Plants carbon consumption revealed by atomic bomb tests

Plants take up a lot more carbon from the air than we first thought, a new study has shown. But, unfortunately, it’s not all good news: they hang onto it for a lot less long too.  Imperial College’s Heather Graven made the discovery by using the high levels of radioactive carbon - also called Carbon-14 - added to the atmosphere by the atomic bomb tests of the 1950s and 60s. By following this radioactive signature out of the air, into plants and back out into the environment, she was able to produce much more accurate figures for the carbon cycle, which will come in very handy when we’re making future predictions about climate change and the impact of greenhouse gas emissions…

Heather - We were studying how quickly plants take up and release carbon, using radiocarbon that was produced by nuclear bomb testing in the 1960s. So radiocarbon is an isotope of carbon and it's naturally occurring, but the bombs produced extra radiocarbon. So we were able to see how quickly plants took up the extra radiocarbon from the air in the 1960s. We found that plants take up carbon more quickly than we thought, but that also means the carbon is stored in plants for a shorter time because it's before it's lost. again, as the plants die or they lose leaves, branches or other materials. Plants are actually more productive than we thought. It also means that the removal of CO2 from the air to store in ecosystems which occurs naturally, but is also of interest for enhancing to reach net zero emissions targets, will actually not store the carbon for as long.

Chris - And how far adrift in our calculations were we, if we take what we thought was going on with the carbon budget before you came along with this research and then you've got your study, what's the disparity?

Heather - Yeah, so what we're looking at is the carbon that plants take in to form their tissues. So it could be wood, leaves, roots, or things like fruit or seeds. There's a lot of different ways that plants can use that carbon. So actually if you go out into the field and look at a particular plant it is hard to track all of that carbon. So previously there haven't been very many measurements like that. So the ones that have been taken were scaled up to come up with an estimate for all the plants on the globe. And so our approach is different in looking at this radiocarbon that was produced by the bomb testing and then seeing how quickly that was taken up. The results of our study suggests that the uptake of carbon into plants and their materials and products is about a third more than what was previously estimated.

Chris - Is that good news or is that bad news, Heather? The fact is the plants are taking up more CO2 than we thought. Does that actually mean that we've got more to play with from a climate change perspective? Or does it throw our sums totally out of the window?

Heather - Well, it's telling us that the carbon is actually cycling through the system more quickly. So the plants are taking in more carbon, but then they're also releasing more carbon. So then actually the carbon that's in the vegetation, in the plants, in their different tissues is actually not going to stay there as long before it comes back out again. So as the plant dies or the leaves or branches fall to the soil, that turnover is happening more quickly than we thought.

Chris - When people therefore say, well I'm going to do some carbon offsetting, I'm going to plant a tree or a million, is that therefore a flawed philosophy? Because not only are they going to release that CO2 more quickly than we first thought, they're also not going to lock it away for any long term period. Full stop.

Heather - So yeah, what we're saying is that the carbon isn't going to be stored for as long. So if the plant is taking up that carbon, then compared to what we thought previously, it's not going to be locked away as long as we thought. So that will likely kind of limit some of the potential of carbon reduction CO2 removal strategies that are trying to lock the carbon away in the ecosystem. And really emphasise that to limit climate change, we need to reduce fossil fuel emissions as quickly as possible.

Chris - Have you plugged your revised numbers into any of the models that are being used by organisations, climate scientists and theorists, planet wide in order to compute where we think the climate's going in the future in order to see how this might affect predictions that we're all working to.

Heather - So if we want to predict what's going to happen in the future, we really have to understand how those processes are taking up and storing the carbon. So from our study, we suggest that the carbon is taking up into the plants more quickly and then cycled more quickly. So we do need to take that into account in these future projections as well as how the carbon uptake in turnover is going to respond to changes in the climate, in future, and other environmental changes that are happening.