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Home , Biological pump, Carbon sequestration, Carbon sink, Solubility pump, Total carbon

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Laurent Bopp*, The Ocean: Chris Bowler*, Lionel Guidi, Éric Karsenti, a Colomban de Vargas

Pump *lead authors

The ocean contains 50 times more carbon than the and exchanges large amounts of CO2 with the atmosphere every year. In the past decades, the ocean has slowed down the rate of by absorbing about 30% of human emissions. While this absorption of anthropogenic CO2 is today the result of physical-chemical processes, marine plays a key role in the ocean by sequestering carbon in the deep ocean. Changes in any of these physical, chemical and biological processes may result in climate feedbacks that either increase or decrease the rate of climate change, although knowledge of such interconnections is today still limited. The feedback between climate, the ocean, and its require a better understanding in order to predict the co- evolution of atmospheric CO2 and climate change more reliably as well as to understand the characteristics of a future ocean.

A MAJOR ROLE FOR THE OCEAN IN of atmospheric CO2 by absorbing a significant fraction of CO emitted into the atmosphere by THE EVOLUTION OF ATMOSPHERIC 2 human activities, and burning of fuels. During the past decade (2004-2013), CO2 the global ocean has absorbed 2.6 billion tonnes The carbon cycle involves a wide range of physico- of carbon per year, representing nearly 30% of chemical and biological processes contributing anthropogenic emissions over this period. Since to a series of interconnected carbon reservoirs 1870, the amount of carbon absorbed by the in the Earth System. A schematic diagram of ocean has reached 150 billion tonnes – also the global carbon cycle highlights the relative representing 30% of anthropogenic emissions importance of each of these processes as shown over this period. By absorbing this greenhouse in Figure 1. The global cycle was roughly balanced gas, the ocean thus contributes to slowing down before the industrial era. During the past 200 years, human-induced climate change. atmospheric CO2 has increased from under 0.03% to over 0.04%, as a result of burning, A NATURAL OCEAN CARBON production, deforestation and other changes in . It is considered that such CYCLE INVOLVING PHYSICO- a rapid change is at least ten times faster than CHEMICAL AND BIOLOGICAL any other that has happened during the past 65 million years (Portner et al., 2014; Rhein et al., 2014). PROCESSES

Since the beginning of the industrial era, the Anthropogenic carbon absorbed by the ocean ocean has played a key role in the evolution feeds a considerable natural carbon reservoir.

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The ocean contains about 40,000 billion tonnes atmospheric carbon fixed by photosynthetic of carbon (40,000 PgC), mainly in the form of undergoes a series of transformations: inorganic carbon dissolved in . This can be directly consumed by amount represents 50 times the size of the , or indirectly by heterotrophic atmospheric reservoir. Each year, the ocean , which will in turn be eaten by larger naturally exchanges with the atmosphere almost organisms. During this process, a fraction of a hundred billion tonnes of carbon as CO2. the (average value of 10%) ends up as detrital matter, fecal pellets or This carbon, represented essentially in the form of dead cells which compose the stock of marine ions (HCO3), is not evenly distributed particles. In turn, a fraction of these particles (in in the ocean, as dissolved carbon concentrations suspension or sinking) also undergoes a series of are higher at depth than at the surface. The transformations before reaching the of the spatial distribution of carbon with depth controls mesopelagic layer (typically 1000m depth), thus atmospheric CO2 levels, as only the inorganic sequestering atmospheric CO2 for thousands of carbon from the surface is in contact with the years. It is generally believed that 0.1 to 1% of atmosphere and contributes to the exchange of the carbon-containing material at the surface

