Wildfires and the Global Carbon Cycle

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This year marks the first time that there have been three consecutive years of record-breaking hottest temperatures on Earth. You don’t need to look any further than a quick google search to find headlines such as “99 Percent Chance 2016 Will Be the Hottest Year on Record” (Scientific American) or “We Just Broke the Record for Hottest Year Nine Straight Times” (The Guardian). So it is no surprise that wildfires are having a heyday in 2016, with Canada’s Fort McMurray wildfire arguably as the “poster child” in Canada, and more recently several smaller fires in Southwest Nova Scotia, close to Eosense’s home turf. This isn’t likely to be an anomaly for 2016 as Natural Resources Canada states that:

Fire-prone conditions are predicted to increase across Canada. This could potentially result in a doubling of the amount of area burned by the end of this century, compared with amounts burned in recent decades. Boreal forests, which have been greatly influenced by fire through history, will likely be especially affected by this change.

About 40% of the global carbon stores are found in northern latitude biomass and soils; boreal forests being a large part of this (Mack et al. 2008). It is well known that fires in these areas are an important part of the natural cycle, however, human-induced climate drying and warming are exacerbating this natural cycle, with increased burn area sizes and burn severity (Alexander and Mack 2016; Beck et al. 2011; Turetsky et al. 2010).

There is concern about the positive feedback loop of increased fire frequency and fire-driven greenhouse gas emissions (i.e. CO2) that ultimately go on to ‘feed the fire’ by absorbing energy in the atmosphere and further changing climate. Everyone is well aware that when trees burn they release CO2 (and a variety of other emissions) but the typical assumption is that, as the forest regenerates, the same amount of carbon is re-stored by the ecosystem and therefore the fires have a net-zero effect. However the story is, not surprisingly, more complicated than that. In this post we’re going to look at work by a few researchers who are studying the effects of fire on carbon in forest and tundra ecosystems and try to get a better handle on what should be considered when evaluating the effects of these fires on global climate.

We looked at publications by leading researchers and found that there were two common points amongst the researchers on whether the “net zero” effect exists in these fire-affected ecosystems. The first and most direct point is that the carbon neutrality of the forest after the fire really depends a lot on which species of plant regenerates in that area. Since different plants have a different storage capacity for carbon, the effect is often not the same if a burned Spruce forest is replaced with Aspen, for example.

Second is that the carbon stored in soils is an often unconsidered component, but in areas where intense fires burn the soil litter and organic layers, it can take hundreds of years for the carbon stockpile to regenerate. Not surprisingly there is also a role for the tree species in the re-accumulation of soil organic matter, where if deciduous species are dominant after the fire the organic matter buildup is slow because the litter from these species is very easy for micro and macrofauna in the soil to use as a food source.

So the short answer is that the “net zero” assumption is (not surprisingly) too simple and generally the forest and landscapes that get burnt tend to behave as net sources of greenhouse gases for many years after the fire has occurred. That being said, there are a lot of factors (including human influences) that determine when and how these landscapes are regenerated and therefore ultimately affect the local greenhouse gas balances into the future. Now we’re going to take a short look at an often overlooked ecosystem that also experiences fires.

Burn scar from the recent fire in the Yukon-Kuskokwim Delta region. Photo by Jocelyn Egan.

Burn scar from the recent fire in the Yukon-Kuskokwim Delta region. Photo by Jocelyn Egan.

As Canadians, we are lucky to have a lot of forests and most Canadians can remember a large forest fire from their childhood (or adulthood), but we often don’t think about the fact that other types of ecosystems also frequently burn. As an example of this, about 40% of Canada’s land mass is above the tree line, and is dominated by Tundra ecosystems. These ecosystems often have fires as well, but are generally unheard of by the population because of the remote nature of the landscape. While there are no trees, a significant amount of biomass does get burned in the tundra as well but importantly fires in tundra environments can also directly and indirectly cause loss of permafrost leading to a longer term fire induced change in greenhouse gas emissions. FluxLab and Dalhousie University researcher Jocelyn Egan is working with a group from Woods Hole Research Center, looking at the effect of these tundra fires on greenhouse gas emissions in Alaska – here’s what she had to say:

The story becomes even more complicated in the Arctic, where most of the ground is underlain with permafrost, which contains large amounts of carbon. Wildfires are predicted to increase in frequency, severity and extent in the Arctic as the climate becomes warmer and dryer. The summer of 2015 was the second worst fire year on record in Alaska. A few of the 2015 Alaskan fires burned within the Yukon-Kuskokwim Delta (YK Delta), where more area was burned in 2015 than in the previous 74 years combined.

As a majority of arctic wildfire studies are centered in the boreal forest, this YK Delta tundra fire served as a great opportunity to examine the immediate effects of fire on carbon storage and export from the YK Delta; and by extension, to gain critical insights into how the carbon balance of arctic deltas will change over the coming decades In September 2015, a group of researchers from Woods Hole Research Center (Falmouth, Massachusetts) traveled to the YK Delta to take initial measurements of permafrost thaw depth and vegetation after the fire.

One year after the fire, in June 2016, I traveled with the WHRC group back to the YK Delta to measure terrestrial and aquatic carbon outputs associated with the burn. I was examining how the fire might have impacted the contribution of “old” permafrost carbon to the ecosystem, as fire can cause the removal of insulating soil organic layers, which increased permafrost thaw in the burn scar compared to unburned areas. We’ll be working on the results of this field work over the coming year to get a better idea what exactly happens in the tundra, so that we’re able to better account for this disturbance and apply it for a more thorough understanding global climate change.

Jocelyn will be presenting the preliminary results of this work at the 2016 AGU Fall Meeting in the session “The role of fire in the carbon cycle: emissions, fluxes and sequestration potential” (B084).

Sampling soil flux for Radiocarbon content in the YK Delta. Photo by Jocelyn Egan.

Sampling soil flux for Radiocarbon content in the YK Delta. Photo by Jocelyn Egan.