I had a request from Richard Betts to do a cartoon on this paper in Nature about soil CO2 emissions. The abstract says soil emits "60 petagrams of carbon per year to the atmosphere as carbon dioxide”.
It made me wonder why they talked about 'carbon' and not 'carbon dioxide' - after all, carbon is not a greenhouse gas. And why use petagrams and not gigatons, or is it gigatonnes?
I found out that a petagram is the same as a gigatonne or metric gigaton, but not a UK or US gigaton. Also the reason scientists use 'carbon' and not 'carbon dioxide' is that they are referring to the carbon cycle (carbon as fossil fuel burns to create CO2 which is reabsorbed by plants and converted back to carbon).
But between the scientists, the politicians and the media it is easy to get a bit confused. For example at Information is Beautiful you get this lovely infographic where they talk about 39 gigatons(!) of CO2 but call it ‘our carbon budget’. Their actual number for a 'carbon budget’ should be 10.6 gigatonnes not 39 (though they do note the confusion in a footnote). Richard Betts says the amount of carbon in manmade CO2 emissions from fossil fuels are, in fact, 8 gigatonnes per year.
When you look at the data behind the infographic they add to the confusion by talking about 'carbon emissions’.
Back to the Nature paper, we learn that "the response of soil microbial communities to changing temperatures has the potential to either decrease or increase warming-induced carbon losses substantially.”
Add this to the 60 petagrams of soil emissions, which dwarfs the manmade emissions of 8 Petagrams, and you have a lot more uncertainty - of the natural kind.
Many thanks to Richard Betts and Nic Lewis for helping with the research, educating me and correcting mistakes.