From the Paris climate agreement coming into force to the election of Trump, this month has seen both significant steps forward and substantial setbacks for global cooperation on tackling climate change. In light of these events, a 5-year comprehensive study on Arctic ecosystems and societies published on Friday, suggesting that we may be headed for uncontrollable changes, has reinforced the necessity for global action. As already highlighted throughout this blog, significant changes are taking place in the Arctic and there is a clear interconnectedness within and between Arctic biophysical systems, as well as the rest of the world. The Arctic Resilience Report outlines 19 potentially irrevocable tipping points that may already have been passed or will be exceeded soon.
A 'social-ecological systems' framework was applied, recognising that human and natural systems are closely intertwined and local interactions are embedded within wider regional and global dynamics. Although individual drivers may be predictable, the triggering of multiple, interacting drivers and feedback loops make the eventual outcome difficult to predict. Thus, in an uncertain future understanding thresholds and society resilience is fundamental.
Similarly to Arctic sea ice, the extent of Greenland Ice Sheet (GIS) melting was above average during 2016, ranking as the 10th highest in the 38-year satellite record. Summer air pressure was higher than average, a trend observed during the last several years, inducing drier and warmer conditions and surface melting, particularly along the western coast. Areas of darker ice with lower albedo, frequent along the west coast, were exposed acting to further enhance warming and ice sheet melting (NSIDC, 2016).
(1) Average daily melt area anomaly for Greenland comparing melt area in each year to the 1981-2010 average
(2) Cumulative melt day area for 2015 and 2016 against the highest ice melting extent years (2007, 2010 and 2012) (NSIDC, 2016)
Why is the GIS so important?
The GIS covers most of Greenland (~81%) and at around 1.7 million km2 is the second largest ice body, following the Antarctic Ice Sheet. The biggest concern over Greenland is that the complete melting of the ice sheet could cause a 7m+ sea level rise, enough to submerge 1.14 million km2 of land with populations of 375 million people.
However, sea level rise is not the only major consequence of the ice sheet melting. A fast rate of melting would produce large quantities of freshwater in the North Atlantic on top of the heavier salt water. Such an alteration in sea salinity could depress the Gulf Stream and fundamentally alter ocean circulation (Driesschaert et al., 2007). Atmospheric circulation would also be liable to significant change, with declines in heat transfer from equatorial to polar regions because of reduced global temperature difference.
Response to warming
Mass loss of the GIS is occurring through two main mechanisms: integrated surface mass balance decreases (difference between surface accumulation and ablation) and glacial acceleration inducing increased discharge (Fyke et al., 2014).
Furthermore, with the enhanced melting of the GIS, an increase in the formation of supraglacial lakes (meltwater in the surface depressions of an ice sheet) during the melt season has been apparent over the past two decades. The presence of supraglacial lakes leads to a reduction in albedo and, acting as a positive feedback, enhances surface melt (Ignéczi et al., 2016). Water reaching the ice sheet base through the drainage of these lakes transports warmth to the base of the ice sheet, enhancing basal sliding and ice velocity (Johansson et al., 2013).
The video below nicely summarises the mechanisms contributing to the decline of the GIS and the observations that have been recorded over the last few decades:
The carbon cycle
In addition to releasing vast quantities of freshwater, the melting of the GIS is exporting many nutrients, including dissolved silica. Dissolved silica is essential in sustaining diatom communities, which act as a carbon pump in the oceans. Meire et al. (2016) recently investigated the extent of dissolved silica export through the physical and chemical weathering by glaciers. Previously considered inactive in the global silica cycle, meltwater from the GIS was shown to strongly enrich surface waters with silicate. An increased supply of between 20-160% to coastal areas was predicted by the end of this century.
The decay of the GIS may also have carbon cycle feedbacks counteracting the reduction in CO2 caused by potentially higher ocean primary productivity. Ryu and Jacobson (2012) investigated the significance of glacial meltwater in transporting inorganic and organic carbon to the oceans and atmosphere. They found that, although rather modest compared to other sources, rivers draining the GIS were contributing to increased CO2 in the atmosphere. It is likely that soils, vegetation, and microbial metabolism have created this carbon reservoir beneath the GIS. Scenarios inferred by simple models suggest that CO2 released from the GIS could act as a rapid positive feedback mechanism. However, many of the parameters used are presently highly uncertain, for example the size of the CO2 reservoir beneath the ice sheet, rate of chemical weathering, and timing and trend of melting. Uncertainties are also apparent in the scaling of carbon fluxes measured from the Akuliarusiarsuup Kuua River (drains the Isunnguata and Russell Glaciers) to the entire GIS.
