Canada's wildfires released unprecedented amount of carbon

A new study has found that carbon emissions from the Canadian wildfires of 2023 exceeded the annual fossil fuel emissions of seven of the ten largest emitting countries in 2022. Only three nations: China, India and the USA emitted more fossil-fuel based carbon in that period. The study - which has been published in Nature - was carried out by Brendan Byrne at Nasa’s Jet Propulsion Laboratory.

Brendan - Last summer we saw that there were these really enormous fires in Canada that went through the entire summer. And so we wanted to estimate how much carbon was released from all those trees burning. And in addition to that, we wanted to understand why this happened in 2023 and not some other year. So we also looked at climate data to see if there was something unusual, in particular about 2023.

Chris - How do you go about doing a study like this to actually get numbers that are a reasonably good guesstimate of what happened?

Brendan - When fires burn, they release a lot of smoke and aerosols into the atmosphere. And one of the things they release is carbon monoxide, which is produced by fires. There are satellites that measure the amount of carbon monoxide in the atmosphere. So what we did is we took satellite data of the amount of carbon monoxide emitted by the fires and then we were able to use that to back calculate how much carbon must have been emitted when they burned.

Chris - Were you watching one particular patch of space then, or do you assume that the carbon monoxide is only around for a little while? So anything that gets added is just been added and you can measure the background level everywhere and that gives you that approximation.

Brendan - Well, the nice thing about carbon monoxide is that, in general, forests are not releasing very much when there isn't a fire. It's really a signal of fires burning. And so from there you can really see it kind of like you would see a smoke plume coming off the fires. So there's pretty low background levels.

Chris - Does it hang around for long? Because how good a register is it of what's happening here and now? Because if you produce some carbon monoxide, does it not sort of go up and then saturate in the atmosphere and then you lose what else goes on since? Or does it go up and come down quite quick, which means you really have got a moment to moment index of what is happening on the ground?

Brendan - The nice thing about carbon monoxide is that it lasts for about a month in the atmosphere. It's converted into CO2 through chemistry in the atmosphere. So it doesn't have the real long lifetime of CO2, which gets really mixed into the background or the very short lifetime of ozone, which is just there momentarily. So it's kind of a perfect lifetime to be able to track fires with.

Chris - And what did that reveal then about how much you think came off from Canada when the fires raged?

Brendan - Yeah, we provided an estimate range of 570 to 727 megatonnes of carbon. I think this is better contextualised when you look at a country's fossil fuel emissions. So this would've ranked as the fourth largest emitting country after China, the US, and India. So we saw really unprecedented amounts of carbon being released from these fires over Canada.

Chris - We always regard trees as a good thing to have. We plant trees, they soak up CO2. But then if you have events like this, you liberate all of the good work that's been done and put the CO2 back into the atmosphere. Is that sort of what was in your mind when you were doing these calculations?

Brendan - Yes, to an extent. Everything else being equal, if you have a forest fire, you'll release a bunch of carbon, but that forest will regrow over the coming decades. So you'd expect it to reabsorb all of that carbon, but we may not see that with climate change.

Chris - In what respect?

Brendan - When we looked at this event, we found that these really extreme emissions coincided with the hottest and driest year for Canadian forests in the climate records we looked at, that were 44 years long. And so, you know, we're seeing under hotter and drier conditions that will potentially have more extreme fires.

Chris - So more fires, but potentially also less regrowth then if the conditions are more stressful for the trees and they're not going to lock away as much CO2 as normal. So it's potentially a double whammy then?

Brendan - Yes. So what we see is that, you know, this year was very unusual with the heat and drought relative to the 44 year climate record. But if we look at climate model projections, by the time we get to the 2050s, we find that this becomes kind of a normal summer temperature-wise for Canada. So we may potentially see a shift to having more frequent intense fires that could potentially change how the ecosystems are able to store carbon.