Aragonite is a key component of the shells and skeletons of many economically and ecologically important marine species. It is an essential mineral that enables such species to grow sufficiently strong shells for their self-defense. Unfortunately, rising changes in oceanic pH (which are directly linked to increasing levels of atmospheric carbon dioxide) are negatively affecting the availability of aragonite in our oceans.

Roughly a third of the CO₂ released into the atmosphere since the industrial revolution has been absorbed by the ocean. This uptake of CO₂ is causing an ongoing change in oceanic carbonate chemistry. Ocean acidification (OA), a phenomenon reflected by falling oceanic pH levels, is worsening. This leads to declines in the saturation state of calcium carbonate (CaCO₃) minerals such as aragonite.

A decline in this aragonite ‘saturation state’ (Ω_arag) can slow down calcification, negatively affecting organisms that rely on it to grow shells. Stony corals, for example, show declined calcification with decreasing aragonite saturation, making them more vulnerable to damage by tropical storms and more prone to erosion. Pteropods present at higher latitudes are projected to be affected severely by OA, and their distribution might be limited because of their inability to grow a shell or their shell being too weak as a defensive mechanism (Orr et al., 2005).

To predict OA impacts on these and other important organisms, accurate projections of Ω_arag are needed. In this research we report on two projections in which Ω_arag was modeled using state of the art global circulation models included in the Intergovernmental Panel on Climate Change Fifth Assessment Report.

The first projection was made using the relative concentration pathway (RCP) 4.5, which is a scenario of rapid initial growth of greenhouse gas (GHG) concentrations, but stabilizing concentrations from 2070 onward. The CO₂ equivalent concentration in 2100 is 580 ppm. This pathway results in a total forcing of 4.5 W.m^-2 in 2100.

The second projection was made using the RCP 8.5, a pathway of continuing growth of GHG concentrations in the atmosphere and one that results in an additional forcing of 8.5 W.m^-2 in 2100 at a CO₂ concentration of 1230 ppm.

Currently emissions are tracking above the RCP 8.5 scenario. In all four previous generations of emission scenarios employed by the Intergovernmental Panel on Climate Change (IPCC) for their Assessment Reports, emissions always exceeded the worst case scenario. Both pathways project a 395 ppm CO₂ concentration in 2013 as a midyear average, which is lower than actual measurements of CO₂ concentration for that year. Measurements of CO₂ in 2014 at Mauna Loa peaked above 400 ppm, a concentration not seen in the earth’s atmosphere for millions of years.

The projections show that without serious efforts to cut greenhouse gas emissions, ocean acidification will be pronounced, with possibly enormous effects on ecosystems and economies alike.