By David L. Brown
One of the things that helps slow the rise of carbon dioxide in our atmosphere—and the greenhouse warming that results from higher CO2—is the fact that more than a third of the carbon released into the air since the beginning of the Industrial Revolution has been absorbed in our oceans and sequestered in sediments. That’s a good thing, right?
Well, it turns out that’s not necessarily the case. It is true that the oceans can absorb large quantities of carbon … but only over long periods of time. Recent studies of a 3.2 kilometer core drilled from the Antacrtic ice cap, as reported today by BBC News (read it here), provide a record of temperature and atmospheric CO2 going back 800,000 years. The core reveals a direct cause-and-effect relationship between atmospheric levels of CO2 and global temperatures. When CO2 goes up, temperatures also rise.
In that entire 800,000 years, the fastest rate of change in atmospheric carbon that was found was an increase of 30 parts per million per 1000 years.
We have seen that much carbon added to our atmosphere just since 1989, a mere 17 years ago—and the rate of change is becoming ever faster as humans proliferate and continue to burn more and more oil, gas and coal.
The oceans cannot respond fast enough to sequester this unprecedented onslaught of carbon in sediments, and instead more and more of the CO2 reacts with the water to form carbonic acid. According to the BBC story,
…[m]ore CO2 absorbed by the oceans will raise their acidity, and a number of recent studies have concluded that this will eventually disrupt the ability of marine micro-organisms to use the calcium carbonate in the water to produce their hard parts.
What is at stake is not only the entire foundation of the global food chain, which starts with the phytoplankton and other micro-organisms that live in the sea, but also a major factor in the delicate balance of oxygen and CO2 in our atmosphere. Much of the planet’s fresh oxygen is produced through he process of photosynthesis by algae and bacteria that share the oceans with those tiny animals that capture carbon and eventually sequester it on the sea bottom in their skeletons.
The subject of ocean acidity was recently explored in a cover story in New Scientist magazine (“The Other CO2 Problem,” 5 August, 2006, pgs. 28-33). According to that article (read it here, subscription required),
The potential seriousness of the effect was underlined in 2005 by the work of James Zachos of the University of California at Santa Cruz and his colleagues, who studied [one of several rare catastrophic events over the past 300 million years]. They showed that the mass extinction of huge numbers of deep-sea creatures around 55 million years ago was caused by ocean acidification after the release of around 4500 gigatonnes of carbon (New Scientist, 18 June 2005, p 19). It took over 100,000 years for the oceans to return to their normal alkalinity.
Something similar to that catastrophic event of 55 million years ago seems to be taking place today, thanks to human-induced CO2 emissions from the burning of fossil fuels. In effect, we are digging up vast quantities of carbon—quantities that it has taken Nature hundreds of millions of years to remove from the air and sequester in the earth—and releasing it again over a period of a few lifetimes, a mere hiccup in geological time. We are only now becoming fully aware how human activity is changing our air, and now it seems that we may be destroying the oceans as well.
Already there are signs of damage and destruction of tropical coral reefs due to rising temperatures and acidity. Speculation that a warmer ocean might offset the higher acid content have proved unfounded, because when tropical corals are surrounded by water just 1 degree C. warmer than normal, they reject the algae on which they depend in a symbiotic relationship, becoming “bleached” and eventually dying. But the effects will be even more pronounced in cooler regions. According to the New Scientist article:
If the outlook for tropical corals is bleak, the consequences of acidification for organisms in more southerly and northerly waters causes even more concern. “Tropical surface waters will be affected by ocean acidification last,” says Ulf Riebesell of the Leibniz Institute of Marine Sciences in Kiel, Germany. “In higher latitudes the waters could tip much sooner into being corrosive.”
Early studies suggested that high-latitude surface waters would become undersaturated with respect to aragonite [the soluble form of calcium carbonate that forms the skeletons of coral, molluscs and other marine organisms] only if atmospheric CO2 reached four times pre-industrial levels. But in September 2005, Orr, Fabry and colleagues published evidence suggesting that some polar and sub-polar surface waters will become undersaturated at just twice pre-industrial levels – conditions that are likely to occur within the next 50 years.
In shipboard experiments, they found that the shells of pteropods ["sea butterflies," tiny shelled plankton] started dissolving after just two days in water at the pH predicted for 2050. This is worrying because pteropods are an important part of the ecosystems in the Southern Ocean and Arctic and sub-Arctic waters, where animals such as cod, salmon and whales eat them. “In standard ocean surveys their abundance is used as an indicator of ecosystem health,” says Orr.
Could the pteropods simply move to warmer waters that are not approaching the saturation horizon so fast? “We think it unlikely, as they would have to outcompete organisms already living there,” says Orr. The fate of all the creatures that feed on pteropods will depend on whether species less vulnerable to acidification take their place in the food chain.
Such an event of new species arising like a deus ex machina to take the place of pteropods and other threatened creatures in any reasonable time frame is extremely unlikely in my opinion. Evolutionary adaptation does not work in the short term, but through millennia and thousands of generations. What the Earth is experiencing is no less than a Great Extinction, only the sixth in the entire history of the Earth, and the geological record shows that the effects of such events last for literally millions of years.
According to the New Scientist article, there is also great concern for cold water corals, lesser known than their colorful tropical relatives and found deep in the waters. Only in recent years has their existence been documented and their importance realized. According to New Scientist, “…one system stretches from Norway down to the coast of Africa. At around 4500 kilometres, it is roughly two-and-a-half times as long as Australia’s Great Barrier Reef. The richness of these reefs is also astonishing. In terms of biomass production and even biodiversity, cold-water corals may be as important as warm-water corals.”
Increasing CO2 in the oceans could affect a broad range of other life forms, including fish and mammals which might not be able to adapt to more acidic conditions. In searching for a silver lining in this otherwise gloomy scenario, New Scientist noted speculation that the increased CO2 might cause microbes to produce larger quantities of volatile organic compounds such as dimethyl sulphide, which could enter the atmosphere and induce cloud seeding, which in turn might shield the planet from some of the Sun’s rays and thus reduce the impact of global warming. Another effect might be to speed up the process of sequstering carbon on the sea bottom by stimulating growth of some siimple organisms in the ocean. However, this is in question and needs more study.
In fact, the whole subject involves many factors about which we know little, and which have the potential to create even more problems for our endangered planet. As a measure of how recently scientists have become aware of this new threat, it was only in 2003 that Ken Caldeira of the Carnegie Institution and Michael Wickett of the Lawrence Livermore National Laboratory calculated that the absorption of fossil CO2 “could make the oceans more acid over the next few centuries than they have been for 300 million years, with the possible exception of rare catastrophic events.”
It was in their Nature paper that the phrase “ocean acidification” appeared in the scientific literature for the first time. Just three years ago. How little we know; how much is yet to be learned. And perhaps most pertinent of all, how much time do we have left? Meantime, the human circus continues to play and in the center ring the antic clowns in the capitals of the world command the audience’s full attention.