4   Ocean acidification

 

4.1.1   CO₂-input

The oceans hold around 38,000 gigatonnes of carbon (Gt C). They presently store about 50 times more CO2 than the atmosphere and 20 times more than the terrestrial biosphere and soils (Fig. 4.1-1). However, the ocean is not only an important CO2 reservoir, but also the most important long-term CO2 sink. Driven by the difference in the partial pressure of CO2 between the atmosphere and seawater, a portion of the anthropogenic CO2 dissolves in the surface layer of the sea and, over periods ranging from decades to centuries, is finally transported into the deep sea by ocean currents.
     There has already been a demonstrable increase in CO2 concentrations in the upper layer of the sea over recent decades (Sabine et al., 2004) that can be attributed to the proportional rise of CO2 in the atmosphere. The ocean is presently taking up 2 Gt of carbon annually, which is equivalent to about 30 per cent of the anthropogenic CO2 emissions (IPCC, 2001a). Altogether, between 1800 and 1995, the oceans have absorbed around 118 Gt C ± 19 Gt C. That figure corresponds to about 48 per cent of the cumulative CO2 emissions from fossil fuels (including cement production), or 27–34 per cent of the total anthropogenic CO2 emissions (including those from land-use changes; Sabine et al., 2004). The anthropogenic CO2 signal in the sea can be traced, on the average, to a water depth of approximately 1000 m. Due to the slow mixing of ocean layers it has not yet reached the deep sea in most parts of the ocean. In the North Atlantic, however, due to the formation of deep water there, the anthropogenic CO2 signal already extends down to 3000 m.
     In the atmosphere CO2 behaves chemically neutral, that is, it does not react with other gases, but it contributes to climate change through its strong interaction with infrared radiation. But in the ocean CO2 is chemically active. Dissolved CO2 contributes to the reduction of the pH value, or an acidification of seawater. This effect can already be measured: since the onset of industrialization the pH value of the ocean surface water has dropped by an average of about 0.11 units. This is equivalent to an increase in the concentration of hydrogen ions (H⁺ ions) by around 30 per cent. Starting from a slightly alkaline pre-industrial pH value of 8.18 (Raven et al., 2005), the acidity of the ocean has thus increased at the surface. The various IPCC emission scenarios indicate that if the atmospheric CO2 concentration reaches 650 ppm by the year 2100, a decrease in the average pH value by 0.30 units can be expected compared to pre-industrial values. With an atmospheric concentration of 970ppm, the pH value would drop by 0.46 units. But if the CO2 in the atmosphere can be limited to 450 ppm, then the pH reduction will only amount to 0.17 units (Caldeira and Wickett, 2005).

Figure 4.1-1
Overview of the global carbon cycle. Values for the carbon reservoirs are given in Gt C (numbers in bold-print). Values for the average carbon fluxes are given in Gt C per year (numbers in normal-print). Mean residence times are in parentheses. Flux into soils amounts to around 1.5 Gt C per year. DOC = dissolved organic carbon, DIC = dissolved inorganic carbon.
Sources: adapted after Schlesinger, 1997 and WBGU, 2003.
Numbers expanded and updated for ocean and fossil fuels: Sabine et al., 2003; marine sediments: Raven et al., 2005; atmosphere: NOAA-ESRL, 2006

Figure 4.1-3
Projections of different CO2 concentrations (a) and their effects on the carbonate budget of the Southern Ocean (b). The variation according to various IPCC scenarios is shown.
Source: Orr et al., 2005

4.1.3   Special role of CO₂

The acidification of the sea is an effect that can be exclusively attributed to the CO2 increase in the atmosphere. In this it is different from climate change, which is caused by the radiative effect of atmospheric CO2 increase, but also of the increase of methane, nitrous oxide and several other radiatively active gases. With respect to climate change, calculations are often made in terms of CO2 equivalents, that is, the radiative forcing attributable to the various gases is recalculated to the corresponding forcing of CO2. The argument is that for climate protection it does not make any difference whether the radiative effect is caused by CO2 or by any other emitted greenhouse gas. But this is not true for the effect of ocean acidification. To protect the oceans, reducing CO2 emissions is relevant for two reasons: to limit both global warming and ocean acidification.
     Acidification is, above all, a consequence of the rapid increase of the quantities of CO2 in the ocean. With a slow input of CO2, as has repeatedly occurred in the Earth’s history (such as the end of the last ice age when the CO2 concentration rose by 80 ppm over a period of 6,000 years), or in climate epochs with elevated CO2 concentrations (around 100–200 million years ago) the CO2 mixes down into the deep sea, where a slow dissolution of carbonate sediments counteracts the acidification. In such constellations the pH value of the sea remains almost constant (Raven et al., 2005).

