Scott Goetz/Woods Hole Research Center
Scientists are warning that intense wildfires in the northernmost areas of North America are changing the composition of the tundra ecosystem, degrading permafrost and contributing to a northward migration of trees, all of which have serious implications for the future of the climate.
Warming air masses resulting from climate change create the conditions for intense forest fires in the cold north, explains Scott Goetz, a senior scientist and deputy director at the Woods Hole Research Center in Massachusetts.
The severity of the fires is an important factor in determining how forests change in the future — and forests in the Arctic are changing in significant ways.
First, the hot air masses and the intense fires they cause reduce the productivity of the forests: they simply don’t grow as well during very hot summers, Geotz says.
“We call it ‘browning,’ which we can observe in satellite images — in another words, declining productivity trends,” he explains. Goetz and others have mapped the ‘browning’ over a large area of northern forest that extends from the central part of Alaska, and across the southern range of boreal forest in Canada.
Scott Goetz/Woods Hole Research Center
But not everywhere is browning, Goetz says, which is the second big change. In the higher Arctic, a lot of greening is taking place — that is, increases in shrub and tundra vegetation growth. In addition, there is pretty good evidence for the northward movement of trees. Even conservative climate models indicate trees will continue to migrate northward over the next 30 to 50 years from their current range into what is currently tundra vegetation, Goetz says.
“There's a lot of interest in keeping track of this because it has large implications for climate, in terms of feedbacks to climate from changes in the surface energy balance related to the density of trees or shrubs,” he explains.
Since trees are much darker than other vegetation in the northern regions, they absorb a lot more solar energy. Trees retain this energy, which in turn warms the surface. While the presence of trees does help to remove some carbon from the atmosphere, the net effect on the climate is different from what it might be in southern areas, according to Goetz.
“In the tropics, it's almost certainly true that more trees are a good thing, because they take a tremendous amount of carbon out of the atmosphere and regulate the climate system,” Goetz explains. “In northern regions, the presence of trees removes some carbon from the atmosphere, but [because] they’re not highly productive systems, what really matters [in these areas] is how the trees change the energy balance.”
Intense fires in the north also have adverse affects on the soil and, even more alarming, on the condition of permafrost. During intense fires, the soil itself actually burns, which changes what comes back after the fire.
“In North America, we often see that [burning soil] produces a shift from what is currently an evergreen conifer forest to a deciduous forest that persists for decades,” Goetz says. “Again, that changes the energy balance; it changes the amount of water [the forests] transpire to the atmosphere; it changes nitrogen nutrient cycling; and it changes the whole productivity of the system — and that, again, persists for decades.”
Permafrost degradation as a result of intense forest fires is even more concerning. “Fire has great potential to degrade permafrost much more rapidly, because it's burning off the layer of soil and peat that insulates the permafrost — these frozen ground areas — from the atmosphere,” Goetz says.
Scientists estimate that northern permafrost stores more than twice the carbon than is currently in the Earth’s atmosphere. Loss of permafrost combined with severe fires can produce the equivalent of another United States, in terms of total emissions from fossil fuels.
Reducing theses harmful trends begins with making policymakers more aware of their enormous ramifications, Goetz says.
“I think it needs to get on the radar screen of the policymakers and enter the discussions of how to mitigate additional climate change and to cut fossil fuel emissions,” Goetz says.