Measuring exchange between the atmosphere and ocean
For dependable climate predictions it is extremely important to determine exactly how much CO2 is absorbed by the ocean sink. Researchers have therefore developed a variety of independent methods to quantify the present role of the ocean in the anthropogenically impacted carbon cycle. These have greatly contributed to the present-day understanding of the interrelationships. Two procedures in particular have played an important role:
- The first method (atmosphere-ocean flux) is based on the measurement of CO2 partial-pressure differences between the ocean surface and the atmosphere. Partial pressure is the amount of pressure that a particular gas such as CO2 within a gas mixture (the atmosphere) contributes to the total pressure. Partial pressure is thus also one possibility for quantitatively describing the composition of the atmosphere. If more of this gas is present, its partial pressure is higher. If two bodies, such as the atmosphere and the near-surface layers of the ocean, are in contact with each other, then a gas exchange between them can occur. In the case of a partial-pressure difference between the two media, there is a net exchange of CO2. The gas flows from the body with the higher partial pressure into that of lower pressure. This net gas exchange can be calculated when the global distribution of the CO2 partial-pressure difference is known. Considering the size of the world ocean this requires an enormous measurement effort. The worldwide fleet of research vessels is not nearly large enough for this task. A significant number of merchant vessels were therefore outfitted with measurement instruments that automatically carry out CO2 measurements and store the data during their voyages or even transmit them daily via satellite. This “Voluntary Observing Ship” project (VOS) has been developed and expanded over the last two decades and employs dozens of ships worldwide. It is fundamentally very difficult to adequately record the CO2 exchange in the world ocean, because it is constantly changing through space and time. Thanks to the existing VOS network, however, it has been possible to obtain measurements to provide an initial important basis. The database, covering over three decades, is sufficient to calculate the average annual gas exchange over the total surface of the oceans with some confidence. It is given as average annual CO2 flux density (expressed in mol C/m2/year), that is the net flux of CO2 per square meter of ocean surface per year, which can be integrated to yield the total annual CO2 uptake of the world ocean.
- Our present picture is based on around three million measurements that were collected and calculated for the CO2 net flux. The data were recorded between 1970 and 2007, and most of the values from the past decade were obtained through the VOS programme. Regions that are important for world climate such as the subpolar North Atlantic and the subpolar Pacific have been reasonably well covered. For other ocean regions, on the other hand, there are still only limited numbers of measurements. For these undersampled regions, the database is presently insufficient for a precise calculation. Still, scientists have been able to use the available data to fairly well quantify the oceanic CO2 sink. For the reference year 2000 the sink accounts for 1.4 Gt C.
- This value represents the net balance of the natural carbon flux out of the ocean into the atmosphere and, conversely, the transport of anthropogenic carbon from the atmosphere into the ocean. Now, as before, the annual natural pre-industrial amount of 0.6 Gt C is flowing out of the ocean. Conversely, around 2.0 Gt C of anthropogenic carbon is entering the ocean every year, leading to the observed balance uptake of 1.4 Gt C per year. Because of the still rather limited amount of data, this method has had to be restricted so far to the climatological CO2 flux, i.e., a long-term average over the entire observation period. Only now are studies beginning to approach the possibility of looking at interannual variability for this CO2 sink in especially well-covered regions. The North Atlantic is a first prominent example. Surprisingly, the data shows significant variations between individual years. Presumably, this is attributable to natural climate cycles such as the North Atlantic Oscillation, which have a considerable impact on the natural carbon cycle. Understanding such natural variability of the ocean is a prerequisite for reliable projections of future development and change of the oceanic sink for CO2.
- 2.4 > The world ocean takes up anthropogenic CO2 everywhere across its surface. The transport into the interior ocean, however, primarily takes place in the North Atlantic and in a belt between 30 and 50 degrees south latitude. The values indicate the total uptake from the beginning of the industrial revolution until the year 1994.
