Permafrost thaw driven changes in hydrology and vegetation cover increase trace gas emissions and climate forcing in Stordalen Mire from 1970 to 2014

Abstract

Permafrost thaw increases active layer thickness, changes landscape hydrology and influences vegetation species composition. These changes alter belowground microbial and geochemical processes, affecting production, consumption and net emission rates of climate forcing trace gases. Net carbon dioxide (CO₂) and methane (CH₄) fluxes determine the radiative forcing contribution from these climate-sensitive ecosystems. Permafrost peatlands may be a mosaic of dry frozen hummocks, semi-thawed or perched sphagnum dominated areas, wet permafrost-free sedge dominated sites and open water ponds. We revisited estimates of climate forcing made for 1970 and 2000 for Stordalen Mire in northern Sweden and found the trend of increasing forcing continued into 2014. The Mire continued to transition from dry permafrost to sedge and open water areas, increasing by 100% and 35%, respectively, over the 45-year period, causing the net radiative forcing of Stordalen Mire to shift from negative to positive. This trend is driven by transitioning vegetation community composition, improved estimates of annual CO₂ and CH₄ exchange and a 22% increase in the IPCC's 100-year global warming potential (GWP_100) value for CH₄. These results indicate that discontinuous permafrost ecosystems, while still remaining a net overall sink of C, can become a positive feedback to climate change on decadal timescales. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

Keywords: Arctic; landcover; methane; permafrost; radiative forcing; remote sensing.

Figure 1.

(a) Our study site was Stordalen Mire located in northern Sweden. (b) Vegetation map overlayed on a WV-2 satellite image from 2014 [11]. The rectangle represents the unpersoned aerial system (UAS) training area for the 2014 vegetation analysis. The training area was then applied to the Palsa Mire area (outlined polygon) which represents the area from the Christensen et al. [1] and Malmer et al. [28] comparison datasets. (Online version in colour.)

Figure 2.

Landcover area (ha) for the Stordalen Mire (black outlined area in figure 1) for 1970, 2000 and 2014 (this study). Landcover specific areas for 1970 and 2000 are from Christensen et al. [1]. Numbers indicate the overall per cent change of each landcover class between 1970 and 2014 with negative values indicating a loss in landcover area and positive values indicating an increase in landcover area. (Online version in colour.)

Figure 3.

Fluxes of carbon by landcover types for (a) CO₂ (gC m⁻² d⁻¹) and (b) CH₄ (gC m⁻² d⁻¹) at Stordalen Mire. Positive values represent net emission from the land surface and negative values represent net uptake by the land surface. Previous study fluxes are from Christensen et al. [1] and Johansson et al. [32]. See electronic supplementary material, table S2 for values. (Online version in colour.)

Figure 4.

Annual CH₄ and CO₂ exchange and net ecosystem carbon balance (kg C yr⁻¹) and radiative forcing (GWP_100 in kg CO_2-equiv yr⁻¹) of Stordalen Mire, Sweden for 1970, 2000 and 2014.

Figure 5.

Time series of landcover, CH₄, CO₂ and net radiative forcing represented as g CO_2-equivm⁻²yr⁻¹ at Stordalen Mire, a permafrost peatland in northern Sweden for 1970, 2000 and 2014. See black outline in figure 1 for location. (Online version in colour.)