Around half of the world’s forest areas consist of conifers – and they are also increasingly affected by forest fires. This applies even in relatively northern climes, as demonstrated, among other things, by the extensive forest fires in Canada in 2023. But even when the fires are over and the forest appears to have recovered, long-term consequences remain. A study now shows that they continue to influence the region’s climate decades later.
Forest fires in the boreal coniferous forests of North America, Siberia and Northern Europe are increasing in number and are also becoming more severe. An example of this was the devastating wildfires in Canada in 2023, in which 140,000 square kilometers of forest went up in flames. Such fires not only release immense amounts of CO2 that was previously stored in the wood, they also cause damage to vegetation and landscapes that will last for years.
Climate effect even decades later
A team led by Manuel Helbig from the GFZ Helmholtz Center for Georesearch has now analyzed how the consequences of forest fires in North American coniferous forests affect the local climate. To do this, the team analyzed satellite data from 142 areas that were affected by forest fires between 1928 and 2014. The satellite data and additional data from ground-based measurements provided information about, among other things, the temperature of the soil and air, the density of vegetation and the heat exchange between the forest floor and the atmosphere.
It turned out that in the first decade after the forest fire, the surface temperature of the affected areas during the snow-free period at midday was 4.7 degrees higher than that of untouched forest areas, as Helbig and his team determined. Only in the fifth decade after the forest fire did the surface temperature increase by less than one degree. The reason for this warming effect is a reduced air exchange because the forest fire causes tree tops to be missing or not yet fully developed. This reduces the surface roughness of the forest and leads to less air turbulence, which causes the earth’s surface to heat up more, as the team explains.
Effect also on evaporation and albedo
Another factor is evaporation: it initially drops drastically immediately after the fire, only to gradually increase again over three decades because the leaf density of the regrowing forest increases. As a result, some of the evaporative cooling is missing in these areas and this also contributes to their warming.
In winter, however, the long-term effect of the forest fires is reversed: the affected areas in Canada were on average 0.02 degrees colder than the untouched forest areas, as Helbig and his colleagues found. They see the primary cause as the increased albedo of the partially regenerated areas: Because snow can cover the regrowing bushes and young trees better than a fully grown forest, these areas reflect sunlight better than an intact forest. This means they absorb less heat and stay cooler.
There is a threat of positive feedback
Taken together, these results show that forest fires continue to influence the local climate decades later. In particular, the warming effect in summer could further increase the effects of climate change: “For a scenario with a sharp increase in the area burned, we estimate that the annual warming caused by fire will increase by a third by 2050,” write the researchers. This in turn could encourage further fires. “Our studies also make it clear how important it is to reduce greenhouse gas emissions globally. By accelerating global warming, they also increase the risk of forest fires and thus the thawing of permafrost and the release of further carbon dioxide and methane from the soil,” explains Helbig.
Source: GFZ Helmholtz Center for Georesearch; Specialist article: AGU Advances, doi: 10.1029/2024AV001327