4.2   Future development of the oceans as a carbon sink

As discussed in Section 4.1, the oceans are the most important net sink for CO2. Without oceanic uptake of anthropogenic CO2, the relative CO2 concentration in the atmosphere would lie more than 55 ppm above the present level (Sabine et al., 2004). The future development of the oceans as a CO2 sink will therefore determine in large part how strongly anthropogenic CO2 emissions are reflected as an increase in the atmospheric concentration of carbon dioxide. Over the long term, that is, a period of several centuries (in which mixing takes place throughout the world’s oceans), the ocean can take up about 65–80 per cent of the anthropogenic CO2, depending on the total quantity of carbon emitted. At even longer time scales this proportion increases to 85–92 per cent due to the dissolution of carbonate sediments (Caldeira, 2005). In the coming decades and centuries, however, only a portion of this great sink potential can be effective: the limiting factor is the transport of carbon taken up at the surface into the deeper ocean layers. In fact, the oceans have so far only absorbed 30 per cent of the amount of anthropogenic carbon that they could take up over a long time period at present atmospheric concentrations (Sabine et al., 2004).
     The great importance of the ocean as a sink is not applicable to the other greenhouse gases regulated by the Kyoto Protocol: the strongest sink for methane as well as for HFCs, for example, is the chemical reaction with the hydroxyl radical OH in the lower atmosphere, while N2O is destroyed primarily in the stratosphere by UV radiation from the sun. The industrial gases PFCs and SF6 do not decay until they are above the stratosphere. It is worth noting, however, that the sea is an important source of N2O, whose future development in response to climate change is unclear.
     Before industrialization the ocean was at a state of near equilibrium, and not a CO2 sink. At its surface it gave off around 0.6 Gt C annually to the atmosphere, while at the same time approximately the same amount of carbon entered the ocean from the terrestrial biosphere (and therefore ultimately from the atmosphere) in the form of organic matter flowing in from rivers (Watson and Orr, 2003). The proportion of atmospheric CO2 did not change under these conditions, remaining constant over millennia at around 280ppm. The reason for the present function of the ocean as a sink is the anthropogenic perturbation of the carbon cycle: when the CO2 concentration of the atmosphere increases, the ocean takes up CO2 until the partial pressures of the surface water and the atmosphere are in equilibrium. Since the beginning of industrialization the atmospheric CO2 concentration has risen almost exponentially. This has caused an annual increase in the CO2 uptake by the oceans since that time, in quantities almost proportional to the atmospheric CO2 concentrations, as model studies indicate (Gloor et al., 2003). For various reasons, however, this cannot be carried over into the future, which will be discussed below.
     When one compares the quantities of CO2 taken up by the ocean with anthropogenic emissions, the efficiency of the ocean sink appears to be falling already: Sabine et al. (2004), based on an analysis of observational data, show that from 1800 to 1994 the ocean absorbed 28–34 per cent of the anthropogenic emissions, while from 1980 to 1999 this value was only 26 per cent. Due to the large uncertainty in the determination of the global carbon balance, this decrease is not statistically significant, but on the basis of known geochemical processes it is also not unexpected. The more CO2 that has been taken up by the ocean, the lower the carbonate concentration in the surface layer becomes (Section 4.1.2). This decreases its capacity to take up additional CO2. Modelling studies show that the relative CO2 uptake by the ocean (that is, the proportion of anthropogenic emissions absorbed by the ocean in the course of a few decades) is reduced by this effect by several per cent when an atmospheric CO2 concentration of 450 ppm is reached. At 750 ppm of CO2 in the atmosphere the relative CO2 uptake falls by as much as 10 per cent (Le Quéré, personal communication). This geochemical effect is fully considered in models of the carbon cycle and is therefore rarely expressly discussed (Gruber et al., 2004). This effect is also active in the extreme long term, that is, time periods in which the ocean completely mixes, so that the proportion of anthropogenic CO2 emissions remaining in the atmosphere continues to increase as more CO2 has been emitted.
     Climate change resulting from greenhouse gas emissions further affects the capacity of the ocean sink: the solubility of CO2 in seawater decreases with rising temperature. Through this effect, by the end of this century the cumulative CO2 uptake could fall by 9–14 per cent of what it would be without a temperature change (Greenblatt and Sarmiento, 2004). This effect is well-understood; the uncertainty predominantly results from the uncertainty of the degree of expected temperature change.
     A further effect of climate change is an increasing ocean stratification, that is, the vertical mixing will be reduced. This has a number of complex effects. For one, the transport of carbon-enriched surface water to greater depths as well as the transport of carbon-depleted water to the surface will be weakened, resulting in an overall decrease of the sink effect of the ocean. For another, there could be changes in biological productivity through altered nutrient availability. Biological productivity is of great importance for the carbon balance of the ocean surface layer: CO2 is taken up by marine organisms through photosynthesis and incorporated into organic substance; dead organisms sink and then decay in different water depths. Part of the released nutrients and carbon return to the surface through vertical mixing, but the net export to the deep sea is considerable. Ten gigatonnes of carbon are transferred annually by this ‘biological pump’ from the ocean surface layer to the deep sea. The combined effect of increased stratification and altered biological productivity on the sink effect of the ocean is highly uncertain. Greenblatt and Sarmiento (2004) give a range of -2 per cent (decreased sink function) to +10 percent (increased sink function) for the change in cumulative CO2 uptake through this effect by the end of the century.
     Many of the effects discussed are still difficult to quantify, but it is likely that climate change will contribute to a considerable overall weakening of the efficiency of the sea as a carbon sink. According to an overview based on various modelling studies by Greenblatt and Sarmiento (2004), the cumulative CO2 uptake by the ocean could be 4–15 per cent lower by the end of the century due to the climate-related influences discussed above (temperature rise, increased stratification, and biological effects) than it would be without these. This attenuation of the CO2 uptake has to be added to the geochemical effects that already lead to a weakening of the relative sink with a similar order of magnitude.
     As already indicated, biological processes represent the greatest uncertainty in estimating the future development of the ocean sink. These biological processes include the impacts of anthropogenic interference with the atmosphere and ocean acidification on marine primary production, the biological pump and calcification (Section 4.3.5). A weakening of the ocean sink due to changes in the wind-driven rise of water at the equator (‘equatorial upwelling’) is a further aspect under debate (Winguth et al., 2005). In addition, non-linear events that are difficult to predict such as a strong decrease in oceanic convection or in the thermohaline circulation, or biological regime shifts (Section 2.2.1) could have a considerable influence.
     In summary it can be stated that, with increasing atmospheric CO2 concentrations, the proportion of anthropogenic CO2 emissions taken up by the ocean will decrease, even if the absolute rate of uptake increases (IPCC, 2001a). 

