Warming Temperatures, Disappearing Ice

The Arctic has clearly undergone significant changes in average temperatures and sea ice extent during the last couple of decades but what does the future hold?

Overland et al. (2013) investigated Arctic climate over the next 30 years using recent climate model projections (CMIP5) for mitigation and business-as-usual scenarios. The results suggested that a seasonally nearly ice-free Arctic Ocean is highly likely before 2050. A mitigation emission scenario could lead to a 7°C temperature increase in late Autumn by the end of the century compared to a 13°C rise for business-as-usual. There is a large gap between modelled results and extrapolated sea ice loss from observations. However, what is clear, is that 'it is very likely that the Arctic climate will continue to show major changes over the next decades' with expanded areas of open water for longer periods, permafrost degradation and dramatic ecological consequences.

Difficulties in projections

Projecting future sea ice loss from global climate models comes with major difficulties and uncertainties, and this difference depends on the proposed timescale in question. Firstly, different models hindcast and forecast different results due to variations in formulas (sea ice physics, clouds, radiation, atmospheric and ocean dynamics) and parameterisation. Internal variability also means that results can differ from modelled near-term projections despite similar initial conditions. For the CMIP5 model used, 80% of the 56 ensemble 1979-2011 member trends are smaller than observed.

1966 to 2005 annual mean SAT based on observations (left) and the 36 CMIP5 models ensemble mean (right) (Overland et al., 2013)

In contrast, longer-term projections are heavily dependent on the future emission pathway used:

Monthly Arctic temperature anomalies for emission scenarios RCP4.5 (blue) and RCP8.5 (red) (Overland et al., 2013)

Sept 1900 to 2100 Arctic sea ice extent based on model simulations and compared against historical and observed records (Overland et al., 2013)

An Unseasonally Warm Christmas

Articles about an Arctic heat wave and record lows in sea ice have covered the news over the past month with many climate scientists linking the unseasonable weather to anthropogenically-induced climate change. During November sea ice extent was at a record low (1.95 million km² below the 1981-2010 average) and even declined by 50,000 km² during the middle of the month, an exceptional occurrence. 

While North Pole temperatures have been significantly higher than average and are expected to be up to 20°C higher than previous Christmas Eve temperatures, continental areas (Alaska and Siberia) have been unseasonally cold. This complies with the controversial idea 'Warm Arctic, Cold Continents' in which changes to the Arctic atmosphere (due to warming and sea ice loss) alter large-scale atmospheric patterns and effectively the cold and warm air masses are swapped.  

Daily Arctic sea ice extent for 2012-2016 against the 1981-2010 average (NSIDC, 2016)

Monthly November Arctic sea ice extent from 1978-2016 (NSIDC, 2016)

Arctic Pollution: Confounding Climate Change

After considering the impacts of Arctic climate change, one of the issues I wanted to cover was the extent to which confounding factors play a role in inducing ecosystem responses. Although modern conditions are comparable to the Holocene Thermal Maximum (HTM), the unprecedented nature of ecological shifts recorded during the 20^th century, for example in Arctic lakes, question the involvement of additional factors (Holmgren et al., 2010).

Despite being regarded as pristine wilderness; the Arctic is as vulnerable to pollution as industrial regions of Europe and North America. The long-range transport of atmospheric pollutants (sulphur and nitrogen, black carbon, aerosols, heavy metals and organic compounds) from the industrialised mid-latitudes mean that remoteness from direct anthropogenic activity does not guarantee that Arctic ecosystems will be unaffected.

We have already seen how, in addition to increasing GHG concentrations, aerosols and black carbon have contributed to the recent warming in the Arctic. In this post, I want to investigate how pollution, alongside climate change, can synergistically force ecological change.

Arctic haze is a powerful reminder of the extent of anthropogenically-released atmospheric pollutants

Arctic acidification

Ice cores retrieved from the Arctic reveal enhanced concentrations of the major acidifying pollutants during the late 20^th century. Impacts resulting from acidification have mostly been recorded on the Kola Peninsula, Fennoscandia and in the Taymir region of Russia. Following legislation implemented to counteract acidification of surface waters, SO2 emissions declined by ~67% in Europe from 1980-2000. As a result, the recovery of lakes has been observed in the Barents region, in particular on the Kola Peninsula.

