuming a "business-as-usual" IPCC CO2 emission scenario, predictive models of ocean biogeochemistry project that surface waters of the Arctic and Southern Oceans will become undersaturated with aragonite (a more soluble form of calcium carbonate) within a few decades, meaning that these waters will become highly corrosive to the shells and skeletons of aragonite-producing marine calcifiers like planktonic marine snails known as pteropods.