4.3   Effects of acidification on marine ecosystems

CO2 input into the sea leads to shifts in the carbonate system of the seawater and to a decrease in pH value, and thus to acidification of the ocean (Section 4.1.1; Turley et al., 2006). Without counteractive measures this change in the carbonate system could reach a state during this century that has probably not been seen for several million years (Feely et al., 2004). Humans are significantly interfering with the chemical balance of the ocean, and this will not remain without consequences for marine organisms and ecosystems.    

4.3.1   Physiological effects on marine organisms

A strong increase of CO2 concentration (hypercapnia) has many adverse physiological effects that have been investigated experimentally on various marine organisms. Numerous changes in marine organisms have been identified, for example, in the productivity of algae, metabolic rates of zooplankton and fish, oxygen supply of squid, reproduction in clams, nitrification by microorganisms, and the uptake of metals (for a survey, see Pörtner, 2005). Many of these experiments, however, were carried out with CO2 concentrations much higher than what could be expected in emission scenarios under discussion today for the time frame up to 2100. Further studies are therefore necessary in order to be able to estimate the short- and medium-term effects of acidification (Section 4.6). From today’s viewpoint it seems improbable that marine organisms will suffer from acute poisoning at expected future CO2 levels (Pörtner, 2005).
     Doubling the present CO2 concentration leads to an increase in the rate of photosynthesis in many phytoplankton species by about 10 per cent (Raven et al., 2005). However, the various groups of phytoplankton exhibit different sensitivities to increased CO2 concentrations with respect to photosynthesis, which is due to differences in carbon uptake (CO2 versus HCO₃^–) and a different saturation behaviour of the photosynthetic rates. The interactions between photosynthesis, primary production of phytoplankton, microbial respiration, and the resulting effects on the food web are, however, compounded by a number of other factors (temperature, light and nutrient supply, disparate feeding risk from zooplankton, adaptive processes, etc.). With the present state of knowledge no clear conclusions can be drawn regarding the effects of acidification on growth rates and assemblage compositions of the phytoplankton.

4.3.2   Effects on calcifying organisms

Next to photosynthesis, calcification is the most important physiological process influenced by the increase of CO2 concentration. It has far-reaching consequences for the ecological function of marine ecosystems, and can also have feedbacks on the atmospheric concentration of CO2 and thus on the climate system (Section 4.3.5).
     For their skeletons or shell structure, many marine organisms use calcium carbonate, which has to be extracted from seawater. This is only possible while the seawater is supersaturated with calcium carbonate, which is why the increasing CO2 concentration and falling pH value hampers calcification (Raven et al., 2005). This causes a weakening of the skeletal structure or – when a level below the saturation concentration is reached – even their dissolution. Calcium carbonate is employed as a construction material for organisms in different crystalline forms: aragonite and calcite are the two most important (Table 4.3-1). Organisms that use aragonite for their shells or skeletons are the first to be adversely affected by acidification, because aragonite dissolves more easily under the changing conditions due to its different crystal structure.