CO2 between the atmosphere and the ocean. This finally reaches the base of the mesopelagic vertical gradient of carbon can be explained by zone, then the sediment where it can turn into both physico-chemical and biological processes. fossil fuel deposits. The remaining

is remineralized through respiration, and CO2 • Biological Processes returns to the atmosphere. Each year, nearly 10 Phytoplankton living in the sunlit layer of the ocean billion tonnes of carbon are exported from the use light energy to perform . They surface layer and are responsible for most of the take up as well as dissolved inorganic carbon vertical gradient. All of these processes carbon to produce organic matter. The production that contribute to the governing role of marine of these carbon-based materials supported by biology on the carbon cycle in the ocean are solar energy is called . It part of the so called biological carbon pump represents the base of the trophic chains from (Figure 1). which other non- photosynthetic organisms can feed on. Photosynthetic activity is therefore an Although only a small fraction (~ 0.2PgCyr-1) of efficient mechanism for extracting CO2 from the the carbon exported by biological processes atmosphere and transferring the carbon into from the surface reaches the sea floor, the fact living organisms. Surprisingly, the organisms that that it can be stored in sediments for millennia contribute to primary production represent only a small organic atmospheric CO carbon pool (~3PgC), but they atmospheric 2 CO CO are capable of generating large 2 2 carbon dissolved amounts of dissolved organic at the surface carbon (DOC: ~700PgC) to sustain deep the food chains because their euphotic zone phytoplankton carbon turnover is very rapid, from a few zooplankton upwelling 100m dissolved in deep waters days to several weeks. sinking organic A fraction of produced organic material exits the surface 400m dissolved carbon of organic layer as sinking particles, thus detritus transferring the surface carbon towards the deep layers of the ocean (Figure 1). Before being Fig.1— Natural carbon cycle and representation of biological and sequestered to the deep the physical pumps (Bopp et al. 2002).

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and longer (Denman et al., 2007; Ciais et al., explain the anthropogenic observed

2014) means that this is the today. In fact, excess CO2 in the atmosphere most important biological mechanism in the will lead to a net carbon flux to the ocean

Earth System allowing CO2 to be removed from due to the disproportion induced between the carbon cycle for very long periods of time. atmospheric and oceanic CO2 concentrations.

Subsequently, once the anthropogenic CO2 Over geological time-scales, the biological car- enters surface waters, it is transported by ocean bon pump has formed oil deposits that today currents and progressively mixed with the sub- fuel our economy. In addition, biochemical se- surface waters. dimentary rocks such as are derived principally from calcifying corals, molluscs, and IS THE OCEANIC CARBON SINK , while the considerable reserves of deep sea hydrates (or clathrates) are GOING TO SATURATE? similarly the result of hundreds of millions of years of activity of methanogenic microbial consortia. To date, and since the beginning of the indus- Considering that, each day, large amounts of trial era, the ocean has continuously absorbed

CO2 that have been trapped for millions of years a relatively constant part of the amount of CO2 are discharged into the atmosphere (the order emitted by human activities. However, many stu- of magnitude is now probably about a million dies based on theoretical considerations, in situ years of trapped carbon burned by humankind observations, controlled laboratory experiments, each year), it is easier to understand the rapi- or supported by models, suggest that several dity at which present climate change is taking processes may lessen or slow-down this natural place. Consequently, there is a dramatic diffe- carbon sink. rence between the rate of CO2 sequestration by photosynthesis and the rate of CO2 discharge The first series of processes is related to the che- into the atmosphere. The anthropogenic emis- mistry of (exchanges between CO2, 2- sions will therefore need to be redistributed by and CO3 ) and can eventually lead to a satu- the global carbon cycle until a new steady ration of the oceanic carbon sink. Indeed, the state is reached. dissolution of anthropogenic de- creases the ocean ion content and therefore the buffer effect of the ocean, which

