The oceans – the largest CO2-reservoir
The mutability of carbon
Carbon is the element of life. The human body structure is based on it, and other animal and plant biomass such as leaves and wood consist predominantly of carbon (C). Plants on land and algae in the ocean assimilate it in the form of carbon dioxide (CO2) from the atmosphere or water, and transform it through photosynthesis into energy-rich molecules such as sugars and starches. Carbon constantly changes its state through the metabolism of organisms and by natural chemical processes. Carbon can be stored in and exchanges between particulate and dissolved inorganic and organic forms and exchanged with the the atmosphere as CO2. The oceans store much more carbon than the atmosphere and the terrestrial biosphere (plants and animals). Even more carbon, however, is stored in the lithosphere, i.e. the rocks on the planet, including limestones (calcium carbonate, CaCO3).
- The three most important repositories within the context of anthropogenic climate change – atmosphere, terrestrial biosphere and ocean – are constantly exchanging carbon. This process can occur over time spans of up to centuries, which at first glance appears quite slow. But considering that carbon remains bound up in the rocks of the Earth’s crust for millions of years, then the exchange between the atmosphere, terrestrial biosphere and ocean carbon reservoirs could actually be described as relatively rapid. Today scientists can estimate fairly accurately how much carbon is stored in the individual reservoirs. The ocean, with around 38,000 gigatons (Gt) of carbon (1 gigaton = 1 billion tons), contains 16 times as much carbon as the terrestrial biosphere, that is all plant and the underlying soils on our planet, and around 60 times as much as the pre-industrial atmosphere, i.e., at a time before people began to drastically alter the atmospheric CO2 content by the increased burning of coal, oil and gas. At that time the carbon content of the atmosphere was only around 600 gigatons of carbon. The ocean is therefore the greatest of the carbon reservoirs, and essentially determines the atmospheric CO2 content. The carbon, however, requires centuries to penetrate into the deep ocean, because the mixing of the oceans is a rather slow (Chapter 1). Consequently, changes in atmospheric carbon content that are induced by the oceans also occur over a time frame of centuries. In geological time that is quite fast, but from a human perspective it is too slow to extensively buffer climate change.
- With respect to climate change, the greenhouse gas CO2 is of primary interest in the global carbon cycle. Today, we know that the CO2 concentration in the atmosphere changed only slightly during the 12,000 years between the last ice age and the onset of the industrial revolution at the beginning of the 19th century. This relatively stable CO2 concentration suggests that the pre-industrial carbon cycle was largely in equilibrium with the atmosphere. It is assumed that, in this pre-industrial equilibrium state, the ocean released around 0.6 gigatons of carbon per year to the atmosphere. This is a result of the input of carbon from land plants carried by rivers to the ocean and, after decomposition by bacteria, released into the atmosphere as CO2, as well as from inorganic carbon from the weathering of continental rocks such as limestones. This transport presumably still occurs today at rates essentially unchanged. Since the beginning of the industrial age, increasing amounts of additional carbon have entered the atmosphere annually in the form of carbon dioxide.
- 2.1 > The carbon cycle in the 1990s with the sizes of the various reservoirs (in gigatons of carbon, Gt C), as well as the annual fluxes between these. Pre-industrial natural fluxes are shown in black, anthropogenic changes in red. The loss of 140 Gt C in the terrestrial biosphere reflects the cumulative CO2 emissions from land-use change (primarily slash and burn agriculture in the tropical rainforests), and is added to the 244 Gt C emitted by the burning of fossil fuels. The terrestrial sink for anthropogenic CO2 of 101 Gt C is not directly verifiable, but is derived from the difference between cumulative emissions (244 + 140 = 384 Gt C) and the combination of atmospheric increase (165 Gt C) and oceanic sinks (100 + 18 = 118 Gt C).
- The causes for this, in addition to the burning of fossil fuels (about 6.4 Gt C per year in the 1990s and more than 8 Gt C since 2006), include changes in land-use practices such as intensive slash and burn agriculture in the tropical rainforests (1.6 Gt C annually). From the early 19th to the end of the 20th century, humankind released around 400 Gt C in the form of carbon dioxide. This has created a serious imbalance in today’s carbon cycle. The additional input of carbon produces offsets between the carbon reservoirs, which lead to differences in the flux between reservoirs when compared to preindustrial times. In addition to the atmosphere, the oceans and presumably also land plants permanently absorb a portion of this anthropogenic CO2 (produced by human activity).
The ocean as a sink for anthropogenic CO2
As soon as CO2 migrates from the atmosphere into the water, it can react chemically with water molecules to form carbonic acid, which causes a shift in the concentrations of the hydrogen carbonate (HCO3-) and carbonate (CO32-) ions, which are derived from the carbonic acid. Because carbon dioxide is thus immediately processed in the sea, the CO2 capacity of the oceans is ten times higher than that of freshwater, and they therefore can absorb large quantities of it. Scientists refer to this kind of assimilation of CO2 as a sink. The ocean absorbs human-made atmospheric CO2, and this special property of seawater is primarily attributable to carbonation, which, at 10 per cent, represents a significant proportion of the dissolved inorganic carbon in the ocean. In the ocean, the carbon dissolved in the form of CO2, bicarbonate and carbonate is referred to as inorganic carbon. When a new carbon equilibrium between the atmosphere and the world ocean is re-established in the future, then the oceanic reservoir will have assimilated around 80 per cent of the anthropogenic CO2 from the atmosphere, primarily due to the reaction with carbonate. The buffering effect of deep-sea calcium carbonate sediments is also important. These ancient carbonates neutralize large amounts of CO2 by reacting with it, and dissolving to some extent. Thanks to these processes, the oceans could ultimately absorb around 95 per cent of the anthropogenic emissions. Because of the slow mixing of the ocean, however, it would take centuries before equilibrium is established. The very gradual buffering of CO2 by the reaction with carbonate sediments might even take millennia. For today’s situation this means that a marked carbon disequilibrium between the ocean and atmosphere will continue to exist for the decades and centuries to come. The world ocean cannot absorb the greenhouse gas as rapidly as it is emitted into the atmosphere by humans. The absorptive capacity of the oceans through chemical processes in the water is directly dependent on the rate of mixing in the world ocean. The current oceanic uptake of CO2 thus lags significantly behind its chemical capacity as the present-day CO2 emissions occur much faster than they can be processed by the ocean. >