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Ocean Acidification: Predictions for the Future

Models are currently being used that compare current rates of ocean acidification with previous known rates, to try and predict the possible consequences that our lifestyles may have. Dr Andy Ridgwell and colleague Dr Daniela Schmidt, from the University of Bristol, have developed an earth systems model, GENIE-1, to compare current predictions of ocean acidification with those that occurred during the PETM, 55 million years ago. They are able to reconstruct the ocean chemistry during the Palaeocene-Eocene Thermal Maximum (PETM) event using data on temperature, ocean circulation and atmospheric interactions. They then compared the results with what is expected for the future. Full results can be viewed at: http://www.nature.com/ngeo/journal/v3/n3/suppinfo/ngeo755_S1.html

The results found that the future calcite saturation horizon rises rapidly to a depth of only about 600m following the peak in atmospheric carbon dioxide around the year 2150. This would cause extensive undersaturation and a decrease in acidification of over 0.3pH units throughout the whole of the ocean. The decline in carbonate saturation was much less pronounced in the PETM. The rapid decline predicted by this model may have dramatic effects on the biodiversity of the marine environment, leading to the possible extinctions of many more organisms than seen during the PETM.

Laboratory experiments suggest that if the pH continues to fall at its current rate, we may start to see impacts such as the dissolution of carbonate shells of marine organisms, slower growth, muscle wastage, dwarfism or reduced activity. In experiments carried out on sea urchins and brittlestars, at carbon dioxide levels just 200ppm higher than today, scientists have found that they were unable to compensate for long-term changes in ocean acidification. Their muscle wastage and reduced growth and survival is a direct cause of them trying to increase their calcification as the pH of the water decreased. Many of these marine organisms play important roles within ecosystems so that changes to their diversity or efficiency of function could affect the ecosystems and food webs within them. It is predicted that the aragonite skeletons of coral reefs will experience dissolution in the corrosive waters, potentially leading to the breakdown of reef structure and loss of habitat for other organisms.

Survival during these environmental changes will depend on the impact on nutrient availability and the species ability to evolve and adapt fast enough. This is what allowed pelagic organisms to survive during the PETM. However, the predicted future rate of surface environmental change is faster than that during the PETM and therefore will create a more severe adaptive pressure on all the organisms. Whether future extinctions will occur cannot be determined with any certainty at present, although it is thought that the rate of change to the ocean chemistry will be too fast for many benthic and pelagic organisms to evolve and survive.

It is clear that better experimentally-based understanding of acidification impacts at both the organism and ecosystem levels and how this translates to the global scale is needed for improvements to be made in model predictions of future fossil fuel carbon dioxide uptake. This is important because many people and industries around the world rely on the productivity of these marine environments for their well being. A decline in their function and efficiency will have global social and economic effects, especially in developing countries where there is no alternative way of life.

Whilst past events shows that calcium carbonate from sediments, combined with silicate weathering feedback, could eventually neutralize the acidity of oceans produced by ocean acidification this will take tens of thousands of years, by which time many marine organisms and environments will have been lost, resulting in huge economical decline in many areas of the world. The only solution to reduce the effect of ocean acidification is to drastically reduce anthropogenic carbon dioxide emissions.


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