This contains extra information and data concerning the possibility of carbon dioxide emissions altering the pH of the oceans

Ocean acidification revisited

In addition to what has been discussed several times in the group, there are extra points to make.

1.       The distribution of carbon in the atmosphere/ocean system is: 820 Gt in the atmosphere, 1020 Gt in surface water, and 38100 Gt in deep water. This gives the respective ratios: 1 : 1.24 : 46.5 or respective percentage contents as 2.1%, 2.5% and 95.4% of the total inorganic carbon species in the system. The pre-industrial value for the atmospheric content of CO2 is one that is closer to true equilibrium, 285 ppmv = 607 Gt C. That alters the above ratios and percentages to: 1 : 1.68 : 62.8, 1.5%, 2.6% and 95.9%.

2.       The above content refers to the present time with a CO2 concentration of ~385 ppmv and the arguments that are being made by some people imply that a concentration of CO2 of 570 ppmv, i.e., that which is twice the pre-industrial value, should cause significant acidification of the oceans. I use the term 'acidification' in its literal sense; an increase in the hydrogen ion concentration, although bearing in mind that the oceans are alkaline, i.e., they have pH values greater than 7.0. The Brookhaven Laboratory's computer programme [CO2SYS_calc_DOS_Original: Lewis, E., and D. W. R. Wallace. 1998. Program Developed for CO2 System Calculations. ORNL/CDIAC-105. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee] for carbonate equilibria in the oceans in contact with an atmosphere containing CO2 gives the following results.

 

 

 

This shows the equilibrium pressure of CO2 in contact with seawater at 288 K

 

If anthropogenic activity should produce an atmospheric concentration of CO2 as much as 570 ppmv, the equilibrium pH of the oceans might be expected to be around 7.9. Such values have been mentioned in published papers in reputable journals and have led to conclusions of damage to various forms of marine life. What has been ignored is that even if the CO2 concentration did rise to 570 ppmv there would not be equilibrium between the atmosphere and the aqueous phase, at least not for a very long time. For this to happen between the atmosphere and the surface water there would have to be provision of a great amount of carbon and it would take a considerable time. Using the figure of 55% of fossil fuel carbon remaining in the atmosphere every year, to increase the concentration of CO2 in the atmosphere from 385 ppmv to 570 ppmv, 716 Gt C would have to be burned. To maintain the 1 : 1.68 ratio between the atmosphere and the surface water another 1203 Gt C would have to be burned. So, to maintain the equilibrium the total burning of 1919 Gt C would be required.

The above scenario is clearly impossible and reliable measurements indicate that only 2 Gt C are entering the surface water per annum. With the current emissions amounting to ~9 Gt C the system is moving further and further away from an equilibrium state and there would have to be 9 x 1.68 = 15.1 Gt C entering the surface water every year to maintain equilibrium. And, of course, the transfer of carbon from the surface water to the deep water, ignored in this argument so far, being only ~2 Gt C per annum, makes any kind of equilibrium calculation irrelevant.

 

3.       Another point in the ocean acidification discussion that has possibly been overlooked is the other effect of melting ice, apart from altering the sea level. The scenario might be (i) extra CO2 or some other factor causes the atmosphere/surface to warm up, (ii) this causes some ice to melt, (iii) the melt water dilutes the ocean, (iv) this causes the ocean to have a greater capacity for dissolved CO2, and (v) this does not necessarily alter the pH, but removes CO2 from the atmosphere and causes some cooling to offset the initial warming; a negative feedback.

A detailed consideration of the problem using the programme CO2SYS gave the following results:

 

Atmospheric

concentration of CO2

285 ppmv

385 ppmv

570 ppmv

H2CO3 (mmol/kg)

11

14

21

HCO3- (mmol/kg)

1776

1870

1976

CO32- (mmol/kg)

211

173

131

Total dissolved inorganic

carbon (mmol/kg)

1998

2057

2128

Average pH of surface

oceans

8.17

8.07

7.92

 

Taking the 385 ppmv and 570 ppmv figures, the total inorganic carbon increases by 71 mmol/kg. Based on the top 100 m of the oceans, which have a mass of 3.7 ´ 1019 kg, to increase the total inorganic carbon content by 71 mmol/kg would require 32 Gt C. An approximate indication of the fate of fossil fuel carbon is that 57% remains in the atmosphere, 13% is absorbed by the biosphere and 32% is absorbed by the oceans. Thus, to increase the CO2 content from the present 385 ppmv to 570 ppmv would require the burning of 690 Gt C taking a considerable time, about 76 years at the present rate of consumption. Assuming that the oceans absorb 32% of this extra carbon, 221 Gt C would be removed from the atmosphere by this route. That would be more than adequate for the provision of the extra 32 Gt C needed to adjust the ocean carbon content to that expected from the calculations. The remaining 189 Gt C would either have to be assimilated into phytoplankton [but that assimilated carbon would be recycled in the surface waters], transferred to the deep oceans or be converted into solid carbonates, all these processes taking considerable time.

            Estimates of the annual carbon fluxes from the atmosphere to the surface water and from the surface water to deep water are ~2 Gt C and ~1.6 Gt C. If the carbon content of the surface layer is building up at 0.4 Gt C per annum it will take 32/0.4 = 80 years to provide the 32 Gt C needed to maintain equilibrium. Bearing in mind the uncertainties in these figures, it is just possible for the atmosphere/surface water system to be in equilibrium. If the top 200 metres of surface waters were to be used as the basis of the calculation, it could be that the requirements for equilibrium are not satisfied. It's an uncertain world!

            The programme used to produce the above results ignores several major issues. The programme itself is based on thermodynamics of equilibria that are set up very quickly. They ignore the inflows of river waters, material ejected by volcanoes, reactions with basaltic magma along the mid-ocean ridges and the depositions of solid carbonates. They ignore the buffering capacities of clay minerals. They ignore the massive amount of photosynthesis that occurs in the surface waters and the corresponding effects of cell death. These points lead to the opinion that the immediate changes of ocean pH that occur as the result of a change in the atmospheric CO2 content are governed by thermodynamics as expressed in the computer programmes, but the longer term changes are governed by the inflows of river waters, the precipitation of carbonates and the buffering activities of clays.

            The current range of ocean pH values encompass the above possible changes given by the computer programme and there seems to be very little excuse for yet another ecological panic.