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The Ocean: a Carbon Pump

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The Ocean: a Carbon Pump www.ocean-climate.org Laurent Bopp*, The Ocean: (LSCE, Gif-sur-Yvette) Chris Bowler*, (ENS, Paris) a Carbon Lionel Guidi, (CNRS UPMC, Villefranche-sur-Mer) Pump Éric Karsenti, (EMBL) Colomban de Vargas (CNRS UPMC, Roscoff) *lead authors The ocean contains 50 times more carbon than the atmosphere and is exchanging large amounts of CO2 with the atmosphere every year. In the past decades, the ocean has slowed down the rate of climate change by absorbing about 30% of human emissions. Whereas this absorption of anthropogenic CO2 is today the result of physical-chemical processes, ma- rine biology is playing an important role in the ocean carbon cycle 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 feedbacks between climate, the ocean, and its ecosystems need to be better understood in order to predict the co-evolution of atmospheric CO2 and climate change more reliably and also to understand the characteristics of a future ocean. A MAJOR ROLE FOR THE OCEAN IN than any other that has happened during the THE EVOLUTION OF ATMOSPHERIC past 65 million years (Portner et al., 2014; Rhein et al., 2014). CO2 Since the beginning of the industrial era, the ocean The cycling of carbon involves a wide range of has played a key role in the evolution of atmos- physico-chemical and biological processes contri- pheric CO2 by absorbing a significant fraction buting to a series of interconnected carbon re- of CO2 emitted into the atmosphere by human servoirs in the Earth System. A schematic diagram activities, deforestation and burning of fossil fuels. of the global carbon cycle showing the relative During the past decade (2004-2013), the global importance of each of these processes is shown in ocean has absorbed 2.6 billion tonnes of carbon Figure 1. The global cycle was roughly balanced per year, representing nearly 30% of anthropogenic before the industrial era. During the past 200 years, emissions over this period. Since 1870, the amount atmospheric CO2 has increased from less than of carbon absorbed by the ocean has reached 0.03% to more than 0.04%, as a result of fossil fuel 150 billion tonnes – also representing 30% of anthro- burning, cement production, deforestation and pogenic emissions over this period. By absorbing other changes in land use. It is considered that this greenhouse gas, the ocean thus contributes such a rapid change is at least ten times faster to slow down human-induced climate change. 12 www.ocean-climate.org A NATURAL OCEAN CARBON organic carbon pool (~3PgC), but they are ca- CYCLE INVOLVING PHYSICO- pable of generating large amounts of dissolved organic carbon (DOC: ~700PgC) to sustain the CHEMICAL AND BIOLOGICAL food chains because their turnover is very rapid, PROCESSES from a few days to several weeks. A fraction of produced organic material exits the surface layer as sinking particles, thus trans- Anthropogenic carbon absorbed by the ocean ferring the surface carbon towards the deep feeds a considerable natural carbon reservoir. layers of the ocean (Figure). Before being se- The ocean contains about 40,000 billion tonnes questered to the deep the atmospheric carbon of carbon (40,000PgC), mainly in the form of fixed by photosynthetic organisms undergoes a inorganic carbon dissolved in seawater. This series of transformations: phytoplankton can be amount represents 50 times the size of the at- directly consumed by zooplankton, or indirectly mospheric reservoir. Each year, the ocean natu- by heterotrophic bacteria, which will in turn be rally exchanges with the atmosphere almost a eaten by larger organisms. During this process, hundred billion tonnes of carbon as CO2. a fraction of the total carbon biomass (average value of 10%) ends up as detrital matter, fecal In the ocean, this carbon, which prevails essen- pellets or dead cells which compose the stock of - tially in the form of bicarbonate ions (HCO3 ), marine particles. In turn, a fraction of these par- is not evenly distributed, as dissolved carbon ticles (in suspension or sinking) also undergoes concentrations are higher at depth than at the a series of transformations before reaching the surface. The spatial distribution of carbon with base of the mesopelagic layer (typically 1000m depth controls atmospheric CO2 levels, as only depth), thus sequestering atmospheric CO2 for the inorganic carbon from the sea surface is in thousands of years.2 It is generally believed that contact with the atmosphere and contributes 0.1 to 1% of the carbon-containing material at to the exchange of CO2 between the atmos- the surface finally reaches the base of the me- phere and the ocean. This vertical gradient of sopelagic zone, then the sediment where