Acidification in the Arctic is at most a subregional issue, impacting poorly buffered areas where acid deposition levels exceed the neutralising capacity of the system (AMAP, 2006). Interaction between pollutants increases the complexity in system response and may mean that ecological changes are not simply due to acidification. Kashulina et al. (2003) showed that the widespread environmental damage from nickel industries on the Kola Peninsula was spatially variable and toxic elements, in addition to SO2, acted as a driving mechanism of vegetation damage. With SO2 the dominant component of emissions and exceeding critical levels, previous studies linked ecosystem degradation in the region to the acidifying impact of SO2. However, evidence of widespread acidification is unconvincing and ecogeochemical mapping indicates a complex interaction between pollutants, with elements of low volatility potentially resulting in greater damage despite being emitted in much lower concentrations. Furthermore, in the study areas the increase in base cations (Na⁺, Ca²⁺, Mg²⁺, K⁺) exceeded the deposition of acid anions (SO4^2-, Cl^-) counteracting acidification.

Acid rain has also led to changes in Arctic terrestrial ecosystems

Nutrient enrichment

Although sulphur has previously been of major concern with its role as a primary acidifying agent, major reductions in sulphur emissions over the last decades and less significant declines in nitrogen has brought further attention to the impact of nitrogen deposition. Terrestrial and aquatic studies have investigated both the acidifying role of nitrogen and more recently concerns have grown over its eutrophying potential in these typically nutrient-limited ecosystems. 

Holmgren et al. (2010) compared recent diatom and geochemical records against data from the early to mid-Holocene for four lakes in western Spitsbergen. Increases in diatom valve and chrysophyte cyst concentrations occurring during the past century, with the most pronouncing alterations in the last few decades, indicate enhanced primary production and taxonomic richness. The recent dramatic shifts in diatom assemblages and significant increase in taxonomic richness contrast the relatively sparse flora recorded during the HTM, pointing to additional explanatory variables. Progressive depletion in nitrogen isotopic composition by ~2% during the last part of the 20^th century coincides with community trends, suggesting both climate change and N deposition are synergistically driving shifts towards new ecological states that have not been recorded in the Holocene.

Relative abundance of diatoms in four lakes in western Spitsbergen, Svalbard, alongside concentrations of diatom valves and chrysophyte cysts (Holmgren et al., 2010)

With the possibility of future accelerations in soil mineralisation, increased nitrogen availability (despite declining atmospheric deposition rates) could further threaten Arctic ecosystems (Hobbie et al., 1999). Similarly to acidification, ecological responses to nitrogen deposition are highly variable because of high spatial heterogeneity in lake characteristics, nutrient limitation, community structure and N supply amongst arctic lakes (Hogan et al., 2014).

Heavy metals and persistent organic pollutants (POPs)

Deriving from mining and fossil fuel combustion, the accumulation of heavy metals in biota and transferal up food chains threatens Arctic ecosystems. Concentrations of heavy metals (Pb, Cd, Cu, Fe, Mn and Zn) exceeding levels in the Earth's upper crust and local rocks have been recorded in cryoconites (aggregates of mineral particles, organic substances and microorganisms on glacial surfaces). This has vital consequences for Arctic ecosystems as contaminants have already been reported in higher-trophic-level species including seals, polar bears and reindeer (Łokas et al., 2016).

Similarly to heavy metals, POPs (organic compounds found in pesticides, industrial chemicals and products of burning that do not break down naturally) bioaccumulate and can have major health implications for wildlife and people (Vorkamp and Rigét, 2014). In response, several global and regional conventions are responsible for the elimination and reduction of POPs and therefore much of the contamination occurring today is from 'legacy' POPs. Temporal trends of some legacy POPs in Arctic biota was examined by Rigét et al. (2010), discovering that many of the  compounds in biota show significant declining trends over the last few decades consistent with the decreasing trends recorded in Arctic air.

