Our Global Ocean
by Joe Romm
Ocean Acidification Threatens Our Ability to Feed Humanity
The geological record of ocean acidification
The world’s oceans may be turning acidic faster today from human carbon emissions than they did during major extinctions in the last 300 million years, when natural pulses of carbon sent global temperatures soaring. This is the conclusion of a 21-author study in a major scientific journal, the first to survey the geologic record for evidence of ocean acidification over this vast time period (The Geological Record of Ocean Acidification, Hönisch, et al. Science 2 March 2012: 1058-1063). Senior author Bärbel Hönisch, a paleoceanographer at Columbia University states:
What we’re doing today really stands out, 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 study looked back over 300 million years to reveal that “the unprecedented rapidity of CO2 release currently taking place” has put marine life at risk in a frighteningly unique way:
The current rate of (mainly fossil fuel) CO2 release stands out as capable of driving a combination and magnitude of ocean geochemical changes potentially unparalleled in at least the last ~300 My of Earth history, raising the possibility that we are entering an unknown territory of marine ecosystem change.
It’s not just that acidifying oceans spell marine biological meltdown by end of century. We are also warming the ocean and decreasing dissolved oxygen concentration. That is a recipe for mass extinction.
The past as prologue: paleo-climate examples
The oceans act like a sponge to draw down excess carbon dioxide from the air. The gas reacts with seawater to form carbonic acid, which over time is neutralized by fossil carbonate shells on the seafloor. But if carbon dioxide goes into the oceans too quickly, it depletes the carbonate ions that corals, mollusks and some plankton need for reef and shell-building. That is happening now.
In a review of hundreds of paleoceanographic studies, the team found evidence for only one period in the last 300 million years when the oceans changed as fast as today: the Paleocene-Eocene Thermal Maximum, or PETM, some 56 million years ago.
In the early 1990s, scientists extracting sediments from the seafloor off Antarctica found a layer of mud from this period wedged between thick deposits of white plankton fossils. In a span of about 5,000 years, a mysterious doubling of atmospheric carbon concentrations pushed average global temperatures up by 6°C--dramatically changing the ecological landscape. Carbonate plankton shells littering the seafloor dissolved, leaving a brown layer of mud. As many as half of all species of single-celled organisms that live at the ocean bottom went extinct, and organisms higher in the food chain must also have disappeared. During this time, ocean pH fell as much as 0.45 units, and acidity rose accordingly. In the last hundred years, atmospheric CO2 has risen some 30%, to 393 parts per million, and ocean pH has fallen by 0.1 unit so far: to pH8.1. The current ocean acidification rate is 10 times faster than 56 million years ago, says Hönisch. We could soon see ocean changes similar to those observed during the PETM.
More catastrophic events have shaken the Earth before, though they did not happen as rapidly as in the PETM. The new study found two other times of potential ocean acidification: the extinctions triggered by massive volcanic eruptions at the end of the Permian and Triassic eras, about 252 million and 201 million years ago respectively. During the end-Permian, about 252 million years ago, massive volcano activity led to a rise in atmospheric carbon, and the extinction of 96% of marine life. At the end of the Triassic, about 201 million years ago, another burst of mass volcanism doubled atmospheric carbon. Coral reefs collapsed and many sea creatures vanished.
For now, the effects of ocean acidification are overshadowed by problems such as sewage pollution and hotter summer temperatures that threaten corals with disease and bleaching. However, scientists studying the effects of acidic water alone (in the laboratory) find that lower pH levels harm a range of marine life, from reef and shell-building organisms to the tiny snails favored by salmon. Fish larvae lose their ability to sniff out predators and find their way home when pH drops below 7.8.
It may take decades before the full effect of ocean acidification on marine life shows itself. Until then, the past is a good way to foresee the future, says Richard Feely, an oceanographer at the National Oceanic and Atmospheric Administration:
These studies give you a sense of the timing involved in past ocean acidification events. Decisions we make over the next few decades could have significant implications on a geologic timescale.
Joe Romm PhD edits the invaluable blog Climate Progress