Organisms	Photosynethetic	Crystal form of the carbonat	Habitat
Coccolithophores	yes	Calcite	Planktonic
Macroalgae*	yes	Aragonite or Calcite	Benthic
Foraminifera	no some	Calcite Calcite	Benthic Planktonic
Corals warm-water cold-water	yes (in symbiosis) no	Aragonite Aragonite	Benthic Benthic
Pteropods	no	Aragonite	Planktonic
Non-pteropod molluscs*	no	Aragonite or Calcite	Benthic or planktonic
Echinoderms	no	Mg-calcite	Benthic
Crustaceans*	no	Calcite	Benthic or planktonic

Table 4.3-1
Groups of calcifying marine organisms. Calcium carbonate occurs in different crystal forms. Aragonite dissolves more quickly than calcite at low carbonate ion concentrations, but more slowly than magnesium-rich calcite (Mg-calcite).
* not all species of this group are calcifiers.
Source: after Raven et al., 2005

     Acidification has an impact on all marine calcifying species, such as certain plankton groups, clams, snails and corals. Echinoderms (for example, starfish and sea cucumbers) are especially threatened, because their calcite structures contain larger amounts of magnesium and therefore dissolve even more easily than aragonite under increased CO2 conditions (Shirayama and Thornton, 2005). Although corals are the most conspicuous and well-known marine calcifying organisms and are especially threatened by acidification as aragonite producers (Section 2.4), they only contribute 10 per cent of the annual global marine carbonate production of 0.64–2 Gt C (Zondervan et al., 2001). The simulations of Guinotte et al. (2003) indicate that at an atmospheric CO2 concentration of just under 520 ppm, which could already be reached by the middle of this century, almost all of today’s warm-water coral reef locations will barely still be suitable for coral growth because of insufficient aragonite saturation (Fig. 4.3-1).

Figure 4.3-1
Aragonite saturation and present occurrence of reef locations for warm-water corals (blue dots). (a) pre-industrial values (around 1870, atmospheric CO2 concentration 280ppm), (b) present (around 2005, 375 ppm CO2), (c) future (around 2065, 517 ppm CO2). The degree of aragonite saturation (Ω) indicates the relative proportion between the product of the concentrations of calcium and carbonate ions and the solubility product for aragonite. Locations with an aragonite saturation below 3.5 are only marginally suitable for reef-forming warm-water corals, below 3 they are not suitable.
Source: Steffen et al., 2004

     Around three-fourths of the global marine calcium carbonate production is carried out by planktonic organisms, primarily coccolithophores, foraminifera, and pteropods. Of these, the coccolithophores are of particular importance because these one-celled primary producers, which can create large-area plankton blooms with only a few species, can greatly contribute to the export of calcium carbonate to the deep sea and thereby play a significant role in the global carbon cycle (Riebesell et al., 2000; Zondervan et al., 2001; Section 4.3.5). In experiments with both monocultures and natural plankton communities it has been shown that the calcification by coccolithophores clearly decreases with increased atmospheric CO2 concentrations (Riebesell et al., 2000; Riebesell, 2004). Pteropods are important components of the marine food webs in polar and sub-polar latitudes where they form dense populations (up to 1000 individuals per m3) and serve as nutrition for the upper trophic layers of the food web. In these regions they are responsible for a significant portion of the export of particulate carbon to greater depths. When the carbonate saturation in seawater drops below a critical value, it is likely that these animals are no longer able to form a shell. For important parts of their habitat in the Southern Ocean an undersaturation with respect to aragonite (under assumptions of the IS92 scenario of the IPCC) is predicted beginning in 2050, so that their distribution area will be severely limited (Orr et al., 2005; Raven et al., 2005).
     There is great uncertainty about the capacity of the organisms to adapt to these changes, as too few long-term experiments have been carried out (Raven et al., 2005; Pörtner, 2005).

