Long-term studies show carbon dioxide levels in the atmosphere are rising. Radiocarbon carbon dioxide peaked in the 1960s but has since decreased. Much has been taken up by the ocean. Dr Kim Currie of NIWA is one of the scientists measuring this.

Radiocarbon in the atmosphere

In 1954, Athol Rafter and Gordon Fergusson started measuring radiocarbon carbon dioxide (14CO2) in the air at Makara, near Wellington. Sampling has since moved a few kilometres to Baring Head and different scientists are involved, but data is still collected every month. This makes it the longest continuous record of its kind in the world. Since 1970, ‘normal’ carbon dioxide has also been measured – the longest running record in the southern hemisphere. The only atmospheric carbon dioxide recording that has been going on for longer is at Mauna Loa, Hawaii, which started in 1958.

Kim Currie of NIWA, based in Dunedin, is one of the scientists involved in carrying on the measurement of atmospheric 14CO2. This ties in with her other research on CO2 in the ocean around New Zealand. It is difficult to study one without the other, as carbon dioxide is constantly being exchanged between the ocean and the atmosphere as part of the natural carbon cycle

The data collected by Kim and her team is added to models that try to explain changes observed over the last 60 years. These models, built up from data from all round the world, are also used to try and predict future changes in the atmosphere and ocean.

Bomb radiocarbon

Atoms of the element carbon come in several forms, called isotopes. Most carbon has a nucleus with 6 neutrons and 6 protons, written 12C. Some carbon atoms have 8 neutrons and 6 protons, written 14C. This isotope is radioactive and is often called radiocarbon. The radioactivity of 14C means that it can be detected amongst all the non-radioactive 12C.

Most carbon dioxide in the atmosphere is made from 12C, but small amounts of 14C carbon dioxide (14CO2) also occur naturally. In the late 1950s and early 1960s, humans added a lot more radioactive 14C to the atmosphere through the testing of nuclear weapons. This is often called ‘bomb radiocarbon’. There was a Test Ban Treaty in 1963, but France carried on testing until 1968 and China until 1980. One good thing came of all this – scientists have been able to study the carbon cycle by following what happens to the extra 14C.

14CO2 in the carbon cycle

12CO2 in the atmosphere has risen steadily since the 1950s, as shown by data from Baring Head and other places around the world. On the other hand, 14CO2 has been decreasing since testing of nuclear weapons stopped.

So where is all the 14CO2 going? It will take thousands of years for it all to be lost through radioactive decay, so there must be other reasons for the decrease. Research shows that most of the bomb radiocarbon has been taken up by the ocean and most of the rest by plants and soil on land. Early models suggested that the ocean would take up so much 14CO2 that it would start giving it back to the atmosphere. This proved to be true – by the early 2000s, the level of 14CO2 in the atmosphere levelled out, suggesting that the amounts of 14CO2 moving from ocean to atmosphere and from atmosphere to ocean were balanced.

Another clue to what has made the 14CO2 in the atmosphere go down is that the 12CO2 has gone up. The addition of lots of 12CO2 to the atmosphere, mainly from the burning of fossil fuels by humans, has diluted the 14CO2. Fossil fuels do not contain 14C because it all decayed away long ago.

Nature of science

Not all progress in science is made by sudden ‘discoveries’. It is only because of long-term collection of data at places such as Baring Head, Wellington, that scientists have been able to observe important patterns. With the current interest in global warming, the graph of atmospheric carbon dioxide increase at Manua Loa has become famous. It is called the Keeling curve, named after the person who started the project.

Published 18 June 2010