Marine climate change impact & adaptation. Report card Australia 2009- Ocean acidification

Anthropogenic CO2 emissions arise mainly from fossil-fuel combustion, land-use practices, and concrete production during and since the industrial revolution. These emissions first enter the atmosphere, but a large proportion of them are then absorbed into the ocean by physical and biological processes that are normal parts of the natural carbon cycle. The result is more CO2 dissolved in the world’s oceans. The ocean is a weakly-alkaline solution (with a pH of ~ 8.1), but this extra CO2 changes the carbonate chemistry of the surface ocean, driving ocean pH lower. The term ‘ocean acidification’ refers to the fact that the CO2 forms a weak acid (carbonic acid) in water, making the ocean more acidic.

This process of ocean acidification is already underway and discernible in the ocean (Feely et al. 2004). Acidification has lowered the pH of the ocean from its pre-industrial state by about 0.1 pH units. By the end of this century pH levels are likely to drop 0.2 – 0.3 units below pre-industrial pH. The level of atmospheric CO2 is now higher than at any time in at least the past 650,000 years, and probably has not been as high as present levels for 20 million years. The current rate of increase of CO2 in the atmosphere is one hundred times greater than the most rapid increases during major climate changes over the last 650,000 years. Approximately half the fossil-fuel CO2 emitted by man has now dissolved into the ocean.

CO2- driven acidification, in addition to lowering seawater pH, shifts the proportion of dissolved carbon dioxide away from carbonate ion and to bicarbonate ion. It is the carbonate ion that calcium carbonate shell-making organisms require for calcification. Similarly, a range of physiological processes are sensitive to pH itself. Most conclusions about the biological response to ocean acidification in Australian waters come from laboratory manipulations rather than in situ observations. However observational data documenting already-underway changes in calcification in Southern Ocean zooplankton (Moy et al. 2009) and in Great Barrier Reef corals (Cooper et al. 2008, De’Ath et al. 2009) indicate acidification has already begun to have detectable impact on biological processes.

The major scientific knowledge gaps in the physical response lie in projecting the spatial and temporal variability in the progress of acidification. In particular there is a critical need for regional and local-scale data on carbonate chemistry variability and vulnerability. The major scientific knowledge gaps in biological and ecological responses lie in understanding inter-specific and intra-specific differences in response to acidification (“winners” versus “losers”), the resilience of organisms to acidification, and in the implications for the structure of ecosystems.

Howard, W. R., Havenhand, J., Parker, L., Raftos, D., Ross, P., Williamson, J. & Matear, R., 2009. Ocean acidification. In A Marine Climate Change Impacts and Adaptation Report Card for Australia 2009 (Eds. E.S. Poloczanska, A.J. Hobday and A.J. Richardson), NCCARF Publication 05/09, ISBN 978-1-921609-03-9. More information & the report.

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