Carbon Loss from an Unprecedented Arctic Tundra Wildfire

As increased frequency and severity of wildfires in historically fire-prone areas pose one set of threats to our ever-more concerning climate situation, scientists have identified a new threat to rising global atmospheric CO₂ levels. Not since the early Holocene epoch has there been any significant wildfire activity or the presence of typical fire regimes within the Arctic tundra biome. As global temperatures rise, changing climatic conditions have introduced wildfire-induced carbon (C) releases in the Arctic tundra that have not been observed in many millennia (perhaps 10,000 years or more). Mack et al. (2011) examined the Anaktuvuk River fire that burned 1,039 km2 of Arctic tundra on the North Slope of the Brooks Range in Alaska, USA, in 2007; this single fire burned more than double the cumulative area burned in the region over the past half-century. They concluded that the C released from this one fire supports the hypothesis that tundra fires have the potential to significantly amplify global warming through the release of concentrated C pools into the atmosphere that in some cases are thousands of years old. –Lindon Pronto

Mack, Michelle C., Bret-Harte, M. Syndonia, Hollingsworth, Teresa N., Jandt, Randi R., Schuur, Edward A. G., Shaver, Gaius R., Verbyla, David L., 2011. Carbon Loss from an Unprecedented Arctic Tundra Wildfire. Nature 475, 489–492.

Mack et al. report that the Anaktuvuk River fire burned 1,039 km² removing 2,016±435 g Cm−2 and 64gNm−2 (or about 400 years of N accumulation) from the ecosystem, an amount they say is two orders of magnitude larger than annual net C exchange in undisturbed tundra. Furthermore they report that “the approximately 2.1 teragrams of C [released] into the atmosphere, was an amount similar in magnitude to the annual net C sink for the entire Arctic tundra biome averaged over the last quarter of the twentieth century.” Approximately 60% of this C loss was from soil organic matter. Radiocarbon dating of residual soil layers showed the maximum age of soil C that was lost in the fire, was 50 years old.

The study area was underlain by permafrost and had a mean annual temperature of –10°C and an average yearly precipitation of 30 cm. The pre-fire vegetation composition of the study area was 54% moist acidic tundra (MAT), 15% moist non-acidic tundra, and 30% shrubland. The study focused on the MAT classification because of its wide distribution and because it had a higher immediate survivability than the other fuel types and therefore was able to provided a benchmark of pre-fire soil organic matter depth and plant biomass. Eleven MAT sites outside the burn area and 20 MAT sites within the burn were sampled. Sites were tested in order to compare pre-fire soil organic layer depth and depth versus bulk density, C or N concentrations and to determine the radiocarbon date of the post-fire soil surface to see whether the fire burned into old and likely irreplaceable soil C pools. The objective was to observe the soil C and N content approximated at pre and post-fire locations to determine the emissions of the particular fire event and to put the results in context with a broader understanding of tundra biome historical norms and characteristics of the climate.

Independent of the transfer of C from the tundra soils to the atmosphere is the threat that any significant disturbances by wildfire have the potential to change local thresholds and alter the ecosystems structure and function through the alteration of surface reflectance (albedo) and energy balance of landscapes that are underlain by permafrost. For example, lake sediment cores showed that there was no observable wildfire activity within the study area over the past 5,000 years. A wildfire event of the magnitude of the Anaktuvuk River fire, has the potential to destabilize the underlying permafrost allowing it to release additional C into the atmosphere during the subsequent decomposition process (as a result of exposure), and adding significant potential for contributing to positive feedback to high-latitude warming. An additional important consideration are the increased concentrations of C stored at increased depths in peat soils, for when drying does occur in this fuel type, the fire doesn’t only burn in a greater radius but can do more vertical damage as well in the semi-combustible soils; an image of the burn scar conveys this phenomenon very well. In this study in particular, though there were areas that had a soil depth range of 12.3–43.3 cm, the maximum burn scar areas were no greater than 15 cm in depth.

Mack et al. conclude that even a single surficially burning wildfire in the Arctic tundra biome can offset or even reverse biome-scale C uptake. Furthermore, C released into the atmosphere from fire occurs at a rate of 30–50 times greater than C release through natural decomposition mechanism such as, for example, stimulated soil organic matter decomposition from a 5°C increase in mean annual temperature. One possible implication raised by the authors is that changing local thresholds may lead to succession patterns that replace the current biome organic soil and vegetation composition with more shrubs. Such a shift would have the potential to …“trigger additional positive feedbacks to climate warming because shrub-dominated ecosystems have higher productivity and plant biomass offset by lower soil C stocks.” Although scientific knowledge and experience with the effects of fire in the Arctic tundra biome are very limited, an increase in this phenomenon has led studies such as this one to conclude that the possible near-future effects of fire in the Arctic can have catastrophic implications for atmospheric carbon levels as well as terrestrial carbon capturing and storing. As seen in the last 20 years, this dangerous positive feedback system of climate change is accentuated—from the high latitude warming resulting in melting snowpack and permafrost, retreating sea ice, to the drying-induced fire having varying consequences from albedo loss to instantaneous mass C releases from age old stocks.