- The second method attempts, with the application of rather elaborate geochemical or statistical procedures, to calculate how much of the CO2 in the ocean is derived from natural sources and how much is from anthropogenic sources, although from a chemical aspect the two are basically identical, and cannot be clearly distinguished. Actually, several procedures are available today that allow this difficult differentiation, and they generally provide very consistent results. These methods differ, however, in detail, depending on the assumptions and approximations associated with a particular method. The most profound basis for estimating anthropogenic CO2 in the ocean is the global hydrographic GLODAP dataset (Global Ocean Data Analysis Project), which was obtained from 1990 to 1998 through large international research projects. This dataset:
All of these data were corrected and subjected to multilevel quality control measures in an elaborate process. This provided for the greatest possible consistency and comparability of data from a number of different laboratories. Even today, the GLODAP dataset still provides the most exact and comprehensive view of the marine carbon cycle. For the first time, based on this dataset, reliable estimateshave been made of how much anthropogenic carbon dioxide has been taken up from the atmosphere by the ocean sink. From the beginning of industrialization to the year 1994, the oceanic uptake of anthropogenic carbon dioxide amounts to 118 ± 19 Gt C. The results indicate that anthropogenic CO2, which is taken up everywhere across the ocean’s surface flows into the ocean’s interior from the atmosphere primarily in two regions. One of these is the subpolar North Atlantic, where the CO2 submerges with deep-water formation to the ocean depths. The other area of CO2 flux into the ocean is a belt between around 30 and 50 degrees of southern latitude. Here the surface water sinks because of the formation of water that spreads to intermediate depths in the ocean.
The CO2 input derived from the GLODAP dataset to some extent represents a snapshot of a long-term transition to a new equilibrium. Although the anthropogenic carbon dioxide continuously enters the ocean from the surface, the gas has not penetrated the entire ocean by any means. The GLODAP data show that the world ocean has so far only absorbed around 40 per cent of the carbon dioxide discharged by humans into the atmosphere between 1800 and 1995. The maximum capacity of the world ocean of more than 80 per cent is therefore far from being achieved.
- includes quality-controlled data on a suite of carbon and other relevant parameters;
- is based on analyses of more than 300,000 water samples;
- contains data that were collected on nearly 100 expeditions and almost 10,000 hydrographic stations in the ocean.
- 2.5 > In order to determine the effect of increasing atmospheric CO2 concentrations on the ocean, an international research team enriched seawater with CO2 in floating tanks off Spitsbergen, and studied the effects on organisms.
How climate change impacts the marine carbon cycle
The natural carbon cycle transports many billions of tons of carbon annually. In a physical sense, the carbon is spatially transported by ocean currents. Chemically, it changes from one state to another, for example, from inorganic to organic chemical compounds or vice versa. The foundation for this continuous transport and conversion is made up of a great number of biological, chemical and physical processes that constitute what is also known as carbon pumps. These processes are driven by climatic factors, or at least strongly influenced by them. One example is the metabolism of living organisms, which is stimulated by rising ambient temperatures. This temperature effect, however, is presumably less significant for the biomass producers (mostly single-celled algae) than for the biomass consumers (primarily the bacteria), which could cause a shift in the local organic carbon balance in some regions. Because many climatic interactions are still not well understood, it is difficult to predict how the carbon cycle and the carbon pumps will react to climate change. The first trends indicating change that have been detected throughout the world ocean are those of water temperature and salinity. In addition, a general decrease in the oxygen content of seawater has been observed, which can be attributed to biological and physical causes such as changes in current flow and higher temperatures. It is also possible that changes in the production and breakdown of biomass in the ocean play a role here.
- Changes in the carbon cycle are also becoming apparent in another way: The increasing accumulation of carbon dioxide in the sea leads to acidification of the oceans or, in chemical terms, a decline in the pH value. This could have a detrimental impact on marine organisms and ecosystems. Carbonate-secreting organisms are particularly susceptible to this because an acidifying environment is less favorable for carbonate production. Laboratory experiments have shown that acidification has a negative effect on the growth of corals and other organisms. The topic of ocean acidification is presently being studied in large research programmes worldwide. Conclusive results relating to the feedback effects between climate and acidification are thus not yet available. This is also the case for the impact of ocean warming. There are many indications for significant feedback effects here, but too little solid knowledge to draw any robust quantitative conclusions.
We will have to carry out focussed scientific studies to see what impact global change will have on the natural carbon cycle in the ocean. It would be naïve to assume that this is insignificant and irrelevant for the futureclimate of our planet. To the contrary, our limited knowledge of the relationships should motivate us to study the ocean even more intensely and to develop new methods of observation.