CO2 between sea and sky

Oceans are huge carbon reservoirs or “sinks”. Without them, the effects of global warming would be much more obvious than they are today – and evident for the past two centuries. Nevertheless, at the present rate of anthropogenic CO2 emissions these gigantic reservoirs seem to be arriving at saturation point. To understand this phenomenon, a European research programme, CarboOcean, has been investigating the oceans of the world in search of indicators making it possible to assess the remaining CO2 absorption potential and the consequences of possible saturation.

Siliceous microalgae (Diatomea) typical of the Kerguelen  oceanic plateau, in the Indian Ocean, studied  by Keops researchers. ©Leanne Armandi Siliceous microalgae (Diatomea) typical of the Kerguelen oceanic plateau, in the Indian Ocean, studied by Keops researchers. ©Leanne Armandi
This expedition, initiated by the CarboOcean researchers, enabled Norwegian pupils to take a trip on the Hans Brattstrøm  scientific vessel, equipped with a plankton net and hydrographic sensors. © Andrea Volbers This expedition, initiated by the CarboOcean researchers, enabled Norwegian pupils to take a trip on the Hans Brattstrøm scientific vessel, equipped with a plankton net and hydrographic sensors. © Andrea Volbers

For CO2, water and air are both pretty much the same. It migrates easily from one to the other, seeking to inhabit these two environments as uniformly as possible. When the carbon concentration in the air increases, as it has been doing for decades, the ocean absorbs the surplus and restores the balance. However, while the behaviour of CO2 in the air is relatively stable, at sea many interactions come into play that essentially relate to two principal mechanisms: the physical pump and the biological pump.

The odyssey of oceanic carbon

The physical pump operates by virtue of the thermohaline ocean circulation, a vast global heat exchanger. At the poles, seawater cools considerably, its salinity increasing as the ice sheet forms which excludes the salt. This colder and more salty water is consequently denser and sinks to the ocean depths. It is there that it begins its long journey to the tropics where it heats up again, rising back to the surface as it becomes less dense before making the return journey to the poles. It is a round trip of around 80 000 km, which it takes a water molecule a thousand years to complete at the rate of a few millimetres a second. This is the ocean “conveyor belt” which draws in part of the carbon dioxide present in our atmosphere like a pump. The cooler the water the more carbon dioxide can be absorbed by it. CO2 is captured in large quantities by the polar waters where it descends to the ocean depths. Several centuries later, when the waters arrive in tropical zones, they heat up and become saturated with CO2 that then returns to the atmosphere.

Nevertheless, part of the carbon dissolved in water does not join this circuit but is consumed directly by the phytoplankton during photosynthesis and then by other marine bodies as part of the food chain. This is the principle of the biological pump. The organic matter that originates from these marine organisms – excrement, corpses – is recycled in the surface waters and, in a period of time ranging from a few days to a few months, the CO2 it contained returns to the atmosphere. However, about a tenth of this organic mass is exported – the term used by the experts – to deeper waters. There the carbon remains for hundreds or thousands of years, or even for geologic timescales if it is deposited on the seabed in the form of marine sediment.

Has the physical pump broken down?

These two mechanisms, especially the physical pump and to a lesser extent the biological pump, are able to store, at least temporarily, a large part of the anthropogenic CO2 emitted into the air. But in what proportion? And until when? And what if the ocean carbon reservoir were to become saturated, what would be the consequences then?

These are all questions that the vast CarboOcean European research programme has been seeking to answer since 1 January 2005. ”We want to better quantify the masses of CO2 that the oceans have absorbed since the beginning of the industrial era (200 years ago), are absorbing today, and will continue to absorb in the future, until around the year 2200,” explains Christoph Heinze, the coordinator of the CarboOcean project. Included under the Sixth Framework Programme and allocated a budget of €14.5 million from the European Commission – out of a total research budget of around €20 million – CarboOcean is focusing mainly on the Atlantic Ocean, including the Arctic and the Southern Ocean. “The first results of our analyses,” continues Christoph Heinze, “suggest that the physical pump in the North Atlantic, which exports CO2 to the ocean depths, is not functioning as well as it used to. A number of hypotheses can explain this, but we are waiting for more information before drawing conclusions about the causes and effects of this slowing. We recently observed the same phenomenon in the Southern Ocean.”

Priming the biological pump?

For many years, researchers have noticed that at certain locations around the world – Southern Ocean, East Equatorial Pacific, North Pacific – the biological pump has also been slowing, due to a lack of phytoplankton. Some believed it would be enough simply to restart it for more CO2 to be absorbed from the atmosphere.

But why was it slowing? Because the phytoplankton lacked iron. Experiments carried out since 1993 show in fact that iron is an essential nutrient for the growth of microalgae. Adding small quantities of iron to the ocean would surely then enable the phytoplankton to regain their strength, thereby increasing the level of CO2 captured by photosynthesis. But for Stéphane Blain, director of the Keops (Kerguelen Ocean and Plateau compared Study) expedition, “these experiments left room for doubt. Because while it is true that this fertilisation results in increased biological activity at the surface – photosynthesis for example – the question of what proportion of absorbed CO2 penetrates to the seabed remains open. And it is this transfer to the seabed that really indicates that the biological pump is operating.” Commercial companies such as Planktos have no hesitation in pushing ahead despite this uncertainty. These operate on the relatively simple principle of selling CO2 credits to polluting companies or authorities so that they can show a zero balance for their CO2 emissions. Planktos then compensates for the emissions of its clients by investing, for example, in reforestation projects on continents. But it also plans shortly to propose the fertilisation of phytoplanktonic zones. However, there is nothing to say that the operation will be profitable, and that CO2 will be exported to the ocean depths in the long term. The initial experiments have in any event left the scientists very sceptical, and that is without taking into account the undesirable effects.

To shed light on this delicate question, the Keops expedition carried out observations at the beginning of 2005 in the area of the Kerguelen oceanic plateau in the Southern Ocean. This area was selected especially for its seasonal plankton bloom. “This is clearly due to an iron influx,” explains Stéphane Blain. “This iron comes naturally from the deepest ocean beds. Studying the reasons for this input was one aspect of our research, but another aspect was to observe the consequences. That made us realise just how effective the natural system is for exporting CO2 to the ocean depths, but also that the conditions for this natural input are completely different from what one can hope to obtain artificially by adding iron to the surface layers. While it is true that an artificial iron contribution boosts the capture of CO2 by surface waters one cannot go further and say that there is a lasting export of CO2 to the deeper waters.”

CO2 + H2O = carbonic acid

When CO2 dissolves in water, the resulting chemical reaction produces carbonic acid. In other words, the CO2 absorbed by the ocean, even naturally, increases the acidity in the water. Before the industrial age, oceanic pH (1) was 8.16, while today it is no more than 8.05. At present rates of carbon emissions into the air and its absorption by the oceans, the pH is likely to be around 7.60 in 2100.

But as Christoph Heinze reminds us, “The survival of many marine organisms, many of which are at the base of the food chain in ocean environments, is partly dependent on pH. Acidification can therefore have the effect of changing the life in the oceans. Organisms with calcareous shells will be the most vulnerable.” “If the oceanic carbon sinks continue to weaken,” concludes the CarboOcean coordinator, “we will probably have to revise present greenhouse gas emission scenarios downwards, meaning communities will have to reduce their energy consumption, the major contributor to atmospheric CO2.”

Matthieu Lethé

  1. On the scale of acidity measurements, the most acid materials have a pH of 0, the more alkaline substances having a pH of 14 and pure water, which is neutral, a pH of 7