4.3.3   Ecosystem structure and higher trophic levels

Over the course of this century the expected pH decreases could have a considerable impact on the calcifying organisms and thus on the total marine biosphere (Orr et al., 2005). Simultaneously, considerable climate-related warming is expected. The two effects are not independent of one another: the CO2 increase, for example, could decrease the temperature tolerance of animals (Pörtner, 2005). The coral ecosystems in particular are an example of such synergistic negative effects (Section 2.4; Hoegh-Guldberg, 2005).
     Acidification impacts on the food web are also conceivable. Different responses to increased CO2 concentrations, with respect to growth rates or reproduction of an organism, could change the spatial as well as temporal distributions of the species through changes in competition (Rost and Süültemeyer, 2003). Impacts that have already been observed in primary producers include a difference in the magnitude of the CO2 fertilization effect (which, for example, favours coccolithophores over siliceous algae) and reduced calcification (which may be a disadvantage for coccolithophores: Riebesell, 2004). In long-term studies in the North Atlantic, it has been observed that changes in the phytoplankton, due to the close coupling with their predators, can be passed on first to the algae-feeding zooplankton and then further to the predatory zooplankton (Richardson and Schoeman, 2004). A change in the species composition of the phytoplankton can thus impact on the zooplankton. In polar ecosystems it is conceivable that reduced calcification by pteropods has effects on the higher levels of the food web, although this is speculative and not easily predictable (Orr et al., 2005). Conclusions about possible adaptive processes at the ecosystem-structure level are also speculative, for example, whether gaps that are created by acidification impacts can be filled by other species without significant effects upon overall productivity.

4.3.4   Effects of acidification on fisheries

Acidification of the world’s oceans could also have an impact on fisheries. Direct toxic effects of increased atmospheric CO2 concentrations on fish are not expected because the threshold of acute sensitivity of fish to CO2 is beyond the predicted concentrations (Pöörtner, 2005; Section 4.3.1). When calcification is reduced, however, this can trigger changes in the species composition of the phytoplankton, and this, in turn, can have an impact all the way to the upper layers of the food web through trophic coupling (Richardson and Schoeman, 2004; Section 4.3.3). It cannot be ruled out that this kind of change in the structure and function of the marine ecosystems can have an impact on the pelagic fisheries, but with the present state of knowledge the prognosis remains very speculative (Raven et al., 2005).
     Changes in growth and competitive conditions for the species in tropical coral reefs will probably also affect another important branch of fishery: millions of people depend on subsistence fishery on coral reefs for their protein supply (Raven, et al., 2005), and the coral reefs themselves are threatened by acidification (Section 2.4). A large-scale loss of coral habitats would doubtless have adverse effects upon this fishery, with socioeconomic consequences that are difficult to predict.

4.3.5   Feedback of changes in calcification on the carbon cycle

Overall, the ecological balances in the sea are shifting to the detriment of calcifying organisms, and this may affect even the global biogeochemical cycles through changes in species compositions in marine phytoplankton. The consequences of changing rates of plankton calcification described here represent only a small sample of all the interactions between the climate system and the ocean, which are reviewed in Section 4.2.
     The annual primary production in the ocean is approximately 50 Gt C, of which approximately. 10 Gt is exported to the deep sea by the biological pump. For this important process in the global carbon cycle, which contributes to the sink function of the ocean, it makes a great difference whether the production is by calcifying species like coccolithophores or by non-calcifying species, for example, siliceous algae.
     Calcification by marine organisms always involves CO2 production:

Ca²⁺ + 2 HCO₃^--> CaCO₃ + CO₂ + H₂O

This carbonate ‘counter-pump’ becomes stronger with increasing atmospheric CO2 concentration as a consequence of the altered carbonate buffer capacity. Assuming constant calcification, this would cause a future weakening of the sink effect of the sea. But if biogenic carbonate formation is reduced as a result of a pH decrease, then this effect can be overcompensated so that the sink effect may even be strengthened. This would, however, only have a minor impact on the CO2 uptake by the ocean (Zondervan et al., 2001). A number of other effects further complicate this picture (Riebesell, 2004): reduced calcification could also reduce the density and thus the sinking rates of particles to the deeper water layers, slowing the carbon export by the biological pump. On the other hand, there is a possible acceleration of the sinking rates caused by the increased formation of extracellular polysaccharides (Engel et al., 2004). Present-day plankton blooms of coccolithophores cover large areas of the sea, up to hundreds of thousands of km2, and lighten the colour of the water because of their carbonate content. Their absence could therefore reduce the global albedo by up to 0.13 per cent, which would slightly accelerate global warming (Tyrell et al., 1999). The magnitude of some of these factors is not clear. The total effect of all of these factors on the interactions between atmospheric CO2 concentration and marine biological production cannot currently be ascertained, which presents a need for increased research efforts in this area (Raven et al., 2005; IMBER, 2005).

4.4   Guard rail: Ocean acidification

4.4.1   Proposed guard rail

To prevent undesirable or high-risk changes to the marine food web due to aragonite undersaturation (Section 4.3), the pH value of the upper ocean layer (surface layer) should not drop more than 0.2 units below the pre-industrial average value of 8.18 in any larger ocean region (nor in the global mean). A pH drop of 0.2 units would correspond to an increase in the H⁺ ion concentration of around 60 per cent compared to pre-industrial values. The decrease in pH so far of 0.11 units since industrialization corresponds to a rise of the H⁺ ion concentration of around 30 per cent. The present average pH value of the ocean surface layer is 8.07 (Raven et al., 2005). Figure 4.4-1 illustrates the WBGU acidification guard rail.

