Ocean acidification: the next big threat to coral reefs?

By Dr. Lizzie Mcleod, Climate Adaptation Scientist for the Nature Conservancy, Asia Pacific Region

As if the long list of threats to coral reefs weren’t enough, we can now add ocean acidification to the list.

Perhaps you’ve seen the gloomy headlines likeOcean Acidification: ‘Evil Twin’ Threatens World’s Oceans, Scientists Warn.

Perhaps it is no wonder that folks think coral reef scientists are never finished “crying wolf” about the next global challenge threatening to wipe out coral reef ecosystems.

How serious is this threat and what can we do to address it? To answer these questions, we decided to enlist the help of some global acidification experts. But first, we have to understand the problem.

What is ocean acidification?

Ocean acidification occurs independently from–but in combination with– climate change. Ocean acidification refers to a decrease in ocean pH over decades or more caused primarily by uptake of atmospheric CO2.

Because human activities (primarily burning of fossil fuels) are releasing CO2 into the atmosphere very quickly, the ocean is taking up CO2 faster today than it has in the past which is causing the chemistry of the world’s oceans to change more quickly than they can handle.

When atmospheric CO2 dissolves into seawater, carbonic acid is formed, and hydrogen ions are released. As a result, the pH of the ocean surface waters decreases, making it more acidic.

When hydrogen ions are released in seawater, they combine with carbonate ions, thus lowering the carbonate ion concentration. Marine calcifiers such as corals, coralline algae, crabs, clams, oysters, and some plankton need carbonate ions to build their skeletons and shells.

Acidification often leads to reduced calcification and dissolution (e.g., affecting a coral’s ability to grow its skeleton, resulting in slower growth and a more fragile structure, thus more vulnerable to erosion).

Why should you care?

Many marine managers are faced with a litany of immediate threats to marine ecosystems and dependent human communities including overfishing and destructive fishing, pollution, sedimentation, and climate driven stressors such as coral bleaching. These threats are obvious as they can be seen directly.

By contrast, ocean acidification is largely an invisible, creeping environmental problem. As such, coral reef managers rarely consider it a feasible management target.

Researchers suggest, however, that by the middle of the century, reefs will be unable to survive in many regions due to ocean acidification. Recent studies indicate that ocean acidification could represent an equal or perhaps even greater threat to marine life and human communities than global warming.

The idea that ocean acidification is the “evil twin of global warming” was developed by Richard Feely of the National Oceanic and Atmospheric Administration (NOAA) because ocean pH has already fallen below 0.1 units, and according to Dr. Feely “unless we curb carbon emissions aggressively, pH will decrease by another 0.3-0.4 units and by 2100, seawater could become 150% more corrosive [than at present] – a situation that hasn’t occurred for more than 65 million years. And it would stay at those levels for millennia.”

Ocean acidification is likely to affect marine food webs which would lead to major changes in commercial fish stocks, threatening protein supply and food security for millions of people and the multi-billion dollar fishing industry. Marine calcifiers, such as corals, are particularly vulnerable to ocean acidification.

If we lose reefs, we lose the benefits they provide such as coastal projection, fish habitat, food and livelihoods for coastal communities, tourism, and medicines from marine organisms.

What is the Conservancy doing to address it?

We knew that we needed help to improve our understanding of the impacts of ocean acidification on marine systems and specifically, how we can address the threat of ocean acidification in our conservation planning and management efforts.

In 2008, we held an experts’ workshop in Honolulu to identify management and policy recommendations to address ocean acidification. Building on this effort, we convened a group of leading climate and marine scientists and coral reef managers in Palau in 2011 to identify factors likely to affect coral reef ecosystem vulnerability to ocean acidification and research priorities necessary to integrate vulnerability to acidification into conservation planning and management. We invited acidification experts from top research institutions including the National Center for Atmospheric Research, Woods Hole, Australian Institute of Marine Science, University of Queensland, James Cook University, SCRIPPS, Stanford, University of Miami, NOAA, and the Great Barrier Reef Marine Park Authority.

What was remarkable about this effort was that after we had shared how we prioritize areas for conservation and took these researchers to different reef environments, they told us that we had changed their approach to their research. Several of them said that it completely opened their mind to the complexity of factors affecting vulnerability to ocean acidification and to identifying conservation priorities in the face of changing ocean chemistry.

After our last day in the field, one researcher joked that we should have put “soul expanding” on the list of workshop objectives on our agenda! (The workshop location, Palau, helped with this one!). By sharing with researchers how we make conservation decisions and what we need to improve on these decisions, we helped to make the research more relevant to our work.

We will continue the work with our partners in Palau, US, and Australia, and are working together to fund a comprehensive research plan to address ocean acidification. We also will continue to work at the policy level to stabilize CO2 emissions and bring them below 350 ppm, and will continue our work on reef resilience – focusing on reducing local stressors.

Mcleod, E., K.R.N. Anthony, A. Andersson, R. Beeden, Y. Golbuu, J. Kleypas, K. Kroeker, D. Manzello, R. Salm, H. Schuttenberg, and J.E. Smith. 2012. Preparing to manage coral reefs for ocean acidification: Lessons from coral bleaching. Frontiers in Ecology and the Environment. doi:10.1890/110240.

Listen to Frontiers in Ecology’s podcast interview with Lizzie.

Elizabeth Mcleod, The Nature Conservancy blog, 20 May 2013. Article.

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