2100- the year the global food web will collapse
If we don't stop acidifying the oceans with CO2 emissions, that is.
"Some species of sea butterfly have shells composed of aragonite, a metastable form of calcium carbonate. These pelagic mollusks are considered sentinels for environmental change; even though they can survive for a couple of days in water depleted of calcium carbonate, their shells already begin to show dissolution marks.
Comeau et al. have collected pteropods (Limacina helicina) from Kongsfjorden, Svalbard, to study their response to the acidification of polar waters that has been predicted will occur as atmospheric carbon dioxide increases, carbonate declines, and ice melts. They measured shell calcium flux by calcein staining and 45Ca uptake, which revealed a 28% decrease in calcification as the pH dropped by 0.3 and aragonite saturation fell to 1.00, which is forecast to happen by the year 2100. Below this threshold, aragonite concentrations in seawater are too low to prevent the dissolution of shelled creatures-conditions that spell doom for more than just the sea butterflies.
Biogeosciences 6, 1877 (2009). "