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Predicted net efflux of radiocarbon from the ocean and increase in atmospheric radiocarbon content

Ken Caldeira, Greg H. Rau, and Philip B. Duffy · Geophysical Research Letters 25, 3811-3814 · 1998

Key finding. A global carbon-cycle model predicts that atmospheric radiocarbon content will reach a minimum and begin to increase within the next few years under a business-as-usual emissions scenario — even though fossil fuels contain no radiocarbon at all — because rising atmospheric CO2 drives a net flux of carbon-14 out of the ocean and land biosphere.

Three stacked line graphs against year from about 1765 to 2100. The top panel plots atmospheric carbon-14 mass in kilomoles from 30 to 70, with a bomb spike near 1965 and all curves rising steeply after 2000. The middle panel plots atmospheric delta carbon-14 in per mil from minus 200 to 800, with the same bomb spike but a long decline to negative values. The bottom panel plots the flux of radiocarbon from atmosphere to ocean in moles per year from minus 400 to 400, with curves crossing zero in the early twenty-first century and continuing downward.
The bottom panel is the paper's point. The flux of radiocarbon from atmosphere to ocean crosses zero and turns negative, so the ocean becomes a net source of carbon-14 to the atmosphere — which is why atmospheric carbon-14 mass in the top panel rises even though fossil fuels contain none. Figure 3 from Caldeira et al. (1998), Geophysical Research Letters 25, 3811-3814. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

Before human interference, radiocarbon production in the stratosphere nearly balanced its flux into the ocean and land biosphere, which in turn nearly balanced radioactive decay. Land-use change, fossil-fuel burning, and atmospheric nuclear tests broke that balance.

Bomb testing raised atmospheric carbon-14 by about 80% by the mid-1960s, and it has been falling since as the excess enters the oceans and biosphere. This paper asked what happens next, and why the historical record does not look the way naive reasoning predicts.

What did we find?

The puzzle the paper starts from is quantitative. Atmospheric radiocarbon fell far less between pre-industrial times and 1950 than the roughly 10% rise in atmospheric CO2 over the same period would lead one to expect from dilution alone.

Most of that discrepancy is explained by radiocarbon fluxes out of the land biosphere and oceans induced by the rising burden of ordinary carbon dioxide — an effect recognized since the 1950s and here quantified in a full carbon-cycle model.

The same mechanism dominates the future. Continued fossil-fuel burning indirectly causes a significant increase in atmospheric carbon-14 content, despite fossil carbon being devoid of it.

Under business-as-usual, the crossover comes soon: the bomb-radiocarbon decline reaches its minimum and reverses within a few years of the paper.

The model puts the turning point at 1998 — the year atmospheric carbon-14 content starts rising again, reversing a decline that began in the mid-1960s, and a prediction the authors note will soon be testable.

Beyond that, the ocean itself changes sign. The fossil-fuel-induced radiocarbon flux out of the ocean is predicted to exceed the natural flux into it by about 2030 and the natural plus bomb flux by about 2050, making the ocean a net source of carbon-14 to the atmosphere by the middle of the century.

Of the increase in atmospheric radiocarbon between 1945 and 1995, about 30% is attributed to land-use change and fossil-fuel burning, with the remaining 70% from atmospheric nuclear detonations.

Why does it matter?

Radiocarbon is the main tracer used to calibrate how fast ocean models ventilate the deep sea, so getting its budget right propagates into every estimate of ocean carbon uptake built on it.

The result is counter-intuitive in a way that is instructive. Adding carbon with no radiocarbon in it raises the atmosphere's radiocarbon content, because the isotopic effect of disturbing the reservoirs outweighs the dilution.

It also bears on radiocarbon dating of recent material, where the atmospheric curve reversing direction means a measured value no longer maps to a single date.

Citation

Ken Caldeira, Greg H. Rau, and Philip B. Duffy (1998). Predicted net efflux of radiocarbon from the ocean and increase in atmospheric radiocarbon content. Geophysical Research Letters 25, 3811-3814.

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