Could Extra CO2 Be Produced by Natural Processes?

ACS Climate Science Toolkit | Narratives

Remark: “In the past century or so, human activities have increased the amount of carbon dioxide in the air and its causing the Earth to warm.”
Reply: “Can you be sure it isn’t the other way around with the Earth warming naturally and causing the oceans to release carbon dioxide into the air?”

Gases, including carbon dioxide, are less soluble in warm water than in cold water. You may have discovered this by opening a warm can of a carbonated drink and having an experience similar to the one illustrated by the photo in the sidebar. A great deal of carbon dioxide is dissolved in the Earth’s oceans. If the oceans are warming, we would expect that they can hold less carbon dioxide and could release some to the air. How can we tell whether the increase of carbon dioxide is a result of human activities, in particular fossil fuel burning, or gas leaving the ocean?

Several kinds of evidence indicate that the great majority of the carbon dioxide added to the air in the past century or so is the result of fossil fuel burning. Let’s focus on one of these pieces of evidence that seems clear-cut and pretty easy to understand. To begin, we consider a story problem that seems to have little connection to carbon dioxide in the air, but will serve as an analogy after we introduce the atoms that make up the molecules of carbon dioxide.

Imagine you have a mixture of 1000 colored candies containing 60 orange candies and 940 yellow candies. Assume you have a sampling scoop that holds 100 candies and you take a sample of the mixture. Since your sample is 1/10 of the mixture, you might expect to find 6 [= 60·(1/10)] orange and 94 yellow candies in the sample. In a relatively small mixture and sample like this, you might not get this ideal result. If you always return the sample to the mixture and repeat the sampling several times, the average number of orange candies you find will likely be 6 and you can estimate the uncertainty in the result from the variations among the samples. What will you observe if you mix 500 more yellow candies into the original mixture and repeat the sampling procedure? The sample is now 1/15 (= 100/1500) of the mixture, so the samples will, on the average, contain 4 [= 60·(1/15)] orange candies. The addition of the extra yellow candies has diluted the mixture so fewer orange candies are observed in the samples.

Now let’s take a look at carbon atoms. Two kinds of carbon atoms, called isotopes, are found in all samples of carbon-containing molecules, including carbon dioxide. These isotopic atoms have almost exactly the same chemical properties, but occur in different amounts in nature. These isotopes are labeled carbon-12 and carbon-13. Almost all carbon atoms are carbon-12. The amount of carbon-13 in a sample varies with the source of the sample. This variation can be used as one of the kinds of evidence for the fossil-fuel source of the carbon dioxide added to the air, but we will focus on another carbon atom isotope, carbon-14.

Carbon-14 is radioactive and, over time, disappears from a carbon-containing sample. It takes 5760 years for half of a given amount of carbon-14 to disappear. You have probably heard of carbon-14 or radiocarbon dating of ancient artifacts containing plant material, a carved wooden totem, for example. This technique is based on measuring the fraction of carbon-14 remaining in the artifact compared to the fraction that would have been present naturally when the plant was alive and using carbon dioxide from the air in photosynthesis.

Until humans began above-ground nuclear bomb testing, all the Earth’s carbon-14 was produced by the reaction of nitrogen in the atmosphere with nuclear particles from the sun. The amount of radiation from the sun is pretty constant over the timescale that carbon-14 is a useful measure. Thus, the fraction of carbon-14 formed by this natural process is pretty constant and is used as the basis for carbon-14 dating. The technique is only good for samples that are younger than about 60,000 years, because essentially all of the carbon-14 in a sample will have disappeared in 600 centuries. Fossil fuels are derived from plants that originally contained carbon-14, but the fuels are millions of years old and no longer contain any carbon-14.

Carbon dioxide dissolved in the ocean has essentially the same fraction of carbon-14 as the carbon dioxide in the air, because the molecules are constantly being interchanged between the air and ocean. If the extra carbon dioxide added to the air over the past century comes from the ocean, no change in the fraction of carbon-14 in the air is expected. Carbon dioxide produced by fossil fuel burning contains no carbon-14, because the fuels contain none. If fossil fuel burning produces the extra carbon dioxide added to the air, the fraction of carbon-14 in the air would be reduced by dilution with the gas containing no carbon-14. This is analogous to the dilution of the mixture of yellow and orange candies by addition of more yellow candies that we discussed above.

Soda can exploding when opened.
Credit: ThinkStock

The problem we face is how to find out what happened to the fraction of carbon-14 in the air as the extra carbon dioxide built up. Did it stay about constant or did it decrease? The method used to answer this question is to analyze the carbon-14 content of tree rings. Remember that all plants, including trees, use carbon dioxide from the air to produce the substances they need to grow. During each growing season, a tree grows a new layer of wood under its bark. If you cut the tree down, you can see these annual growth rings, as shown in the sidebar photo. The tree does not have to be cut down to sample the growth rings—a hollow augur can be drilled into the tree to remove a sample core that extends from the outer ring to the center of the trunk leaving the tree intact to continue growing.

Each ring can be analyzed to find the fraction of carbon-14 in its wood, which is the fraction of carbon-14 in the air during the year the ring formed. The outer ring grew during the year when the sample was taken. The next ring in grew the previous year and so on. Counting rings backwards from the outer ring tells you what year each ring grew, so you can associate the fraction of carbon-14 in each ring with the year the ring formed. The results from this kind of analysis are shown in this figure.

These are the results from measurements of carbon-14 in tree rings from two trees near the Pacific coast in Washington state.
SOURCE: The data are from Stuiver, M. and Quay, P., “Atmospheric 14C changes resulting from fossil fuel CO2 release and cosmic ray flux variability,” Earth Planet. Sci. Lett., 1981, 53, 349-362, replotted in Levin, I. and Hesshaimer, V., “Radiocarbon – A Unique Tracer Of Global Carbon Cycle Dynamics,” Radiocarbon, 2000, 42(1), 69-80.

The values plotted on this graph are related to the fraction of carbon-14 (14C) in the tree ring that grew during that year. A negative value on the graph scale means that there is less carbon-14 in the tree ring compared to a standard sample. Through the 19th century there was a modest decrease in the fraction of carbon-14 in the tree rings. A more precipitous decline began in the 20th century as the pace of energy use and fossil fuel burning, including the introduction of the automobile, picked up and increasing amounts of carbon dioxide containing no carbon-14 were produced.

Unfortunately, above-ground nuclear bomb testing, beginning in 1945, produced man-made carbon-14 that upset the natural carbon-14 fraction and made it impossible to carry these studies beyond 1950. The data in the figure, however, are evidence that the carbon dioxide added to the air in the first century and a half of the Industrial Revolution contained no carbon-14 and came from fossil fuel burning, not from the oceans. Other lines of evidence, some involving the carbon-13 isotope, confirm this conclusion and extend it to show that human activities, including fossil fuel burning, continue to be the major source of the increasing amounts of carbon dioxide in the air.

Part of a cross section of a log from a tree trunk
showing the annual tree ring growth.
Source: Sten Porse, Wikimedia Commons