it can carbon can be explained by both physico-che- turn into fossil fuel deposits. The remaining orga- mical and biological processes. nic matter is remineralized through respiration, and CO2 returns to the atmosphere. Each year, • Biological Processes nearly 10 billion tonnes of carbon are exported Phytoplankton living in the sunlit layer of the from the surface layer and are responsible for ocean use light energy to perform photosyn- most of the carbon vertical gradient. All of these thesis. They take up nutrients as well as dissolved inorganic car- atmospheric CO bon to produce organic matter. atmospheric 2 CO CO The production of these car- 2 2 carbon dissolved bon-based materials supported at the surface by solar energy is called primary deep convection production. It represents the base euphotic zone phytoplankton upwelling carbon of the trophic chains from which zooplankton upwelling 100m dissolved in deep waters other non- photosynthetic or- sinking organic ganisms can feed on. Photosyn- detritus thetic activity is therefore an ef- ficient mechanism for extracting 400m remineralisation dissolved carbon of organic detritus CO2 from the atmosphere and transferring the carbon into living organisms. Surprisingly, the orga- nisms that contribute to primary Natural carbon cycle and representation of biological and physical production represent only a small pumps (Bopp et al. 2002). 13 www.ocean-climate.org processes that contribute to the governing role from further contact with the atmosphere. This of marine biology on the carbon cycle in the process that contributes to the vertical gradient ocean are part of the so called biological car- of ocean carbon is known as the physical pump bon pump (Figure). or solubility pump (Figure). Although only a small fraction (~ 0.2PgCyr-1) of Despite the fact that biological processes are the carbon exported by biological processes responsible for the majority of the vertical gra- from the surface reaches the sea floor, the fact dient of natural carbon in the ocean, the phy- that it can be stored in sediments for millennia sico-chemical processes can nevertheless ex- and longer (Denman et al., 2007; Ciais and al., plain the anthropogenic carbon sink observed 2014) means that this biological pump is the today. Indeed, excess CO2 in the atmosphere most important biological mechanism in the will lead to a net carbon flux to the ocean due Earth System allowing CO2 to be removed from to the disproportion induced between atmos- the carbon cycle for very long periods of time. pheric and oceanic CO2 concentrations. Sub- sequently, once the anthropogenic CO2 enters Over geological time-scales, the biological car- surface waters, it is transported by ocean cur- bon pump has formed oil deposits that today rents and progressively mixed with the sub-sur- fuel our economy. In addition, biochemical se- face waters. dimentary rocks such as limestone are derived principally from calcifying corals, molluscs, and IS THE OCEANIC CARBON SINK foraminifera, while the considerable reserves of deep sea methane 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 of magnitude is now probably about a million studies based on theoretical considerations, in years of trapped carbon burned by humankind situ observations, controlled laboratory expe- each year), it is easier to understand the rapi- riments, or supported by models, suggest that dity at which present climate change is taking several processes may lessen or slow-down this place. Consequently, there is a dramatic diffe- natural carbon sink. rence between the rate of CO2 sequestration by photosynthesis and rate of CO2 discharge into The first series of processes is related to the che- the atmosphere. The anthropogenic emissions mistry of carbonates (exchanges between CO2, 3- 2- will therefore need to be redistributed by the HCO and CO3 ) and can eventually lead to a global carbon cycle until a new steady state is saturation of the oceanic carbon sink. Indeed, reached. the dissolution of anthropogenic carbon dioxide decreases the ocean carbonate ion content and therefore the buffer effect of the ocean, • Physico-Chemical Processes which in turn increases the proportion of CO2 in A second series of processes, comprising phy- comparison to the other forms of dissolved inor- sico-chemical activities, also contributes to the ganic carbon species and thus may reduce the increasing carbon distribution with depth. The efficiency of the natural carbon sink. This phe- cooling of surface waters at high latitudes fa- nomenon occurs in parallel with the process of vours their ability to dissolve atmospheric CO2 ocean acidification, and could potentially have (mainly by increasing the solubility of the gas) serious impacts on life in the ocean.
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