Posted on 25 July 2011 by Doug Mackie

This post is number 10 in a series about ocean acidification. Other posts: Introduction, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.

Welcome to the 10th post in our series about ocean acidification. In the last post we asked "how can we be sure that atmospheric CO₂ is entering the oceans to cause acidification?" That is, how can we be certain that the oceans are not a net source of the increased CO₂ in the atmosphere? Our answer to this question is a modification of an earlier post by one of us (2010) at Skeptical Science.

We can use four  key observations to be confident the extra CO₂ in the atmosphere has come from the combustion of fossil fuels and not from outgassing of CO₂ from the ocean or from soil or land sources: (1) the decrease in atmospheric oxygen corresponds to the usage of fossil fuels, (2) the carbon isotope ratio in the atmosphere indicates that the excess CO₂ comes from fossil fuels, (3) not enough warming of the ocean has occurred, and (4) known emission of CO₂ from fossil fuel usage.

First, a quick word about 'ppm': Scientists use parts per million or ppm for concentrations that are much lower than percent (parts per hundred) or permil (parts per thousand). 1 ppm = 0.0001%.

1. Oxygen decrease

Atmospheric oxygen (O₂) is going down by the same amount that atmospheric CO₂ is going up.

We have seen in previous posts that carbon in the ocean is always bound to oxygen in some way – as H₂CO₃, HCO₃^–, or CO₃^2–. So if the oceans were the source of the extra CO₂ in the atmosphere there would be no change in atmospheric oxygen as the carbon is already bonded to oxygen. On the other hand, we know that the burning of any sort of fuel (including non fossil fuels like wood) requires oxygen and produces CO₂.

This suggests that the burning of some sort of carbon containing fuel is responsible for both the CO₂ increase and the oxygen decrease.

Atmospheric Oxygen is so abundant at about 21% (209,500 parts per million or ppm) that we are in no danger of running out. However, the measured decrease in oxygen corresponds to the amount of oxygen required to burn the amount of fossil fuels known to have been burned. See the IPCC 3^rd Assessment Report (2001) section 3.5.1, especially Figure 3-4 and the IPCC 4^th Assessment Report (2007) Figure 2-3.

But how do we know that fossil fuels are the source of added carbon that is causing the decrease in oxygen? How can we be confident the extra CO₂ has not come from some other source like changes to forests? In a word: Isotopes.

2. Isotope ratios

Isotopes are different forms of the same element that differ in the mass of the nucleus. You may have heard of uranium-235 and uranium-238. These are two (of the many) forms of uranium. The nucleus contains particles with a positive electric charge (protons) and particles with no electric charge (neutrons). Isotopes have the same number of protons but different numbers of neutrons. Mostly the properties of isotopes are the similar except for properties that depend on the mass of the atoms. For example, diffusion of a gas: A heavier gas diffuses more slowly than a lighter gas. This principle was used to separate uranium isotopes during the Second World War's atomic bomb programme, the Manhattan project.

Not all isotopes are radioactive. Isotopes that do not undergo radioactive decay are called stable isotopes There are two common stable isotopes of carbon, ¹²C and ¹³C, and one common radioactive isotope, ¹⁴C. The carbon in all living things is a mixture of all three of these isotopes of carbon.

¹⁴C is very rare; only about 0.0000000001% (1 part per trillion) of the carbon in the atmosphere is ¹⁴C, 98.9% is ¹²C, and 1.1% is ¹³C. Such a relative ratio of ¹²C to ¹³C is usually different for each type of carbon source.

For example, photosynthesis favours ¹²CO₂ for several reasons (including poor diffusion of ¹³CO₂ into cells because it is heavier). This means that living plants and fossil fuels (which are derived from plants) have a relatively low proportion of ¹³CO₂ – chemists say fossil fuels are depleted in ¹³C. Also, because they are so old, fossil fuels contain no ¹⁴C. The half life of ¹⁴C is 5730 years so after a few million years of halving in number every 5730 years there is no ¹⁴C left as it has all decayed. Compared to fossil fuels, seawater water is enriched in ¹³C and ¹⁴C. Isotope ratios can be determined with great precision and have been monitored in the atmosphere and ocean for decades.

Observations show that the isotope ratios of carbon in the atmosphere are changing due to an influx of CO₂ depleted in ¹³C That is, the new isotope ratios of carbon contained in atmospheric CO₂ tells us that the additional carbon must be coming from ¹³C-depleted fossil fuels, not the ¹³C-rich oceans.

3. Not enough warming

You may remember from post 8, that warm water can hold less CO₂ than cold water. Our knowledge of Henry's Law and the CO₂ equilibria allow us to calculate the increase in seawater temperature that would be needed to cause the observed increase in pCO₂ in the atmosphere (i.e. the partial pressure (or 'concentration') of CO₂). The results show that to explain the 100 ppm of additional CO₂ added to the atmosphere since preindustrial times by ocean warming, the average temperature rise of the surface ocean needs to be about 10^o C, much larger than has occurred.

As we noted in post 8, the Henry's Law coefficient, K_H, is dependent on temperature (and salinity to a lesser extent). However, there is no exact expression as seawater is sufficiently complex that the values for K_H for seawater have been experimentally determined.

For constant salinity, the pCO₂ in the atmosphere doubles (i.e. =200% the initial concentration) for every 16^oC increase in seawater temperature*. Atmospheric CO₂ is now 140% of the preindustrial value (it increased by about 110 ppm from 280 to 390 ppm). Thus the temperature change required to sufficiently change the Henry's Law coefficient is 140/200 × 16 = 11^oC.

This calculation shows that the surface ocean would on average have to have warmed by about 10^oC since about 1750 if the oceans had been the source of the CO₂. Plainly the ocean does not have a uniform temperature, so the changes would actually need to be even more extreme in some places. Of course, no such warming has occurred.

*for the interested this is explained in detail in the appendix to: Takahashi et al. "Seasonal variation of CO₂ and nutrients in the high-latitude surface oceans: A comparative study" Global Biogeochemical Cycles, 7(4), 843-878.

4. Known fossil fuel CO₂ emissions

Most obviously, any alternative explanation for the source of the CO₂ in the atmosphere must also come up with where the 30 billion tonnes of CO₂ released yearly by fossil fuel burning goes.

We have a very good idea of the total amount of fossil fuel burned in the last 150 years. But, if we want to stick to really solid data we can limit our discussion to the last 30 odd years. The US Dept Energy publication The International Energy Annual shows that since 1980 to 2006 (the most recent year they have data for) a world total of 603 billion metric tons of CO₂ have been released into the atmosphere from the 'consumption and flaring of fossil fuels'.

However, the amount extra CO₂ that is currently in the atmosphere is less than the known amount of released fossil fuel CO₂. That is, not only have humans released a lot of CO₂ to the atmosphere over the last 30 years, but not all of it is still in the atmosphere. Where is this ‘missing’ CO₂? In the next posts, we calculate the amount of missing CO₂ and explain where it went.

Written by Doug Mackie, Christina McGraw, and Keith Hunter. This post is number 10 in a series about ocean acidification. Other posts: Introduction, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.