Figure 4.4-1
Variability of the average pH value of the oceans in the past and present, as well as a projection for the future for an atmospheric CO2 concentration of approx. 750 ppm. The red line indicates the WBGU guard rail.
Source: after IMBER, 2005

     It is necessary, however, to further specify the spatial and temporal averaging to which the guard rail refers, because the pH value is subject to strong natural variability. According to Haugan and Drange (1996), pH values vary up to 0.5 units worldwide, while local seasonal fluctuations can amount to around 0.1 pH units (in high-production regions even 0.2–0.3 units: Riebesell, personal communication).
     According to simulations by Caldeira and Wickett (2005), a stabilization of the atmospheric CO2 concentration of 540ppm by the year 2100 would already lead to a pH decrease of the ocean surface layer of 0.23 in the global average compared to the pre-industrial level, that is, at this CO2 concentration the acidification guard rail would already be overstepped. A stabilization at 450ppm by 2100 reduces the pH value by 0.17, and so would presumably be consistent with the acidification guard rail. It still needs to be reviewed, however, whether at this stabilization value higher pH reductions could occur locally over longer time periods, which, especially in the Southern Ocean, could lead to undersaturation of the surface layer with respect to aragonite. It should be noted that this refers to the stabilization of CO2 itself and not the stabilization level of greenhouse gases in total, which is described by the CO2 equivalent.

4.4.2   Rationale and feasibility    

The largest threat to marine organisms due to acidification is related to the solubility of calcium carbonate, which they need for the construction of their shells and skeletal structure (Section 4.3). The more easily dissolved variant of calcium carbonate is aragonite, which is used by corals and certain plankton species (Table 4.3-1). Calcifying marine organisms are important components of marine ecosystems, so their endangerment would represent a non-tolerable interference with the Earth System.
     If the concentration of carbonate ions falls below the critical value of 66µmol per kilogram, then the seawater is no longer saturated with respect to aragonite, and marine organisms can no longer build their aragonite shells. This needs to be avoided above all in the surface layer where primary production takes place. The danger of undersaturation for aragonite is especially present in the Southern Ocean. According to Orr et al. (2005), simulations where the pH decrease averages about 0.25 already show a clear reduction of the vertical extent of the saturated layer, and undersaturation in some parts of the Southern Ocean. It is the view of WBGU that such a situation should be avoided.
     The pH value is a critical variable not only for calcification, but also for many other processes in marine systems (for example, availability of nutrients). Within the past 23 million years the natural fluctuations of the average pH value between glacial and interglacial periods lay within a range of slightly more than 0.1 (Fig. 4.4-1), so that over a long time marine organisms were able to adapt to a fairly narrow pH span that very rarely dropped below the minimum in the surface-layer water (IMBER, 2005). This is a further argument for application of the precautionary principle, especially considering the gaps in the scientific knowledge about the impacts of acidification (Section 4.3).
Because of the importance of the consequences of ocean acidification, research in this area should be intensified considerably (Section 4.6). As long as there is no general scientific consensus about the tolerable limit for the effects of acidification, a margin of safety according to the precautionary principle should be observed. The suggestion of WBGU to prevent a pH decrease of more than 0.2 is oriented toward the goal of avoiding an aragonite undersaturation in the ocean surface layer. If it is found that other intolerable damages already occur before reaching aragonite undersaturation, then the guard rail will have to be adjusted accordingly.
     Because the CO2 input into the sea is caused by a rise in atmospheric CO2 concentrations and therefore by anthropogenic CO2 emissions, the pH drop in the ocean can be limited by reducing emissions. Once acidification has occurred, however, it is irreversible – as long as there is no possibility of lowering the atmospheric CO2 concentration the pH value of the surface layer will not rise again in any foreseeable future. Overstepping the guard rail would thus be irreversible, which makes the precautionary principle particularly relevant to this problem.
     Compliance with the guard rail can be verified reliably by scientific means: for one, the pH value of seawater can be determined directly, and for another, the average pH value of the sea can be derived from measurements of atmospheric CO2 concentrations.
     The acidification and climate guard rails could exhibit redundancies with regard to the measures required to obey them, but they are not replaceable by one another: human-induced global warming is caused by a group of greenhouse gases, with 60 per cent of the effect from CO2. The only one of this group responsible for ocean acidification, however, is CO2. Stabilization of CO2 at 450ppm by 2100 would reduce the pH value by 0.17, therefore staying within the allowable range of the acidification guard rail. Compliance with the global warming guard rail of 2°C also requires a stabilization concentration of 450ppm or less, depending on climate sensitivity. Therefore, observance of the climate guard rail would incorporate the acidification guard rail, under the condition that CO2 is adequately taken into account in the emissions reduction.