Ecological implications of nickel mining in the Russian Arctic town, Nikel

Earth's Changing Wobble

Following an earlier post on the melting of the Greenland Ice Sheet, I wanted to look at its implications for the Earth's spin axis. By altering the distribution of weight, the melting of ice sheets has caused a dramatic shift in polar motion towards the UK at a rate of ~18 cm/year (previously the North Pole was moving away from Canada at ~8 cm/year) since 2000. 

The spin axis of the Earth was drifting toward Canada prior to 2000, however, climate-driven ice loss has shifted the direction towards the east (NASA, 2016)

Overall, the melting of the GIS is responsible for ~40% of the polar movement, while 25% is affected by gains in ice volume in East Antarctica and losses in West Antarctica. Redistribution of water on continents due to melting glaciers is also contributing to 25% of the axial wobble. Although this shift is harmless, it is a powerful reminder of the extent to which humanity is affecting the Earth.

Changes to water storage on the continents (NASA, 2016)

The video below summarises the changes in polar motion:

Ocean Acidification: The Other CO2 Problem

Many of Arctic responses to climate change explored in this blog, such as lake ecology regime shifts, terrestrial ecosystem alterations, melting of the Greenland Ice Sheet, and permafrost thawing, have focused on the warming effect of increased atmospheric CO2 levels as a driver for change. However, marine environments are also under threat from enhanced CO2 uptake in the ocean as a result of elevated atmospheric CO2 concentrations (Sabine et al., 2004). Worldwide, there has been an increase in ocean acidity of ~30% over the last 200 years, a rise that is mirrored in the Arctic Ocean (AMAP, 2013). With its low temperatures, sea ice retreat and large input freshwater input, the biological and chemical impacts of ocean acidification are expected to be most pronounced in the Arctic Ocean (Popova et al., 2014).

To what extent will the Arctic Ocean become acidified?

Aragonite reduction

The reaction of excess CO2 with water forms carbonic acid that acts to lower pH and decrease the concentration of carbonate ions (carbon undersaturation) which are used in the formation of the shells and skeletons of some organisms (e.g. corals, coccolithophorids, foraminifera, pteropods). A seasonal reduction in aragonite (one of two forms of calcium carbonate) has already been observed in Arctic surface waters, including the Canada Basin, Arctic shelves, and western Arctic Ocean. The transferral of anthropogenic CO2 to Arctic surface waters has accelerated because of significant reductions in sea ice. In addition, this increased influx of brackish water has led to further declines in the availability of Ca²⁺, decreasing alkalinity and moving us closer toward an aragonite tipping point (AMAP, 2013).

The inorganic carbon system in the Arctic Ocean (AMAP, 2013)

The role of primary production

Coastal and riverine erosion supply a large volume of organic carbon to the Arctic shelves. In some nearshore locations, the release of some of this terrigenous organic carbon store through oxidation to CO2 is not permitted by seasonal ice, however, metabolism still occurs. With some of the highest primary productivity rates observed in the world ocean, metabolism of organic carbon in Arctic shelf waters releases large amounts of CO2, which lowers pH. Upwelling of this CO2-rich water is likely to increase with reductions in sea ice, with these waters in the western Arctic Ocean then transferred to the Pacific Ocean (AMAP, 2013).

Alterations to the marine N cycle

The response of microbially-driven biogeochemical cycling to enhanced CO2 concentrations was investigated by Tait et al. (2014). Fixation of nitrogen gas by diazotrophs is the largest N source to oceans globally, while denitrification through microbial processes (reduction of oxidised N to N gas) and anammox (anaerobic oxidation of ammonium with nitrate) leads to losses of N from oceans. By altering microbially-induced ammonia oxidation sediment-water N fluxes are influenced by ocean acidification. Reductions in pH lead to enhanced nitrate uptake by sediments and increase ammonium release while decreasing release of nitrite. This could have a significant effect on primary production rates by altering the benthic supply of key nutrients.

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.