Oceans are the main carbon sink: more than half of the CO2 emitted since the beginning of the industrial era has been absorbed by the oceans through a complex chemical reaction at the surface of the ocean. CO2 dissolves in the water which creates carbonic acid. From the 1790s to 1994, the concentration of CO2 in the atmosphere has risen by 118 GtC, and 53Gtc from 1980 to 2005. Consequently, the CO2 intake by oceans also increased, so did the concentration of carbonic acid. This resulted in a decrease of the average PH level: for the past twenty years, the PH level has dropped 0.02 per decade. This ocean acidification or decline in alkalinity is problematic for coral reefs: to build their shells, coral reefs create CaCO3, or calcium carbonate. When the water becomes saturated with carbonic acid, Ca2+ particles are under-saturated. The lower calcium carbonate concentration challenges shell-building process for these species. An experiment in Biosphere 2, a self-enclosed and self-sufficient world experiment in Arizona, showed that a drop in saturation of calcium carbonate correlated with a decrease in the growth rate of these organisms.