CO2 emissions acidify oceans, threatening marine life

Triona McGrath is a Postdoctorate Researcher at the National University of Ireland Galway and the Marine Institute Ireland

Atmospheric carbon dioxide (CO2) is continuing to rise due to human activities, primarily the burning of fossil fuels for energy. The world’s oceans play a crucial mitigation role, as they absorb 30 per cent of all the CO2 emitted to the atmosphere through human activities.

However, when CO2 enters the ocean, it undergoes a number of chemical reactions whose net effect is to make the ocean more acidic. And there is general agreement among scientists that, as ocean acidification accelerates over the coming decades, the ecosystem structure will shift, reducing biodiversity. For this reason, this oceans’ uptake of CO2 has been referred to as the evil twin of climate change, since they are both a consequence of fossil fuel CO2 emissions to the atmosphere.

The average acidity of the surface of the ocean has decreased from pH 8.2 in preindustrial times to 8.1 today, representing a 26 per cent increase in acidity. The Intergovernmental Panel on Climate Change (IPCC) has projected that by the end of this century, the ocean surface will reach pH 7.8, equivalent to a 170 per cent increase in ocean acidity. While the oceans themselves will not reach pH 7, the point at which a substance becomes an acid, the rate of acidification is unprecedented in the earth’s history.

A major concern when the ocean absorbs more CO2 and increases acidity, is the decreasing levels of carbonate ions. These are required by calcifying marine species to build their shells, which are made out of calcium carbonate, and lower levels of carbonate ions have been associated with reduced growth rates and weaker calcified structures. Larval stages are particularly vulnerable. Marine species thought to be vulnerable include corals, crustaceans, molluscs and foraminifera.

As more CO2 builds up in the ocean, only an ever-shallower part of the ocean is saturated in calcium carbonate minerals (calcite and aragonite). It is projected that by the end of the century, 70 per cent of all known cold water corals, most of which are in the North Atlantic, will be surrounded by seawater that is undersaturated in aragonite. This could have knock-on effects on the rich biodiversity that cold water corals support, including important fisheries.

It is difficult to determine the impacts of ocean acidification on marine life due to the number of stressors at work simultaneously, such as warming, de-oxygenation, over-fishing and pollution. And complex interactions between species must be considered. Furthermore, the impact of ocean acidification differs even between closely related species and in some cases between populations of the same species.

Researchers have also found that oyster larvae whose parents were exposed to elevated CO2 conditions were in turn more tolerant to elevated CO2, suggesting that sensitive marine organisms may have the capacity to adapt. Some types of marine life, such as sea grasses, have been shown to thrive when there is more CO2 in the ocean.

It remains difficult to quantify the socio-economic impacts of ocean acidification. However it is likely that industries that depend on the ocean’s ecosystems, such as shellfish aquaculture, shore protection and tourism, will be negatively impacted. The only realistic way to reduce ocean acidification globally is to reduce our CO2 emissions, though improved land use can increase the uptake of CO2 by vegetation.

Through continued monitoring of ocean climate variables, scientists increase their understanding of the rates and drivers of ocean acidification. This knowledge is crucial for accurate future projections and for putting in place mitigation and adaptation strategies.

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