Ocean acidification – the other CO2 problem

Within decades increasing atmospheric carbon dioxide levels in the atmosphere could severely damage marine ecosystems, scientists warn.

Global warming affects the oceans not only through rising sea level and warmer surface water. As carbon dioxide levels in the atmosphere increase, so does the uptake of carbon dioxide in ocean surface water, which in turn decreases the pH of the water – the oceans become more acid. Ocean acidification has been referred to as “the other CO₂ problem”. Scientists warn that the effects on marine ecosystems may be severe. It could, among other things, make most regions of the oceans inhospitable to coral reefs and lead to changes in commercial fish stocks, threatening food security for millions of people.

The exchange of gases between the atmosphere and the oceans represents major pathways in the global carbon cycle, and the oceans are enormous carbon sinks, holding about 50 times more CO₂ than soils and the terrestrial biosphere. The present net carbon uptake is about 2 Gt (Gigatonnes) annually, which is equivalent to about 30 per cent of anthropogenic emissions. As a result of this, there has already been a demonstrable increase in CO₂ concentrations in the upper layer of the sea over the last few decades. This effect can be traced to a water depth of 1,000 metres.

The chemical effect of this increase is measurable. Since the onset of industrialization the pH value of the ocean surface water has dropped by an average of about 0.11 units, which is equivalent to an increase in concentration of hydrogen ions (H+) by around 30 per cent. This may not be alarming in itself, but the rate of change is cause for concern. Available data indicates that the ocean has never experienced such a rapid acidification. By the end of this century, if concentrations of CO₂ continue to rise exponentially, we may expect changes in ocean pH larger than ever before experienced on the planet – at least for the past 20 million years.

Human interference with the chemical balance of the ocean will not remain without consequences for marine ecosystems. Of specific concern is calcification, the process by which many marine organisms build their skeletons or shells, using calcium carbonate extracted from the water. Increasing CO₂ concentrations and falling pH value hamper calcification, causing a weakening of the skeletal structure, or even – below a certain pH level – the dissolution of shells and skeletons. This will of course affect all marine calcifying species, including clams, snails and corals. Starfish and other echinoderms are especially threatened, since their calcite structures dissolve more easily than those of other organisms.

Simulations indicate that with an atmospheric CO₂ concentration of just under 520 ppm (which could be reached as early as the middle of this century), almost all of today’s warm weather coral reef locations will be unsuitable for coral growth because of acidification.

Around three-fourths of the global marine calcium carbonate production is carried out by plankton, among which pteropods and coccolitophores are of specific interest in this context. Coccolitophores are single-celled phytoplankton (i.e. they are “green plants”, serving as primary producers in the food webs) with calcite shells. They play a significant role in the global carbon cycle, since the deposition of coccolitophore shells greatly contributes to the export of calcium carbonate to the deep sea. Pteropods are important components in marine food webs in polar and subpolar seas.

Ocean acidification will likely make it impossible for these organisms to form shells. In the case of pteropods, for which the Southern Ocean forms an important part of their habitat, this could take place as early as 2050, severely limiting their distribution area.

The effects of acidification on marine plankton may in turn affect the global carbon cycle and the role of the ocean as a carbon sink.

Through the marine food chains, the adverse effect of ocean acidification on calcification could have considerable impact on the total marine biosphere. It cannot be ruled out that pelagic fisheries, and thus the human food supply, may be affected.

In a common declaration (The Monaco Declaration) 150 top marine scientists recently voiced their deep concerns over “recent, rapid changes in ocean chemistry and their potential, within decades, to severely affect marine organisms, food webs, biodiversity and fisheries.”

Policymakers need to realize that ocean acidification is not a peripheral issue, the scientists say. “It is the other CO₂ problem that must be grappled with alongside climate change.” The statement calls on policymakers to stabilise CO₂ emissions “at a safe level to avoid not only dangerous climate change but also dangerous ocean acidification”. An example given in the declaration indicates that halting ocean acidification may demand even further reductions than indicated by the IPCC to keep global warming within limits: “To stay below an atmospheric CO₂ level of about 550 ppm, the current increase in total CO₂ emissions of three per cent per year must be reversed by 2020. Even steeper reductions will be needed to keep most polar waters from becoming corrosive to the shells of key marine species and to maintain favourable conditions for coral growth.”

Roger Olsson

Further reading:
WBGU: The future oceans – Warming Up, Rising High, Turning Sour. Special report 2006:
http:// www.wbgdu.de
BBC news: Acid oceans need urgent action: http://news.bbc.co.uk/2/hi/science/nature/7860350.stm
The Ocean Acidification Network:
http://www.ocean-acidification.net. The Monaco Declaration can be accessed from this site.