In the few years since ocean acidification came under the public spotlight, it has become an increasingly important issue that is challenging scientists, policy-makers and governments.
Ocean acidification differs from global warming in that its impact derives from the chemistry of carbon dioxide (CO2) in seawater, rather than from its physical action as a greenhouse gas in the atmosphere. This means that even if the climate does not warm, increasing atmospheric CO2 will inevitably increase ocean acidity.
The surface ocean absorbs as much as 30% of anthropogenic (human-made) CO2 emissions, and the gas dissolves through a well understood chemical process, forming a weak acid that raises the acidity of the oceans. Atmospheric CO2 is absorbed by the ocean faster than natural processes can neutralise the increased acidity it causes. The Southern Ocean is particularly vulnerable to the phenomenon, due to the higher solubility of CO2 in cold water. As a result, the current trajectory of carbon emissions will cause a change in ocean acidity during this century that is greater in extent than anything likely to have occurred for millions of years.
The oceanic response to enhanced CO2 levels is likely to have serious consequences for marine ecosystems that are the backbone of important economic and social activities, such as fisheries, aquaculture, and tourism.
Evidence is already emerging of changes in the growth and structure of marine organisms in response to ocean acidification (Australian Antarctic Magazine 10: 26–27, 2006). Recent studies by the Antarctic Climate and Ecosystems Cooperative Research Centre have shown that the shells of some microscopic marine organisms in the Southern Ocean are getting thinner. The evidence is a warning that ocean acidification will have potentially serious impacts within the 21st century for the sustainability and management of many marine ecosystems and the human communities that depend on them.
A significant scientific uncertainty relating to ocean acidification is how key marine species, such as this coccolithophorid (Emiliania huxleyi), will respond to increasing acidity, and how these responses will alter marine food webs and biodiversity. Coccolithophorids are an important component of marine phytoplankton and secrete shells of calcite — a form of calcium carbonate that will become harder to produce as ocean acidity increases.
Unlike ‘geo-engineering’ proposals to mitigate global warming, there are currently no practical technological or engineering solutions for reducing the acidity of oceans or for mitigating its impacts, short of reducing CO2 emissions. Given the possibility that emissions will continue over the coming decades, the scientific response to ocean acidification must therefore focus on anticipating impacts and assisting policy-makers to develop informed responses. Government decision-makers and communities could then prepare adaptation strategies and action plans to address the projected effects of ocean acidification on ecosystems, economies and communities.
Scientific knowledge about ocean acidification and its effects is increasing, but there are still significant uncertainties that make developing ocean acidification-related policy a challenge. The biggest gaps are in understanding:
- The underlying variability of ocean chemistry and ecosystems in Australian seas. This is required to provide references against which future change can be identified.
- How key species in their various life stages will respond to ocean acidification and how those responses will alter marine food webs and affect biodiversity.
- How the combined effects of climate-related changes (increased ocean temperature, nutrient availability and circulation) and commercial activities, such as fishing, tourism and aquaculture, are likely to interact with ocean acidification. For example, ocean acidification may have more severe effects on ecosystems already affected by pollution, warming or overfishing, than on less stressed systems (see The acid test).
- How the vulnerability of low-lying islands to sea-level rise may be exacerbated by ocean acidification. If the growth of reef-forming and reef-stabilising organisms such as corals and coralline algae is compromised, reefs and onshore coastal communities will be more vulnerable to the physical impacts of storm-driven erosion.
- How rapidly change will occur and whether there are thresholds that, once breached, will lead to major, persistent changes in marine ecosystems.
A further policy challenge arises because the only mitigation option available is a reduction in carbon dioxide emissions. Ocean acidification needs to be considered by decision-makers when setting stabilisation targets for atmospheric CO2 and the timeframes in which the targets need to be reached. There is a natural time lag involved in the marine carbon cycle, both in the uptake of CO2 by the ocean as well as in the centuries needed to reverse the acidification already under way. This places a penalty on delaying limits on carbon emissions and a premium on early action.
Another challenge is understanding the connection between ocean acidification and proposed climate mitigation proposals, such as ocean fertilisation. This involves adding nutrients such as iron to the ocean to boost carbon consumption by marine plants. The uptake of CO2 in iron-fertilised systems would be accelerated by dissolving carbonate shells, but at the expense of these organisms and their roles in the ecosystem. On the other hand, if ocean fertilisation can enhance the transfer of carbon from the atmosphere directly to the deep ocean, it would tend to reduce the uptake of CO2 in near-surface waters, where most shell-forming organisms dwell. The impact would be transferred to deeper waters, where organisms such as deep-water corals are important players in the ecosystem. Trade-offs like these have not yet been examined.
As ocean acidification is related to, but is not strictly ‘climate change', it was not addressed in detail in the 2007 Fourth Assessment Report of the Intergovernmental Panel on Climate Change. However, growing recognition of the significance of acidification for the world’s oceans has already prompted some countries to initiate research programs to further explore the scientific uncertainties.
Research programs are now underway in the European Union and there is a Bill before the US Congress that would establish an ocean acidification research and monitoring program to carry out ongoing long-term measurements of ocean chemistry and vulnerable ecosystems.
So how can Australia improve its scientific, policy and management response to this growing threat? Scientists attending an ocean acidification workshop in Hobart earlier this year recommended:
- A coordinated national assessment of the Australian Marine Jurisdiction to provide benchmark data on which to build sustained observation programs for identified high-risk regions. Future impacts can then be detected and responses to the problem assessed quickly and effectively.
- A range of modelling, experimental, and field studies that will enable clear identification of the risk and cost of ocean acidification to marine ecosystems and society.
The global scale of ocean acidification means that the Australian research community cannot fill the knowledge gaps by itself, and must build upon its already strong involvement in international research networks. The policy challenge is similarly global. As the impacts affect many low-lying island nations in our region, options for addressing these impacts need to be incorporated into Australia’s international aid and foreign policy strategies and programs.
Nationally, some of Australia’s iconic natural marine assets, such as the Great Barrier Reef, are at risk. Ocean acidification is thus a challenge to Australian governments at all levels.
WILL HOWARD and ROSEMARY SANDFORD