Nuclear Bomb Testing and Boreal Forest Fires

One important piece of evidence that shows that climate change has been caused by humans is the decrease in the amount of radiocarbon (carbon-14) in CO₂. You may be familiar with carbon-14 because of its use in radiocarbon dating in archaeology for objects up to 60,000 years old. For those unfamiliar, though, carbon-14 is produced in the upper atmosphere by reaction of neutrons from cosmic rays with nitrogen. The carbon-14 produced is then eventually transported to the lower atmosphere, where plants uptake the ¹⁴CO₂ via photosynthesis. Once a plant dies, it stops absorbing carbon dioxide, which then causes any of the carbon-14 in the leftover plant material to radioactively decay. As such, fossil fuels, having been in the ground for millions of years, contain almost no carbon-14, so that when they are burned, the carbon dioxide produced has no carbon-14 content.

Of course, measuring carbon-14 in CO₂ in the atmosphere has several complications associated with it. In an article in the Journal of Geophysical Research, a group of scientists from NASA, NOAA, and various universities around the world measured CO₂ and its carbon-14 content from air samples collected on aircraft campaigns over the Arctic in Canada. As you might expect from my above explanation, CO₂ from right over the Alberta tar sands has both higher concentrations and reduced carbon-14 content compared to typical levels for background CO₂ from emissions from burning the oil extracted. Interestingly, some of the air samples that were far from any influence from civilization also had higher concentrations of CO₂ but increased carbon-14 content. High levels of carbon monoxide in these samples along with some atmospheric modeling (and a little knowledge of local conditions) led the scientists to conclude that these measurements were sampling air from boreal forest fires in the Arctic. Why would this lead to higher carbon-14, though?

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The above plot with the amount of carbon-14 in the atmosphere over the last few decades helps show what likely caused this increase in the carbon-14 content. During the 1950's and into the 1960's, the world's governments conducted many nuclear bomb tests in the atmosphere, which released a lot of neutrons into the atmosphere. Since neutrons cause the formation of carbon-14 from nitrogen as explained above, this nuclear testing caused a so-called "bomb spike" in carbon-14 until atmospheric nuclear testing was banned internationally in the 60's from health concerns about the release of other much more radioactive isotopes of elements such as strontium-90 into the atmosphere. Because plants continuously absorb CO₂ throughout their lifetimes, decades-old trees in boreal forests in the Arctic could retain high levels of carbon-14 until burned by a sudden forest fire, releasing highly enriched carbon-14 back into the atmosphere.

The results in this paper highlight some of the challenges involved in the use of carbon-14 as a tracer of fossil fuel burning. However, it also shows the incredible utility of trace isotope studies in providing information about the chemical processes in the atmosphere. Using isotopes, much more information can be gained about a chemical than from simply looking at the amount present alone.