In a new study marine scientists warn that the rate of ocean acidification presently occurring is unprecedented in the last 300 million years. This is due to dissolving carbon dioxide from the atmosphere, much of which human activity has contributed over the last 200 years through the use of fossil fuels. The extent of the acidification and rate of acidification enhances the prospect for a mass marine extinction event this century.
Related: Dec 2011 - Ocean acidification warning to Durban climate negotiators to act on reducing CO2 emissions | June 2011 - Oceans at high risk of unprecedented Marine extinction scientists warn | April 2010 - Paleo-perspectives on ocean acidification - marine ecosystems under threat | Feb 2010 - Marine Extinction looms with Ocean Acidification increasing
Caption: This graph shows the correlation between rising levels of carbon dioxide (CO2) in the atmosphere at Mauna Loa with rising CO2 levels in the nearby ocean at Station Aloha. As more CO2 accumulates in the ocean, the pH of the ocean decreases. (Modified after R.A. Feely, Bulletin of the American Meteorological Society, July 2008) Image coutesy of NOAA - Hawaii Carbon Dioxide Time-Series
The oceans act as a great carbon sink which has been good for slowing the presence of carbon dioxide in the atmosphere and moderating the greenhouse effect which causes global warming. That is one reason why we have only experienced a 0.8C rise in average global temperatures over the last 100 years. To slow the process of ocean acidification humans need to reduce greenhouse gas emissions, and eventually devise technology to draw down and scrubb carbon dioxide from the atmosphere for sequestration.
"What we're doing today really stands out," said lead author Bärbel Hönisch, a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory. "We know that life during past ocean acidification events was not wiped out—new species evolved to replace those that died off. But if industrial carbon emissions continue at the current pace, we may lose organisms we care about—coral reefs, oysters, salmon."
The chemical process for how the oceans absorb carbon dioxide are well known. CO2 reacts with seawater to form carbonic acid, which over time is neutralized by marine shelled creatures, corals and small planktonic creatures to form carbonate shells which is eventually sequestered on the sea floor. But when CO2 is absorbed too quickly, carbonate ions are depleted resulting in a reduced capacity for corals, mollusks and plankton to build coral reef and their carbonate shells. When the smallest creatures start experiencing problems, this effects ripples through the food chain affecting larger predator species.
The team of researchers from five countries reviewed hundreds of paleoceanographic studies, and found evidence for only one period in the last 300 million years when the oceans changed even remotely as fast as today: the Paleocene-Eocene Thermal Maximum, or PETM, some 56 million years ago. Over a period of about 5000 years there was a surge of carbon dioxide which doubled atmospheric concentrations that resulted in global average temperatures surging up by about 6C. With still rising emissions, unless we start mitigation action we are likely to hit 6C by the end of this century. What was done naturally in 5000 years we are doing in about 100 years.
During the PETM event carbonate plankton shells littering the seafloor dissolved, leaving the brown layer of mud. According to study co-author Ellen Thomas, from Yale University, as many as half of all species of benthic foraminifers went extinct, inferring that many species higher in the food chain may have also disappeared. "It's really unusual that you lose more than 5 to 10 percent of species over less than 20,000 years," she said. "It's usually on the order of a few percent over a million years." During this time, scientists estimate, the ocean pH level — a measure of acidity - may have fallen as much as 0.45 units. As pH falls, acidity rises.
We have already experienced atmospheric CO2 rising about 30 percent, to 393 parts per million in the last 100 years, with ocean pH falling by 0.1 unit, to 8.1. This rate of acidification is 10 times faster than during the PETM event 56 million years ago according to lead author Bärbel Hönisch. Predictions by the Intergovernmental Panel on Climate Change (IPCC) show that pH may fall another 0.3 units to 7.8 by the end of the century which could see a marine mass extinction event similar to those observed during the PETM.
The abstract for the study - The Geological Record of Ocean Acidification says in full:
Ocean acidification may have severe consequences for marine ecosystems; however, assessing its future impact is difficult because laboratory experiments and field observations are limited by their reduced ecologic complexity and sample period, respectively. In contrast, the geological record contains long-term evidence for a variety of global environmental perturbations, including ocean acidification plus their associated biotic responses. We review events exhibiting evidence for elevated atmospheric CO2, global warming, and ocean acidification over the past ~300 million years of Earth’s history, some with contemporaneous extinction or evolutionary turnover among marine calcifiers. Although similarities exist, no past event perfectly parallels future projections in terms of disrupting the balance of ocean carbonate chemistry—a consequence of the unprecedented rapidity of CO2 release currently taking place.
We are already starting to see the effects of ocean acidification.
