Effect of Carbon Emissions on Ocean Chemistry

Much of the current climate change debate has tended to focus on the release of carbon dioxide and other greenhouses gases into the air, along with the modifying effect that shorter-lived pollutants such as nitrogen oxides and carbon monoxide have on the atmosphere.

However, the effect of carbon emissions on the oceans – dubbed the “forgotten issue of global warming” by some climate researchers – is every bit as important, but for reasons which are often not readily appreciated.

Global Carbon Sink

According to work recently done by the Carnegie Institution's Department of Global Ecology and the University of Hawaii, over 30 per cent of the CO2 emissions from human activity over the last two hundred years has been absorbed by the world’s seas.

While this has helped to reduce the planetary carbon footprint, it has had serious consequences for ocean chemistry, the additional carbon dioxide making seawater more acidic – a change of about 0.1 pH units over the levels prior to the industrial revolution.

The world’s oceans are the largest natural carbon sink, acting as a reservoir to collect and contain excess CO2 and with predictions that the seas will absorb nearly 90 per cent of atmospheric emissions over the next millennium, the picture is an increasingly gloomy one.

There is a strong possibility that the levels of carbon dioxide dissolved in the oceanic surface waters will rise to double the pre-industrial values – increasing the acidity by perhaps as much as 0.35 pH units by the middle of the current century. If so, the rate of change would be greater than anything experienced in recent geological history, including the end of the ice ages.

A More Acidic Ocean

The knock-on effect of the acidification of seawater threatens to have significant consequences for marine life – with many organisms relying on a chemical called aragonite – a natural form of calcium carbonate – to build their external skeletons.

Although the changes in ocean chemistry are fairly predictable, the biological response to rising dissolved carbon dioxide levels is less certain, since the pH and carbonate chemistry of the world’s seas has been remarkably stable for many millions of years.

Changes on the scale threatened are unprecedented – making predictions of how species will cope virtually impossible with any degree of certainty.

Laboratory investigations have shown that shifts of as little as 0.2 pH units can alter the ability of important kinds of sea-life, such as corals and some types of plankton, to build properly calcified shells. While this obviously has serious implications for some of the world’s major coral features, including Australia’s Great Barrier Reef, how increased ocean acidity might affect other forms of marine life remains entirely unknown.

Most sea creatures live in the well-lit surface waters – which are the most vulnerable to increased CO2 levels; with dissolved carbon dioxide increasing at record levels, the outcome for the world’s oceans could be potentially devastating.

With coral reefs and the communities they support already pressurised by the wider effects of climate change, over-fishing and broader forms of marine pollution, the further pressures generated by the ongoing acidification of the seas represent a serious challenge to their future. If prompt action is not taken, many marine scientists fear that the coral reefs and the rich variety of life that depends on them will not survive the century.

Climate Change and Ocean Carbon

The effect of carbon dioxide on ocean chemistry is largely independent of its potential to drive climate change, leading many researchers in the field to warn that trying to mitigate global warming without cutting actual emissions will not help avoid the seas becoming more acidic. If their predictions are correct, the only way to achieve this is likely to be even more wide-ranging carbon cuts than previously thought.

With so many voices united in viewing climate change as the most serious threat we currently face, any measure to lessen its impact must be a step in the right direction. However, although it can often be tempting to simplify the issue in order to make the message easier to grasp, the reality of the interactions between the planet’s atmosphere, ocean chemistry and eco-systems is inevitably rather more complex.

The bottom line is that by absorbing around a third of the carbon dioxide we put into the atmosphere, the oceans help slow the rate of global warming, but they do so at a cost to their own well-being. How big this price will ultimately be in terms of the economy and ecology of the world remains to be seen.