A quick update on Arctic sea ice extent that I talked about at the start of the blog. During the second half of October, Arctic sea ice extent set new daily record lows, averaging 6.4 million km2 (2.55 million km2 below the 1981-2010 average) because of unusually high air temperatures and raised sea surface temperatures. This month's data is yet another stark indicator of the extent of Arctic sea ice demise. However, a mild winter may not necessarily result in record-lows next summer, as the dominating weather patterns at the time determine the amount of melting ice (NSIDC, 2016).
Before delving into terrestrial ecosystem responses, it is important to understand the distinct biomes that characterise Arctic environments. Arctic tundra covers around 20% of the Earth's surface and typically has low biotic diversity. It is characterised by plants with shallow root systems including low-growing shrubs, mosses, lichens and grasses that can survive in the shallow active soil layer of thawed permafrost during summer months. The Arctic treeline, marking the boundary between taiga and tundra, represents the northern limit of tree growth and is a sensitive indicator of climate variability.
A warming Arctic climate is inducing earlier thawing, greening and longer growing seasons of tundra plant communities and is likely to promote the advancement of tundra into the polar deserts. Shifts in plant assemblages can influence productivity, nutrient cycling, active layer depth, decomposition, snow distribution and surface albedo. Elmendorf et al. (2012) quantified the climatic effects on tundra ecosystems by investigating responses to experimental warming over time periods of up to 20 years at 27 different locations. The results illustrated increased canopy height, amount of dead material and abundance of shrubs and graminoids (herbaceous plants with grass-like features). On the other hand, species diversity and the abundance of lichens and mosses declined in correspondence with warming.
What's also apparent in this study is the spatial and temporal variation in ecosystem responses. Temporally, the effects of rising temperature influence a range of ecosystem processes including photosynthetic rates, soil organic matter, and biogeochemical cycling and thus the direction and extent of climate change responses vary over time. Spatially, the tundra biome covers a range in average summer temperature of more than 10°C and an array of moisture contents from wetlands to polar desert, with variations in nutrients, organic matter content, pH, and herbivore communities. For example, caribou and muskoxen herbivory counteracts the positive effect on shrub expansion that warming has, favouring the growth of graminoids (Post and Pedersen, 2008). There are also regional variations in the structure and composition of species, for example, vascular species are more abundant at warmer sites and the shrub canopy is generally taller and denser.
Despite the comprehensiveness of the study and agenda to investigate spatial variation in ecosystem responses, sites in the Siberian tundra are lacking. This has led to a bias towards Canada, Greenland and northwest Europe, neglecting a significant tundra environment. In addition, a substantial amount of unexplained variation exists regarding effect size, particularly for long-term impacts.
The position of the Arctic Front (the southern boundary of Arctic air) during the summer determines the position of the northern treeline. Therefore, ocean-atmospheric phenomena and teleconnections are important determinants of the northern treeline (Sulphur et al., 2016). The increase in productivity that has been recorded in correlation with Arctic warming is also linked to the range expansion of birch, willow and alder. This shift to taller, darker canopies, with reduced albedo, and elevated evapotranspiration rates, will act as a positive feedback to warming. It is estimated that boreal forest coverage can reduce surface albedo by 25-50% compared to Arctic tundra (Macdonald et al., 2008).
In addition, the areal coverage of northern terrestrial ecosystems is a fundamental component of the global carbon cycle, influencing the Earth’s radiative balance by acting as a sink. Although expansion of woody species that are more biologically productive, creating more litter, significantly increases the carbon content of above-ground biomass, this may be modest when compared to the potential loss of carbon stored in tundra soils. An increased abundance of fungi associated with increased forest cover, potentially enhanced carbon cycling with litter from new plant species, and increased winter soil temperature may enhance the decomposition of older carbon stores (Parker et al., 2015).
