Can the ocean slow global warming?

The ocean plays an important role in the climate system by regulating the amount of carbon dioxide in the atmosphere. It is generally accepted that more than 80 percent of anthropogenic CO2 emissions will eventually be absorbed naturally by the oceans, primarily through a slow exchange between the atmosphere and the surface waters. This exchange process behaves like a clog in a sink’s drain: drainage occurs too slowly to prevent a large build-up; in this case of CO2 in the atmosphere. Carbon dioxide entering the deep waters of the ocean is removed from interaction with the atmosphere for periods of hundreds of years. A significant fraction of the excess carbon dioxide released by man’s activities enters the deep waters of the ocean and plays no further part in global warming over century timescales.


Last time Earth suffered a carbon-induced fever, it was the oceans that helped save the day, say marine scientists in California. Massive ocean-bottom accumulations of the mineral barite show that the last severe global warming episode 55 million years ago was accompanied by several thousands of years of ocean plant life kicking into high gear. All that productivity captured excessive carbon from the atmosphere and dropped it to the ocean floor, where it was buried — or “sequestered.”

To date, the world’s oceans have absorbed nearly a third of the excess carbon dioxide emitted as a result of anthropogenic activities. One way to reduce CO2 emissions is to capture and store it. An option for storing the captured CO2 is ocean storage, in which CO2 is injected deep into the ocean, where it dissolves, or deposited onto the ocean floor, where it is denser than water and therefore forms a “lake” of CO2. On entering the ocean, carbon dioxide undergoes rapid chemical reactions with the water and only a small fraction remains as carbon dioxide. The carbon dioxide and the associated chemical forms are collectively known as dissolved inorganic carbon or DIC. This chemical partitioning of DIC (‘buffering’) affects the air-sea transfer of carbon dioxide, as only the unreacted carbon dioxide fraction in the sea water takes part in ocean-atmosphere interaction.

It is hypothesized that the rate of carbon dioxide fixation by microscopic plants called phytoplankton that live in the surface waters of the oceans may be limited by the availability of iron. In particular, field experiments in high nutrient, low chlorophyll (a measure of plant biomass) ocean waters such as Southern Ocean and the Equatorial Pacific have shown that addition of iron increased the rate of removal of carbon dioxide through the process of photosynthesis.

Ocean sequestration technologies to reduce carbon emissions may be good news for the atmosphere, but scientists and policymakers are increasingly concerned about the side effect of carbon dioxide absorption: ocean acidification. CO2 could be stored in the ocean but the effects that this may have are not fully understood. It may make the oceans more acidic which would damage sea-life. Although deep ocean storage is possible we have to consider how much excess carbon dioxide can the ocean hold and how will it affect marine life?

Scientists are becoming increasingly worried about ocean acidification, a direct result of the increase in atmospheric CO2 levels and storing captured carbon in ocean. On 30 June 2005, the Royal Society of London published a Report on why this is important:

* Carbon dioxide from the atmosphere dissolves in the ocean, and makes it acidic.
* This is inevitable with high carbon dioxide, no fancy models are involved.
* The oceans are already 30 percent more acid than before the fossil fuel burning started.
* Acidification will kill corals, and probably make many other species (like squid) extinct.
* The overall effects are unknown – there has been no period like this in the last two million years.

Since the industrial revolution, ocean pH has gone down by 0.1 units, which translates into a 30 percent surge in acidity. Scientists predict that pH will go down another 0.14 to 0.35 units by the end of this century. Accompanying the lower pH are lower saturation points of minerals such as calcium carbonate, the primary skeletal material of marine organisms that form the basis of ocean food webs, such as phytoplankton and coral reefs. As the ocean becomes more acidic, calcium carbonate begins to dissolve. The shift in ocean chemistry is so profound that the shells will literally dissolve off the backs of some organisms under the ocean conditions predicted for 2100, according to experiments conducted by Victoria Fabry, of California State University in San Marcos.

Already, ocean acidification is damaging surface waters and having an impact on marine ecosystems. It makes unavailable the compounds necessary for marine organisms to build shells and skeletons, thus impeding the growth of plankton, starfish, urchins, oysters and other shelled organisms as well as coral. Due to ocean acidification, coral reefs will begin to erode more quickly than they can rebuild. Ocean acidification will have a devastating effect on shell fisheries. Research shows that nearly 100 percent of the larval clam community dies within several days of exposure to pH levels already seen now in some regions of the oceans, and that will be seen nearly everywhere by 2100. If none of the larval clams are able to survive, it will only be a matter of years before entire adult clam communities disappear.

The lowered pH will also dissolve several small phytoplankton species that represent the very base of marine food webs, and the disappearance of these microscopic plant species will cascade throughout the entire ocean with devastating consequences. And all these changes are occurring so quickly that marine life will have great difficulty adapting to changing seawater chemistry.

Unlike the situation with other aspects of climate change, there is no controversy over ocean acidification. Thomas Lovejoy, president of the Heinz Centre for Science, Economics and the Environment, has described ocean acidification as “the most profound environmental change I have observed in my entire professional career.” It is highly necessary to get people talking about ocean acidification and immediately formulate research strategies to deal with this potentially catastrophic problem.

Mirza Galib, The Daily Star, 8 February 2008. Article.


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OA-ICC HIGHLIGHTS

Ocean acidification in the IPCC AR5 WG II

OUP book