Carbon dioxide’s fingerprint

In the year 2000, Harro Meijer, Professor of Isotope Physics at the University of Groningen, set up the Lutjewad Measurement Station near Hornhuizen. That is where he and his team have been measuring the composition of the atmosphere, including carbon dioxide (CO₂) concentrations in the air, for over 25 years. By looking at the various forms of carbon in the measurements, researchers from Groningen are mapping where the CO₂ originates and where it ends up.

FSE Science Newsroom | Text Charlotte Vlek | Image Leoni von Ristok

This is the first part in a series about research on the carbon cycle: the cycle in which carbon is exchanged between atmosphere and oceans, plants, and rocks. This cycle becomes unbalanced if we humans release too much CO₂ into the atmosphere. This article is about carbon measurements in the atmosphere.

‘Did you know that plants grow better these days due to higher concentrations of carbon dioxide (CO₂) in the atmosphere?’ says Meijer. ‘We also notice that atmospheric CO₂ concentrations currently fluctuate with greater highs and lows: plants absorb slightly more carbon during the growing season; however, this all returns to the atmosphere at the end of the season.’

Meijer’s measurements reflect how the atmospheric CO₂ concentrations ‘breathe’ along with the seasons. However, the total concentration also increases over the course of several years and that is due to the CO₂ emissions caused by humans. Meijer: ‘The annual human contribution is relatively small compared to the larger total. But since it is not a cycle, it accumulates.’ That is because the carbon that we, people, introduce into the atmosphere through the use of fossil fuels originates from the deep layers of the Earth where it would have been stored for many thousands to millions of years.

Not all carbon is the same

With his research, Meijer intends to map how carbon circulates between atmosphere, plants, oceans, and rocks. He does this by studying the three forms of the carbon atom that occur in nature: carbon-12, carbon 13, and carbon-14. These are the different isotopes of carbon; they differ in the number of particles in the atomic nucleus. Having one neutron more or one less means it is still carbon, only a little heavier or lighter than the other relatives.

The carbon variants that Meijer is measuring in the atmosphere are like a fingerprint that shows the material’s source. For example, it can be clearly deduced that the increase in atmospheric CO₂ is due to the use of fossil fuels. However, Meijer is also interested in the continuation of the cycle, such as how much of the atmospheric carbon ends up in plants and oceans. This is something he can also infer from his measurements.

It is all about ratios: carbon-12 and carbon-13 occur, in principle, in a set ratio and when this ratio deviates from the standard, it means that plants have absorbed some of it. ‘That is because plants prefer carbon-12, whereas oceans have no preference,’ explains Meijer. ‘That is why we look at how much more carbon-13 is proportionally present in the atmosphere and then you know that its counterpart carbon-12 must have been absorbed by plants.’

Measuring human effects

Of course, Meijer also measures 'ordinary' CO₂ concentrations in the atmosphere, for example to check if the actual emissions around an industrial area align with what is recorded on paper. ‘In such a case, we measure to the south of Rotterdam, for example. We measure the baseline concentration of atmospheric CO₂ and when there is a northerly wind, we can see exactly how much the emissions from the industry in Rotterdam have added to this.’

And has Meijer also observed any positive human effects over the years? ‘Certainly, you see, for example, that reforestation reduces the atmospheric CO₂ concentration. However, then there will be forest fires, that are sometimes again the consequence of human activities, and these, conversely, increase the concentration.'

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