Science Under Attack

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The Scientific Method at Work: The Carbon Cycle Revisited

The crucial test of any scientific hypothesis is whether its predictions match real-world observations. If empirical evidence doesn’t confirm the predictions, the hypothesis is falsified. The scientific method then demands that the hypothesis be either tossed out, or modified to fit the evidence. This post illustrates just such an example of the scientific method at work.

The hypothesis in question is a model of the carbon cycle, which describes quantitatively the exchange of carbon between the earth’s land masses, atmosphere and oceans, proposed by physicist Ed Berry and described in a previous post. Berry argues that natural emissions of CO2 since 1750 have increased as the world has warmed, contrary to the CO2 global warming hypothesis, and that only 25% of the increase in atmospheric CO2 after 1750 is due to humans.

One prediction of his model, not described in my earlier post, involves the atmospheric concentration of the radioactive carbon isotope 14C, produced by cosmic rays interacting with nitrogen in the upper atmosphere. It’s the isotope commonly used for radiocarbon dating. With a half-life of 5,730 years, 14C is absorbed by living but not dead biological matter, so the amount of 14C remaining in a dead animal or plant is a measure of the time elapsed since its death. Older fossils contain less 14C than more recent ones.

Berry’s prediction is of the recovery since 1970 of the 14C level in atmospheric CO2, a level that became elevated by radioactive fallout from above-ground nuclear bomb testing in the 1950s and 1960s. The atmospheric concentration of 14C almost doubled following the tests and has since been slowly dropping – at the same time as concentrations of the stable carbon isotopes 12C and 13C, generated by fossil-fuel burning, have been steadily rising. Because the carbon in fossil fuels is millions of years old, all the 14C in fossil-fuel CO2 has decayed away.

The recovery in 14C concentration predicted by Berry’s model is illustrated in the figure below, where the solid line purportedly shows the empirical data and the black dots indicate the model’s predicted values from 1970 onward. It appears that the model closely replicates the experimental observations which, if true, would verify the model.

However, as elucidated recently by physicist David Andrews, the prediction is flawed because the data depicted by the solid line in the figure are not the concentration of 14C, but rather its isotopic or abundance ratio relative to 12C. This ratio is most often expressed as the “delta value” Δ14C, calculated from the isotopic ratio R = 14C/12C as

Δ14C = 1000 x (Rsample/Rstandard – 1), measured in parts per thousand.

The relationship between Δ14C and the 14C concentration is

14C conc = (total carbon conc) x Rstandard x (Δ14C/1000 + 1).

Unfortunately, Berry has failed to distinguish between Δ14C and 14C concentration. As Andrews remarks, “as Δ14C [calculated from measured isotope ratios] approaches zero in 2020, this does not mean that 14C concentrations have nearly returned to 1955 values. It means that the isotope abundance ratio has nearly returned to its previous value. Therefore, since atmospheric 12CO2 has increased by about 30% since 1955, the 14C concentration remains well above its pre-bomb test value.”

This can be seen clearly in the next figure, showing Andrews’ calculations of the atmospheric 14CO2 concentration compared to the experimentally measured concentration of all CO2 isotopes, in parts per million by volume (ppmv), over the last century. The behavior of the 14CO2 concentration after 1970 is unquestionably different from that of Δ14C in the previous figure, the current concentration leveling off at close to 350 ppmv, about 40% higher than its 1955 pre-bomb spike value, rather than reverting to that value. In fact, the 14CO2 concentration is currently increasing.

At first, it seems that the 14CO2 concentration in the atmosphere should decrease with time as fossil-fuel CO2 is added, since fossil fuels are devoid of 14C. The counterintuitive increase arises from the exchange of CO2 between the atmosphere and oceans. Normally, there’s a balance between 14CO2 absorbed from the atmosphere by cooler ocean water at the poles and 14CO2 released into the atmosphere by warmer water at the equator. But the emission of 14C-deficient fossil-fuel CO2 into the atmosphere perturbs this balance, with less 14CO2 now being absorbed by the oceans than released. The net result is a buildup of 14CO2 in the atmosphere.

As the figures above show, the actual 14C concentration data falsify Berry’s model, as well as other similar ones (see here and here). The models, therefore, must be modified in order to accurately describe the carbon cycle, if not discarded altogether.

The importance of hypothesis testing was aptly summed up by Nobel Prize winning physicist Richard Feynman (1918-88), who said in a lecture on the scientific method:

If it [the hypothesis] disagrees with experiment, it’s WRONG.  In that simple statement is the key to science.  It doesn’t make any difference how beautiful your guess is, it doesn’t matter how smart you are, who made the guess, or what his name is … If it disagrees with experiment, it’s wrong.  That’s all there is to it.

Next: How Clouds Hold the Key to Global Warming