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Ocean acidification projections to 2100

Ongoing monitoring of the ocean is imperative to detect changes in ocean chemistry and impacts of ocean acidification.

Photo credit: Sunburst Sensors, LLC

Lead author: Ruben van Hooidonk

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.

Aragonite is a calcium carbonate mineral that is an important component of shells and skeletons of marine species

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’ (Ωarag) 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).

Ocean acidification can lead to aragonite dissolution, leading to significant weakening of the shells and skeleton of many marine species.

To predict OA impacts on these and other important organisms, accurate projections of Ωarag are needed. In this research we report on two projections in which Ωarag 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.

Weakened shells and skeletons negatively affect the health of the marine ecosystem and also harm economically valuable species.

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.

Ocean acidification is expected to increase noticeably by 2100, even under modest projected scenarios, rendering aragonite less accessible to species that rely in it.

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.

There is a latitudinal gradient in reduction of aragonite saturation state, near the equator the reduction is largest.