Arctic's Ticking Time Bomb

Frozen soils in the Arctic prevent the decomposition of organic material, trapping almost 1700 billion tonnes of carbon (more than twice the amount present in the atmosphere). Climatic warming threatens to transform permafrost soils from a carbon sink to a source as permafrost melting will release methane and CO2, initiating a positive feedback mechanism. While previous models suggested abrupt transferrals of carbon to the atmosphere from melting permafrost, new research points towards more gradual and sustained release (Schuur et al., 2015).

Thawing permafrost

While it's normal for the top 30-100cm of permafrost to thaw during Arctic summers, warming is leading to the gradual deepening of the seasonally thawed soil layer. In addition, the creation of thermokarst landscapes when high ice content permafrost soils thaw leads to soil collapses and upward water seepage. The combined influence of active layer deepening and thermokarst creation initiates enhanced organic carbon erosion and mineralisation into CO2 and methane. However, the role of permafrost thaw through thermokarst initiation has previously not been incorporated into models projecting future greenhouse gas release in arctic regions, underestimating the permafrost carbon feedback. 

Melting of ancient permafrost

Thermokarst lakes

Olefeldt et al. (2016) used existing spatial data on the northern boreal and tundra permafrost region to estimate that thermokarst landscapes cover ~20% and act as a store for up to half of its soil organic carbon. In interpreting the maps and data it must be considered that data for certain landscape characteristics (e.g. ground ice content), although spatially extensive, varied in quality and resolution. Hence, the heterogeneity present in reality is not fully captured. Despite this, the results are an important advancement in evaluating the large-scale impacts of thermokarst expansion as a result of climate change.

Types and extent of thermokarst landscapes in the northern boreal and tundra permafrost region (Olefeldt et al., 2016)

Past extreme warming events

Hyperthermals (extreme warming events) occurred around 52-55.5 million years ago superimposed on a long-term warming trend. The Palaeocene Thermal Maximum (PETM) is the largest of these events, in which global temperature increased by ~5°C in a few thousand years. Simulations indicate that organic carbon decomposition in Arctic permafrost, triggered by orbital forcing (high eccentricity and obliquity), corresponds with the timing of hyperthermals. The rapid recovery in temperature following each event is likely to be a result of the replenishment of organic carbon in permafrost soils. Following the PETM, declining permafrost areal extent resulted in a diminished carbon stock and hence smaller hyperthermals thereafter (DeConto et al., 2012).

The High Arctic as a C sink

With increasing temperatures, deeper soils will develop in Arctic regions (e.g. the High Arctic and polar deserts) that were previously characterised by low productivity and lacked liquid water. This could therefore enhance C sinks in these areas (McGowan et al., 2016). 

In addition to temperature, the amount and frequency of precipitation is an important control of the carbon dynamics through alterations in soil water availability and temperature distribution. Heat is delivered to the permafrost table by percolating water, which influences microbial processes at the interface between the active layer and permafrost. Using measurements of C fluxes and sources in northwest Greenland, Lupascu et al. (2014) demonstrated that while warming alone decreased the C sink strength by up to 55%, warming combined with enhanced precipitation reduced loss of old carbon. Thus, there is a potential for parts of the High Arctic to remain strong carbon sinks, partly counteracting the expansion of C source regions at lower latitudes.