• Physico-Chemical Processes in turn increases the proportion of CO2 in com- A second series of processes, comprising parison to the other forms of dissolved inorganic physico-chemical activities, also contributes to carbon species and thus may reduce the effi- the increasing carbon distribution with depth. ciency of the natural carbon sink. This phenome- The cooling of surface waters at high latitudes non occurs in parallel with the process of ocean favours their ability to dissolve atmospheric CO2 acidification, and could potentially have serious (mainly by increasing the of the gas) impacts on life in the ocean. as well as increasing their density. These heavy surface waters plunge down to great depths, The second series of processes is related to the in this way exporting the CO2 and preventing it feedback between climate and the carbon cy- from further contact with the atmosphere. This cle. This concerns the feedback between anthro- process that contributes to the vertical gradient pogenic climate change and different carbon of ocean carbon is known as the physical pump absorption phenomena. As mentioned earlier, or (Figure). climate change leads to modifications in water temperature, ocean currents, and production Despite the fact that biological processes of organic matter in the ocean. Should these are responsible for the majority of the vertical changes boost the carbon sink, they could curb gradient of natural carbon in the ocean, the climate change and induce negative feedback. physico-chemical processes can nevertheless On the contrary, in the event of a weakening of

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the carbon sink, the changes could lead to a rial by phytoplankton is going to be affected by positive feedback that could in turn accelerate changes in environmental conditions of surface the phenomenon. water: for example rising water temperature, mel- ting of sea ice and changes in dissolved Once more, different processes are involved. availability (, ). For example, the increase in the temperature of the ocean weakens the ocean carbon sink. An Modelling approaches predict an overall reduc- increase by 2 or 3°C in tion in global mean NPP as a result of climate decreases the solubility of CO2 by a few percent, change, though with significant latitudinal varia- and thus the capacity of the ocean to absorb car- tions. One of the factors leading to this reduction bon dioxide. Another effect could accentuate is the predicted expansion of oligotrophic gyres this saturation of the carbon sink: in response to as nutrient availability decreases with the inten- rising temperatures, climate models predict an sification of . Predictions indicate increase in vertical stratification of the ocean. increasing NPP at high latitudes (because the In other words, vertical mixing, which tends to amount of available sunlight should increase as homogenize the surface waters with the deep the amount of water covered by ice decreases). water, would diminish and the resulting stratifi- However this would be counterbalanced by a cation would reduce the present penetration of decrease of NPP in temperate and tropical lati- anthropogenic CO2 towards the ocean depths. tudes (because of reduced nutrient supply). The types of species that would dominate The future of the biological pump is difficult to the in altered conditions should also predict. Even a qualitative estimate of the effect be estimated, as the composition of plankton can of changes in marine ecosystems on the ocean significantly affect the intensity of CO2 absorption. carbon sink remains highly speculative. More The role of certain phytoplankton populations, specifically, because the activity of the- biolo such as , can be particularly significant. gical pump is likely to be strongly regulated by They are characterised by relatively large cell net primary production (NPP), it is important to sizes (tens to hundreds of micrometers), which consider the effects of climate change on pho- allows them to sink rapidly. They are therefore tosynthetic activity. On land, as the CO2 supply is responsible for the export of a large fraction of generally limiting for photosynthesis, the increase carbon to the deep ocean. Nonetheless, diatoms in anthropogenic CO2 tends to stimulate plant cannot thrive in nutrient depleted conditions. In growth (known as the carbon dioxide fertiliza- this case they could be replaced by other types tion effect). This does not appear to be the case of smaller (<10 microns) phytoplankton cells that in marine systems because Dissolved Inorganic are better adapted to poor nutrient conditions. Carbon (DIC) is not limiting for by Although such cells are abundant in the ocean, photosynthesis. However, photosynthesis is also due to their small size they are principally recy- strongly affected by temperature, and the up- cled within the surface layer, and thus have a per ocean has significantly warmed during the very minor role in carbon export to the deep. last 150 years. In addition to temperature, light, A decrease in the /small cell community inorganic nutrients, and the density-dependent ratio could thus greatly disrupt the intensity of the stability of the surface (González- biological pump, especially in the polar regions. Taboada and Anadón, 2012; Portner et al., 2014) are all likely to affect photosynthetic activity, as Despite these multiple levels of uncertainty - the are , pH, and salinity. Environmental va- most important being the biological response riability and the displacement of organisms by to climate change - the different predictions ocean currents cause variability in phytoplankton produced by numerical models that couple productivity, competitiveness, and natural selec- the and the carbon cycle all tion, which are also likely to result in changes in point to a declining ocean carbon sink due to .It is therefore crucial to global warming. Even though this ocean sink is estimate how the production of organic mate- unlikely to become a source there is no doubt

