Projected climate change impact on oceanic acidification
© McNeil and Matear; licensee BioMed Central Ltd. 2006
Received: 06 March 2006
Accepted: 27 June 2006
Published: 27 June 2006
Anthropogenic CO2 uptake by the ocean decreases the pH of seawater, leading to an 'acidification' which may have potential detrimental consequences on marine organisms . Ocean warming or circulation alterations induced by climate change has the potential to slowdown the rate of acidification of ocean waters by decreasing the amount of CO2 uptake by the ocean . However, a recent study showed that climate change affected the decrease in pH insignificantly . Here, we examine the sensitivity of future oceanic acidification to climate change feedbacks within a coupled atmosphere-ocean model and find that ocean warming dominates the climate change feedbacks.
Our results show that the direct decrease in pH due to ocean warming is approximately equal to but opposite in magnitude to the indirect increase in pH associated with ocean warming (ie reduced DIC concentration of the upper ocean caused by lower solubility of CO2).
As climate change feedbacks on pH approximately cancel, future oceanic acidification will closely follow future atmospheric CO2 concentrations. This suggests the only way to slowdown or mitigate the potential biological consequences of future ocean acidification is to significantly reduce fossil-fuel emissions of CO2 to the atmosphere.
Rising atmospheric CO2 concentrations via fossil fuel emissions will lead to an increase in oceanic CO2 via thermodynamic equilibration. Carbon chemistry in seawater undergoes the following equilibrium reactions as CO2 enters the ocean.
The pH of seawater is defined by the amount of H+ ions available: pH = -log10[H+]. Increasing CO2 concentrations in the surface ocean via anthropogenic CO2 uptake will have implications for oceanic pH. As shown in equation (1), when CO2 dissolves in water it forms a weak acid (H2CO3), dissociates to bicarbonate generating hydrogen ions (H+), which makes the ocean less basic (pH decreases). Using an ocean-only model forced with atmospheric CO2 projections (IS92a), Caldeira and Wickett predicted a pH drop of 0.4 units by the year 2100 and a further decline of 0.7 by the year 2300.
Future acidification (lowering of pH) may adversely impact marine biota, but our present understanding of the potential biological response is limited . It is recognised however that a decrease in pH will alter the acid-base balance with the cells of marine organisms . Marine organisms regulate intercellular pH by the metabolic interconversion of acids and bases, the passive chemical buffering of intra- and extra-cellular fluids, and the active ion transport (e.g. proton transport by extra-cellular respiratory proteins such as hemoglobin) . Acid-base imbalances in marine organisms can lead to the dissolution of exoskeletal components such as calcareous shells, metabolic suppression, reduced protein synthesis and reduced activity [6, 7]. Experiments to determine the likely response of marine organisms to pH changes have induced large changes in pH under laboratory conditions (>1) [8–13]. Little is known on what the gradual long-term effects of pH lowering will be on marine organisms. As pH changes have the potential to directly impact marine biota it is important to understand the magnitude of these changes under elevated CO2 levels and global warming.
Projections of future decreases in pH have been obtained from an ocean-only model that has not considered the effect of climate change feedbacks on the carbon chemistry of the ocean . Recently, a study explored the role that climate change plays on the extent of ocean acidification . Using three separate climate models they found climate change to insignificantly impact the projected future decreases of pH. However there was no investigation into this outcome even though the same models used predict large reductions in oceanic CO2 uptake due to climate change in association with temperature, circulation and biological feedbacks . In this study we use a climate model to examine, partition and discuss the dominating climate change feedbacks controlling the future surface ocean pH.
Results and discussion
The solubility driven reductions in the growth of surface DIC concentration due to warming increase pH by a magnitude that is almost equal to pH decline directly associated with ocean warming, which cause the two affects to almost cancel each other. In Figure 2, the lines of constant pH are almost parallel to slope of the . As a consequence, the projected global pH decline of the climate change experiment does not differ from the projection made with the control experiment.
The CO2 biological pump within our simulations changed considerably with carbon export decreasing with climate change . These changes would also lead to changes in pH within the water column however in the surface ocean, biologically mediated pH changes were found to be negligible.
Our study confirms previous suggestions that climate change feedbacks do not influence the projected decline in pH. This insensitivity to climate change occurs because the decrease in pH due to warming is nearly equal to but opposite in magnitude to the pH increase associated with reduced growth of DIC concentration in the upper ocean caused by reduced solubility of CO2 with ocean warming (Figure 2). Therefore, projections that neglect climate change  provide a reasonable estimate of the future pH change. Future projections of ocean acidification will therefore mainly be dependent on the future level of atmospheric CO2. The consequences of a small but sustained decrease in oceanic pH on marine phytoplankton are virtually unknown. It will be important for marine ecologists in the future to better understand the sensitivities of phytoplankton growth to pH in particular, so as to better quantify the likely future biological changes at the regional and global scale.
The coupled atmosphere-ice-ocean carbon cycle model developed by the Commonwealth Scientific Industrial Research Organisation (CSIRO) was used for this study . Details of the model are described elsewhere . Climate change feedbacks were quantified by comparing two separate climate model experiments. The 'control' experiment did not include the warming effects of elevated greenhouse gases in the atmosphere (no radiative forcing) while the 'climate change' experiment explicitly includes the radiative forcing of greenhouse gases in the atmosphere. For both experiments atmospheric CO2 levels increased according to observations between 1880 to 1995 then followed IS92a projections until the year 2100 . Differing climate models maintain differing sensitivities to anthropogenic climate forcing. The sensitivity is defined as the global annual temperature change associated with a doubling of atmospheric CO2. The sensitivity of the CSIRO Mark II climate model is 4.3°C , and is at the higher end of global model sensitivities .
We acknowledge the constructive suggestions of the editor, Christopher Sabine, Mark Baird and three anonymous reviewers. B.I.M was supported through a grant from the Australian Research Council while R.J.M was supported through the Australian Greenhouse Office Climate Change Program.
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