- In a recent study, scientists from Stony Brook University found that the larvae of bay scallops and hard clams grow best at pre-industrial pH levels, while their shells corrode at the levels projected for 2100. See The effects of elevated carbon dioxide concentrations on the metamorphosis, size, and survival of larval hard clams (Mercenaria mercenaria), bay scallops (Argopecten irradians), and Eastern oysters (Crassostrea virginica) (abstract | media release)
- Off the U.S. Pacific Northwest, the death of oyster larvae has recently been linked to the upwelling of acidic water there. See Yale 360 November 2011 article - Northwest Oyster Die-offs Show Ocean Acidification Has Arrived
- In a 2011 study of coral reefs off Papua New Guinea, scientists writing in the journal Nature Climate Change found that when pH dropped to 7.8, reef diversity declined by as much as 40 percent. The study concluded "Reef development ceased below pH 7.7. Our empirical data from this unique field setting confirm model predictions that ocean acidification, together with temperature stress, will probably lead to severely reduced diversity, structural complexity and resilience of Indo-Pacific coral reefs within this century." See Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations (abstract)
- Studies with clownfish larvae raised in the lab show that they lose their ability to sniff out predators and find their way home when pH drops below 7.8. See Ocean acidification impairs olfactory discrimination and homing ability of a marine fish (abstract)
- Several other studies show that ocean acidification may be fatal to fish at various stages of their life cycle. See Skeptical Science 22 December 2011 article Ocean Acidification Is Fatal To Fish
- Climate related changes in oceans may also increase jellyfish frequency with impacts on fisheries. See Climate-related increases in jellyfish frequency suggest a more gelatinous future for the North Sea (Climate-related increases in jellyfish frequency suggest a more gelatinous future for the North Sea) and Evidence for impacts by jellyfish on North Sea herring recruitment (Evidence for impacts by jellyfish on North Sea herring recruitment)
Ocean acidification impact on fisheries
Ocean acidification will impact the economic importance of fisheries, with the people most affected being island and developing countries reliant on subsistence fisheries for protein. A 2010 United Nations Environment Program report on Environmental Consequences of Ocean Acidification: a threat to Food Security (PDF) estimates that 3 billion people use fish protein in their diet, with one billion people dependent on subsistence fishing as a primary food source.
"The contribution of marine protein to global food security is substantial. Fish, including shellfish, contribute 15% of animal protein for three billion people worldwide. A further one billion people rely on fisheries for their primary source of protein. Fish are even more important in countries where other protein sources decline seasonally." says the report.
Achim Steiner, UN Under-Secretary General and UNEP Executive Director, said in a 2010 media release: "Ocean acidification is yet another red flag being raised, carrying planetary health warnings about the uncontrolled growth in greenhouse gas emissions. It is a new and emerging piece in the scientific jigsaw puzzle, but one that is triggering rising concern".
"Whether ocean acidification on its own proves to be a major or a minor challenge to the marine environment and its food chain is to date unknown. But the phenomenon comes against a backdrop of already stressed seas and oceans as a result of over-fishing to other forms of environmental degradation. Thus the public might quite rightly ask how many red flags do governments need to see before the message to act gets through," he said at the launch of the report in Cancun in 2010.
This February 2011 video from UNEP explains the potentially catastrophic implications of ocean acidification:
Many commercial fishermen and aquaculture farmers are already aware of ocean acidification and the threat to their livelihoods and culture. It just requires Government's to impose the necessary regulations to limit and reduce carbon emissions. This video from the National Fisheries Conservation Centre (NFCC) and Sustainable Fisheries Partnership highlights the threat to commercial fisheries.
Resolving the problem of ocean acidification will not be easy, and may prove insurmountable in the timeframe we have available for taking mitigation action.
"It's not a problem that can be quickly reversed," said Christopher Langdon, a biological oceanographer at the University of Miami who co-authored the study on Papua New Guinea reefs. "Once a species goes extinct it's gone forever. We're playing a very dangerous game."
Richard Feely, an oceanographer at the National Oceanic and Atmospheric Administration who was not involved in the study commented, "These studies give you a sense of the timing involved in past ocean acidification events — they did not happen quickly. The decisions we make over the next few decades could have significant implications on a geologic timescale."
- The Earth Institute, Columbia University, media release 1st March 2012, Ocean Acidification Rate May Be Unprecedented, Study Says
- Bärbel Hönisch et al, published in Science (AAAS) 2 March 2012: The Geological Record of Ocean Acidification (abstract) DOI: 10.1126/science.1208277
- Talmage and Gobler (2009) The effects of elevated carbon dioxide concentrations on the metamorphosis, size, and survival of larval hard clams (Mercenaria mercenaria), bay scallops (Argopecten irradians), and Eastern oysters (Crassostrea virginica) (abstract)
- Yale 360 November 2011 article - Northwest Oyster Die-offs Show Ocean Acidification Has Arrived
- Katharina E. Fabricius et al, Nature Climate Change, June 2011 - Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations (abstract)
- Philip L. Munday et al, PNAS February 2009 - Ocean acidification impairs olfactory discrimination and homing ability of a marine fish (abstract)
- United Nations Environment Program report, (2010) Emerging Issues - Environmental consequences of ocean acidification: a threat to food security (PDF) | 2010 media release