Ocean acidification is speeding up
15 February 2010, by Sara Coelho
Carbon dioxide released from fossil fuels and dissolved in the ocean is making seawater more acidic and causing trouble for marine life. Now a new model suggests that seawater is acidifying at a rate that exceeds anything seen on Earth over the past 65 million years. The speed of change may in fact be faster than what marine animals can cope with.
Dr Andy Ridgwell and colleague Dr Daniela Schmidt, from the University of Bristol, developed a model to compare current predictions of ocean acidification with what happened during a greenhouse gas event 55 million years ago, called the Palaeocene-Eocene thermal maximum (PETM).
During this event, which saw a 5-6°C increase in surface water temperature, the oceans acidified and a massive amount of carbonate rocks was dissolved as a consequence. All this happened over thousands of years. It may look like a lot of time, but on a geological scale it was very sudden.
The tiny animals and plants that make up the plankton at the surface of the sea did not suffer much during the event, possibly because they migrated to cooler waters or because they had time to adapt to the new conditions. Even so, the Palaeocene-Eocene acidification event was severe enough to cause the extinction of many benthic foraminifers, tiny organisms that live at the bottom of the sea protected by calcium carbonate shells.
The mass extinction has been linked to the high levels of carbon dioxide dissolved in seawater, because it's difficult for the foraminifers to build their shells if the ocean is too acidic on the seafloor.
Nowadays the ocean chemistry might be changing even faster. 'What we found was a geologically unprecedented rate of acidification,' says Ridgwell. The change observed today is occurring at the scale of hundreds of years - and this might be too much for marine life to handle.
'Given that we had an extinction in the past, it's quite possible that this will happen again in the future,' says Ridgwell.
Benthic foraminifers might not attract as much sympathy as giant pandas, but their demise 'has implications for the cycling of nutrients and it's a good indication of the extent to which we're affecting the oceans,' Ridgwell adds, suggesting that even ocean organisms might encounter trouble this time.
Past and future ocean acidification
Scientists have an idea of the consequences of ocean acidification on marine animals such as foraminifers or corals thanks mainly to experiments. But how will animals adapt to changing conditions? 'Experiments don't tell what will happen over 100 years,' says Ridgwell. 'And we can't wait 100 years.'
This is where past global warming events come in. 'The geological record has many examples of what happens to ecosystems during climate change events,' says Ridgwell, 'and we need that to understand the biological response to ocean acidification.'
Ridgwell and Schmidt use earth systems model GENIE-1 to reconstruct the ocean chemistry during the Palaeocene-Eocene event using data on temperature, ocean circulation and atmospheric interactions. They then compared the results with what is expected for the future and published their results yesterday in Nature Geoscience.
Of all the known ancient global warming events, they decided to focus on the Palaeocene-Eocene thermal maximum because it was sudden and transient - the extreme temperatures and levels of carbon dioxide appeared within a few thousand years, and then disappeared over the following few hundred thousand.
Andy Ridgwell and Daniela N. Schmidt. Past constraints on the vulnerability of marine calcifiers to massive carbon dioxide release. Nature Geoscience published online: 14 February 2010. doi: 10.1038/ngeo755
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