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 CO32–. 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 3rd Assessment Report (2001) section 3.5.1, especially Figure 3-4 and the IPCC 4th 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 10o C, much larger than has occurred.
As we noted in post 8, the Henry's Law coefficient, KH, 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 KH for seawater have been experimentally determined.
For constant salinity, the pCO2 in the atmosphere doubles (i.e. =200% the initial concentration) for every 16oC 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 = 11oC.
This calculation shows that the surface ocean would on average have to have warmed by about 10oC 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.
Posted by Doug Mackie on Monday, 25 July, 2011
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