Ocean acidification: Global warming’s evil twin

While there’s much focus on the impacts from warming temperatures, there’s another more direct effect from the burning of fossil fuels and deforestation. More than 30% of the carbon dioxide emitted by humans is dissolved into the oceans, gradually turning ocean water more acidic. Coral reef researcher Ove Hoegh-Guldberg explains the threat of ocean acidification: “Evidence gathered by scientists around the world over the last few years suggests that ocean acidification could represent an equal – or perhaps even greater threat – to the biology of our planet than global warming”. Thus a new paper Paleo-perspectives on ocean acidification (Pelejero et al 2010) labels ocean acidification the ‘evil twin’ of global warming.

As CO2 dissolves in the oceans, it leads to a drop in pH. This change in seawater chemistry affects marine organisms and ecosystems in several ways, especially organisms like corals and shellfish whose shells or skeletons are made from calcium carbonate. Today, the surface waters of the oceans have already acidified by an average of 0.1 pH units from pre-industrial levels and we’re seeing signs of its impact even in the deep oceans.

The past gives us an insight into future effects of ocean acidification, as we continue to emit more CO2 and acidify the ocean even further. Ice cores give us accurate data on the evolution of CO2 in the atmosphere over the last 800,000 years. These reconstructions, together with data derived from foraminifera, find that the pH of ocean surface water was lower during interglacials (high levels of atmospheric CO2). Seawater pH was also higher during glacial periods  when atmospheric CO2 was low. Correspondingly, foraminifera seem to have grown thicker or thinner shells over glacial–interglacial timescales in time with changing CO2 levels.

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