4.5   Recommendations for action: Linking climate protection with marine conservation

In the 1970s and 1980s, the phenomenon of ‘acid rain’ became widely known. This problem is caused by emissions of acid-forming gases (mainly SO₂ and NO_x) from the combustion of fossil fuels. The issue was taken up by the media under the catchphrase ‘Waldsterben’ (forest dieback), and exerted considerable pressure upon policymakers and industry. Great technical and financial effort was subsequently invested to fit large-scale power plants with flue gas scrubbers, mandate catalytic converters for cars, and embark upon broad-scale liming of forests and lakes. A comparison of the problem of acid rain with the already advancing acidification of the oceans shows the latter to be an issue significantly more serious. The media and policymakers, however, are expressing negligible interest compared to acid rain. Indeed, the problem is being practically ignored. Policymakers are therefore called upon to recognize the full impact of ocean acidification and to take measures of a scope and effectiveness comparable to those adopted to tackle acid rain.

4.5.1   Reappraising the role of CO₂ in climate protection policy

The release of CO2 has particularly far-reaching consequences for marine ecosystems. Firstly, CO2 acts as a greenhouse gas, altering the radiation balance of the atmosphere and thus contributing to global atmospheric warming and, as a further consequence, to the warming of the oceans. Secondly, a large proportion of the CO2 emitted by human activities dissolves in seawater, where, in addition to the warming, it causes chemical changes. In view of these particularly harmful effects of CO2 upon the oceans, it is essential that climate policy give special attention to this specific greenhouse gas.

Need for action

Compliance with the WBGU guard rail for ocean acidification will only be possible if the increase of the atmospheric CO2 concentration is limited. Engineering approaches, such as liming the surface layers of the oceans, are unrealistic considering the scale of the problem (Raven et al., 2005). However, over a time scale of centuries, the acidified surface water will mix down into the deep sea through ocean currents. One option for action is to stabilize the atmospheric CO2 concentration; this would lead over the long term to an acidification of deeper waters of the oceans until the pH of the surface layers is reached. An alternative option would be to agree a maximum limit on the total amount of CO2 emitted to the atmosphere by human activities; that approach could cause the atmospheric CO2 concentration to drop again over the medium term and could prevent the acidification of the deep sea.

Legal setting

The present climate policy instruments do not take into account the aspect of ocean acidification caused by CO2 input. WBGU takes the view that the United Nations Framework Convention on Climate Change (UNFCCC) does indeed establish an obligation to take into account the impacts of climate change upon the oceans, regardless of the circumstance that this aspect was not a priority when the UNFCCC was concluded, and was not covered by the Kyoto Protocol when reduction commitments were set. The rationale is as follows: Under Art. 1 para 3 UNFCCC, ‘climate system’ means the totality of the atmosphere, hydrosphere, biosphere and geosphere and their interactions. The term ‘climate system’ is thus defined in such a comprehensive way that it includes the oceans, which are a part of the hydrosphere, as well as the interactions of the oceans with the atmosphere and the biosphere. The UNFCCC objective established in Art. 2, ‘stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system’, thus also covers the impacts of increasing greenhouse gas levels upon the oceans. With respect to acidification, the meaning of Art. 2 UNFCCC can be concretized as follows: CO2 is a greenhouse gas, and an excessive CO2 concentration in the atmosphere leads to dangerous anthropogenic interference with marine ecosystems, for CO2 dissolves in water and causes acidification (Sections 4.1 and 4.3). The oceans are a part of the hydrosphere, and marine organisms are a component of the biosphere. The problem of acidification is thus one of interaction among the atmosphere, hydrosphere and biosphere, all of which are components of the climate system (Art. 1 para 3 UNFCCC). There can thus be no doubt that the objectives of the convention include preventing a dangerous acidification of the oceans. Furthermore, Art. 2 UNFCCC also states that ecosystems should be able to adapt naturally to climate change. The speed of acidification observed today calls compliance with this requirement into question: for instance, the adaptive capacity of marine ecosystems can be overstretched if the aragonite saturation horizons in the Southern Ocean rise to the surface (Section 4.3). This presents an immediate need to limit acidification and adopt appropriate measures under the UNFCCC.

