Jocelyn Turnbull from GNS Science is measuring how much CO2 we're producing in NZ towns - and she's doing it by cutting the grass.

Radiocarbon, grass and the 'who farted' problem are all part of a Carbon Watch NZ project measuring how much carbon dioxide is being produced by New Zealand's cars and trucks.

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How to fingerprint how much CO2 is coming from burning fossil fuel

Measuring how much carbon dioxide is in the air is straightforward. Working out how much of that carbon dioxide has come from vehicles is rather more complicated.

"If we just measure carbon dioxide concentrations there's not only emissions from fossil fuel burning, but there's also this natural exchange," says Jocelyn Turnbull, a radiocarbon scientist with GNS Science.

"Photosynthesis, where plants suck carbon dioxide out of the air to grow and respiration, where ... plants breathe carbon dioxide back out into the atmosphere ... So it can be hard to get at exactly how much carbon dioxide is coming from fossil fuel burning."

The way Jocelyn and her colleagues get around this problem is to measure radiocarbon in carbon dioxide.

Because fossil fuel comes from petroleum reserves that are many millions of years old, all of the radiocarbons they contain have already radioactively decayed. As a result, carbon dioxide emissions released by the burning of fossil fuels decrease the relative amounts of radiocarbon in the atmosphere.

The radiocarbon team at GNS Science use this change in the ratio of heavy carbon isotopes to calculate how much carbon dioxide is being released due to the burning of fossil fuels.

The 'who farted' problem

The final step is to calculate where the emission occurred.

Jocelyn describes the process of identifying the source of carbon dioxide emissions as the 'who farted problem'. Was it a small emission nearby, or a large emission from further away?

To work this out they combine the radiocarbon and carbon dioxide data with a weather model to work out where the emissions have come from.

"That's the problem we're trying to solve with these atmospheric transport models," says Jocelyn. "Where did it come from? Who did it? How big was it?"

The grass record

Measuring a sample of air gives you a carbon dioxide level at a single moment in time, whereas plants turn out to be excellent record keepers over longer periods of time…

Go to this episode on rnz.co.nz for more details