環境：2023年のカナダの山火事による過剰な炭素排出

Carbon emissions from the Canadian wildfires of 2023 exceeded the annual fossil fuel emissions of seven of the ten largest emitting countries in 2022, a study published in Nature reports. Only China, India and the USA emitted more fossil-fuel based carbon in that period. The findings add to concerns about the ability of forests to act as a long-term carbon sink.

The wildfires that burned across Canada in 2023 destroyed over 15 million hectares of forest, approximately 4% of the country’s total forest area. The total burned area was seven times the average of the preceding 40 years. Canada’s forests are a carbon sink and absorb more carbon from the atmosphere than they release. How these forest fire events affect the long-term carbon sink remains uncertain.

Brendan Byrne and colleagues quantified the amount of carbon released by the fires from May–September 2023 using satellite-based observations of carbon monoxide in the smoke plumes. They estimate that the fires released 647 teragrams of carbon (a teragram is one trillion grams), exceeding typical Canadian forest fire emissions (average estimates in the past decade range from 29–121 teragrams). The authors suggest that hot, dry weather was a major driver of fire activity and note that 2023 was the warmest and driest year on record since 1980. While the temperatures experienced in Canada in 2023 were extreme compared to the historical record, Byrne and colleagues note that even under a moderate emissions scenario (SSP2–4.5) these temperatures are projected to be typical during the 2050s. These conditions are likely to drive an increase in fire activity and could suppress carbon uptake by the forests, leaving their role as carbon sinks in doubt.

The authors suggest that to meet climate mitigation targets, the amount of allowed anthropogenic emissions may have to be adjusted to compensate for reduced carbon uptake by forests, impacting Canada’s carbon budget.