Recommendations

WBGU argues against this backdrop that climate policy needs to take all impacts upon the marine habitat into account. In the negotiations on the second commitment period under the Kyoto Protocol now commencing, the German federal government should work to ensure that the direct adverse effects of CO2 emissions upon the oceans are taken into account. The desired stabilization of atmospheric greenhouse gas concentrations should be set in such a way that ocean acidification is adequately limited. This implies that CO2 should not be viewed only as part of a basket of various greenhouse gases. The atmospheric CO2 concentration rather needs to be stabilized specifically, regardless of the reduction of other greenhouse gases – at a level permitting compliance with the WBGU acidification guard rail (Section 4.4).
     To achieve this goal, it may be necessary to define a CO2 emissions ceiling for individual states or groups of states in addition to existing reduction commitments. This CO2 cap would then need to be observed as a complement to the other commitments. The precise effects of this and any further potential instruments still need to be clarified. A particularly important aspect in this regard is the possible need for adaptation of the existing flexible mechanisms (emissions trading, Clean Development Mechanism and Joint Implementation).
It would not, however, be necessary to define a separate ceiling for CO2 if, firstly, the international community were to agree to reduce greenhouse gas emissions to a level ensuring compliance with the WBGU guard rail on climate protection, and, secondly, the relative proportion of CO2 within overall greenhouse gas emissions does not change significantly. The CO2 reduction needed in this case would most probably suffice to prevent transgression of the acidification guard rail.

4.5.2   Taking shipping sector emissions into account

As a part of efforts to stabilize the atmospheric CO2 concentration, the CO2 emissions generated by ocean shipping and international aviation should be integrated more closely into emissions reduction strategies. No quantitative reduction commitments have yet been agreed for either sector. WBGU recommends closing these regulatory gaps by integrating the CO2 emissions generated by international shipping and aviation into negotiations on future reduction commitments within the Kyoto process. Present estimates suggest that worldwide CO2 emissions from shipping amount to about 2 per cent of global emissions. Over the past decade, the rate of increase in shipping emissions was more than twice that for total emissions (Bode et al., 2002; IEA, 2002). This illustrates the urgent need for action.
     Besides emitting CO2, ocean shipping also generates pressures upon marine and coastal ecosystems by emitting pollutants, nutrients and sediment particles. Controls on ocean shipping thus present a starting point for linking climate protection with marine conservation at the level of legal instruments.
In view of the relatively good environmental performance and significant economic importance of ocean shipping, regulatory controls need not aim to reduce the volume of shipping traffic. The goal is rather to create incentives for technological innovations and improvements in environmental management that contribute both to abating ocean pollution and preventing atmospheric CO2 emissions. As a means to this end, WBGU recommends levying charges on the use of the oceans by shipping (WBGU, 2002).
     This instrument highlights the connection between the utilization of the environmental resources represented by ‘the oceans’ and ‘the atmosphere’ and the utilization-related impairment of these resources. A charge signals the scarcity of environmental resources and the cost of their provision. The economic players burdened by a charge receive an incentive to modify their utilization of global environmental goods and to make it more sustainable (WBGU, 2002).
     WBGU has set out the options for designing a user charge system in detail elsewhere (WBGU, 2002). A proposed user charge regime applied to the European Union area alone could generate annual revenues of Euro 400–700 million. WBGU proposes that the financial resources thus received be earmarked for marine conservation purposes, in order to create a substantive link between the generation and reduction of pressures upon the oceans (WBGU, 2002).

4.6   Research recommendations

Acidification and marine ecosystems
The physiological effects of acidification on marine organisms, especially on calcifying ones, and the impacts on the marine ecosystem are insufficiently understood. Physiological experiments are needed with moderately increased CO2 concentrations, as are experiments exploring the effects on marine food webs (trophic coupling among phytoplankton, zooplankton and fish), and studies of possible physiological adaptation processes on an evolutionary basis.

Biogenic calcification and the carbon cycle
Our understanding of the interactions between calcifying plankton, the biological pump, and the global carbon cycle shows similar gaps, so modelling of the net effect is not yet possible. Modelling studies therefore need to be carried out on the acidification-related reduction of biological export production due to decreased mineral ballast (carbonate shells).

Further impacts of climate change
Acidification is probably just one of many changes that will take place in the biogeochemistry of the oceans due to anthropogenic greenhouse gas emissions, or through climate change. Other aspects, such as the effects on oxygen balance and nutrient supply in the sea, are poorly understood and urgently need further study in order to recognize critical developments in good time.

Future CO2 uptake by the ocean
CO2 uptake by the ocean plays a key role in climate change. Interactions among the atmospheric radiation balance, the chemical composition of the atmosphere and the physical, chemical, and biological changes in the ocean should therefore receive increasing attention.

International research programmes
Promotion of projects by international research programmes (e.g., SOLAS, 2004; IMBER, 2005) that address the questions above is recommended.

CO2 and climate protection
In the event that a specific reduction commitment for CO2 proves to be necessary, the potential ways of designing such a commitment need to be developed and evaluated. In addition, the implications for Kyoto mechanisms (especially CDM and emissions trading) need investigation.