The projected contraction of tundra and expansion of taiga will evidently have major impacts on Arctic environments and produce wider global feedbacks, the extent to which will vary on a spatial and temporal scale.
We're all familiar with the concept of a carbon footprint and how our energy usage contributes to rising CO2 emissions, however, new research has put this accountability into context by calculating individual contribution to Arctic sea ice melt. The researchers estimate that a tonne of CO2 contributes to summer sea ice loss of 3m2. This translates to the melting of 24m2 of Arctic sea ice for the average UK resident. What's more, this research calls into question the 2°C global warming target, suggesting that the extra 1000 gigatons of CO2 released with 2°C warming would be enough for Arctic summer sea ice to disappear. The 1.5°C target agreed at COP21 would make the survival of Arctic summer sea ice a more realistic prospect.
Despite the climatic fluctuations characterising past Arctic climate, recent warming over the last 150 years has been unprecedented. It has exceeded warming in other regions and even that experienced during the Pleistocene-Holocene transition (which concurred with large-scale vegetational shifts and faunal extinctions in the Arctic). However, the impacts of climate change on Arctic ecosystems has received relatively little attention compared with tropical, temperate and montane biomes (Post et al., 2009). In looking at the impacts of climate change in the Arctic, I will begin at the local and regional scale, investigating how climate change is affecting lake ecosystem structure and functioning.
Forcings, thresholds and regime shifts
Natural ecosystems respond to a number of external forcings and are sensitive to any changes. However, this response is not necessarily linear and instead alternative stable states may exist, which can be reached when changing conditions are sufficient to force the system past the threshold. What's also interesting is that reversal back to prior conditions may not return the ecosystem to its previous state, instead, the ecosystem must be pushed back beyond another switch point. In addition, gradual changes may not directly cause change but reduce the resilience of the system to deal with perturbation and shifts can then be dramatic and unexpected (Scheffer et al., 2001). This leads to the question, how have arctic lake ecosystems responded to climatic changes? - Has warming reduced system resilience and have any irreversible shifts occurred?
The remoteness of Arctic lakes, as well as the sensitivity of the region to warming, makes their ecological components particularly useful indicators of climatic warming. Both directly and indirectly, lake ecosystems respond to climate warming and integrate terrestrial catchment responses, thus acting as 'sentinels' of change (Adrian et al., 2009).
Declines in ice cover have increased light penetration and lengthened the overturn period in spring, increasing turbulence and nutrient cycling and the availability of pelagic habitats (van Donk and Kilham, 1990). As a result, diatom community changes in response to warming have been recorded across Russia,Greenland and Finland, with a distinct shift from benthic taxa (e.g. Achnanthes and Fragilaria)and tychoplanktonic species (e.g. Aulacoseira) towards planktonic genera (e.g. Asterionella and Cyclotella) (Rühland et al., 2008). In addition, small-celled diatoms (e.g. Cyclotella) with high surface area to volume ratios are increasingly favoured as warming strengthens thermal stratification leading to declines in turbulence and upward nutrient transportation during the summer months (Winder et al., 2009).
Hobbs et al. (2010) investigated diatom compositional changes across 52 lakes in Greenland and North America from the Little Ice Age (~1550-1800) to the present day. Diatom β-diversity was implemented to capture the community turnover and the results indicated high rates in the 20th century that has not previously been seen. This indicates that while natural climatic changes have previously not induced regime shifts, recent environmental factors (climatic warming and nitrogen deposition) have driven dramatic alterations in community dynamics. Lower β-diversity values in regions that have experienced lower rates of warming suggest climatic warming is the primary driver. It is evident that thresholds have been exceeded with regime shifts toward new ecological states.
Cascading effects
Intensifying thermal stratification and subsiding turbulence will continue to disfavour diatoms, as their cellular requirements and silica frustules mean that their cell size is restricted to 2-4μm. Thus, phytoplankton species adapted to such conditions will become increasingly favoured (Winder et al., 2009). As diatoms contribute 20-25% of primary production global, such a significant decline in abundance could have dramatic ecological implications (Rühland et al., 2015).