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that a decrease will affect the evolution of the in a temperature drop due to the enhanced cloud

CO2 in the atmosphere and, ultimately, climate cover. This is a self-regulating negative feedback change itself. By 2100, the feedback between loop. It is an example of regulation that allowed the climate and the carbon cycle (including Lovelock to build the Gaia theory, stipulating that the response of the terrestrial biosphere to several self-regulatory processes, including the sul- climate change) could even be responsible for phur cycle, allow the planet Earth to be conside- an additional increase in atmospheric CO2 of red as a living . several tens of ppm! More than 20 years later, research projects have The future evolution of the oceanic carbon sink, revealed the complexity of the sulphur cycle in as predicted by models coupling the climate the ocean, but have neither confirmed nor re- and carbon cycle at a global scale, still remains futed this hypothesis. It is not yet known how, why very uncertain. The last IPCC report points to and what species of phytoplankton can release a number of poorly constrained processes the precursory sulphur compounds for the forma- that explain the wide range of uncertainties tion of DMS. Knowledge is therefore still lacking associated with the predictions: these primarily to determine whether anthropogenic climate include biotic responses to climate change change will result in a decrease or an increase in and the changes in the biological pump (the DMS emissions from the ocean. complexity of biological processes being extremely difficult to include in climate models). MANIPULATION OF THE CARBON Other processes related to the representation of small-scale features (eddies) and to the PUMP TO OFFSET CO2-INDUCED consideration of particularly complex coastal CLIMATE CHANGE areas are also mentioned in this report.

Humankind has disrupted the steady state A ROLE IN OTHER balance of the global carbon cycle and has BIOGEOCHEMICAL CYCLES brutally contributed to the modification of the composition of Earth’s atmosphere, just as bacteria, protists and the biosphere in general Besides its role in both the carbon cycle and the have played a role in the shaping of the Earth’s evolution of atmospheric CO , it must be empha- 2 atmosphere in the past. As other events have sized that the ocean also plays a key role in other marked the history of our planet in the past, these major biogeochemical cycles, including , present changes provoked by human activities and sulphur that are likely to affect will significantly affect the Earth System. Our the biogeochemical balance of our planet. duty as inhabitants of the planet Earth is now to formulate predictions and to react in the best In the mid-1980s, several scientists including James possible way to avoid disaster. Lovelock suggested that ocean ecosystems, es- pecially phytoplankton, are able to regulate the Studies have suggested that an artificial world climate by releasing the sulphurous gas di- enhancement of the ocean carbon pump might methyl sulphide or DMS. Once in the atmosphere, improve carbon sequestration in the ocean, this gas favours the formation of tiny sulphate par- thus counterbalancing CO -induced climate ticles which play a role as condensation nuclei for 2 change. For example, primary productivity of clouds, thus contributing to an increase in cloud phytoplankton could be stimulated by adding cover. This hypothesis, which is still called the CLAW nutrients such as to surface waters where hypothesis (based on the first letter of the surname they are limiting. There is currently no consensus of each of the authors; Charlson et al., 1987), states on the efficiency of such methods, which are that the ocean ecosystem reacts to an increase limited to a few field experiments. Moreover, in temperature by increasing productivity. This in alternative geoengineering approaches focusing turn leads to increased emissions of DMS, resulting

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on solar radiation management are not capable day. This can only be done by preserving the of resolving the issue of . , their and their planktonic ecosystems. The carbon balance of the To conclude, it remains essential to protect different parts of the carbon cycle also needs to the ocean carbon pump that contributes to be better characterised by carrying out further more than half of the CO2 sequestered each fundamental research in this field.

REFERENCES

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