2.b. case study- Arctic tundra, physical and human factors that affect water and carbon cycles in an arctic tundra

intro and location

arctic tundra occupies some 8mil km² in northern Canada, Alaska and Siberia. it extends from the northern edge of the boreal coniferous forest to the arctic ocean and its southern limit approximates the 10^oc july isotherm (climatic limit of the tree line). climatic conditions are severe and becomes more more extreme with latitude. for 8/9 months a year the tundra has a negative heat balance with average monthly temps below 0. as a result the ground is permanently frozen with only the top metre or so thawing during the Arctic summer.

permafrost underlies much of the tundra and is an important feature of the region’s water cycle. in winter when for several weeks the sun remains below the horizon, temps can plunge below -40^oc. long hours of daylight in summer provide some compensation for brevity of the growing season, mean annual precipitation is low.

few plants and animals have adapted to this extreme environment; biodiversity is low and apart from a few dwarf species; the ecosystem is treeless. in the southern areas- the low arctic- conditions are less severe and vegetation provides a continuous ground cover. further north in the high arctic, plant cover is discontinuous with extensive areas of bare ground

water cycle in tundra

low annual precipitation- 50-350mm with most precipitation falling as snow. small stores of moisture in the atmosphere owing to low temps which reduce absolute humidity. limited transpiration because of the sparseness of the vegetation cover and the short growing season. low rates of evaporation- much of the sun’s energy in summer is expended melting snow so that ground temps remain low and inhibit convection. also surface and soil water are frozen for most of the year. limited groundwater and soil moisture stores- permafrost is a barrier to infiltration, percolation, recharge and groundwater flow. accumulation of snow and river/lake ice during the winter months, melting of snow, river and lake ice and the uppermost active layer of the permafrost in spring and early summer results in a sharp increase in river flow. extensive wetlands, ponds and lakes on the tundra during summer- this temporary store of liquid water is due to permafrost which impedes drainage

carbon cycle in tundra

the permafrost is a vast carbon sink. globally it is estimated to contain 1600 GT carbon. accumulation of carbon is due to low temps which slow decomposition of dead plant material. overall the amount of carbon in tundra soils is 5x greater than in the above ground biomass.

the flux of carbon is concentrated in the summer months when the active layer thaws. plants grow rapidly in the short summer. long hours of daylight allow them to flower and fruit within just a few weeks. nonetheless, net primary productivity is less than 200 grams/m²/year. consequently the tundra biomass is small, ranging between 4 and 29 tonnes/ha depending on the density of vegetation cover

during the growing season tundra plants input carbon rich litter to the soil. the activity of microorganisms increases, releasing CO2 to the atmosphere through respiration. however CO2 and methane emissions are not just confined to the summer. even in winter, pockets of unfrozen soil and water in the permafrost act as sources of CO2 and methane. meanwhile snow cover may insulate microbial organisms and allow some decomposition despite the low temps

in the past the permafrost functioned as a carbon sink. today global warming has raised concerns that is is becoming a carbon source. at the moment the evidence is unclear. while outputs of carbon from the permafrost have increased in recent decades, higher temps have stimulated plant growth in the tundra and greater uptake of CO2. thus in turn has increased the amount of plant litter entering the store. it is possible therefore, that despite the warming arctic climate, the carbon budget remains in balance.

physical factors, seasonal changes affects on stores and flows of water

influenced by temp, relief and rock permeability

average temps all well below 0 for most of the year= water stored as ground ice in permafrost layer. during short summer= shallow active layer thaws and liquid water flows on the surface. meltwater forms mils of pools and shallow lakes which stud the tundra landscape. drainage is poor- water cannot infiltrate the soil because of the permafrost at depth. in winter sub 0 temps prevent evapotranspiration. in summer, some evapotranspiration occurs from standing water, saturated soils and vegetation. humidity is low all year round and precipitation is sparse.

permeability is low owing to the permafrost and the crystalline rocks which dominate the geology of the tundra in arctic and sub arctic Canada

the ancient rock surface which underlies the tundra has been reduced to a gently undulating plain by 100s of mils of years of erosion and weathering. minimal relief and chaotic glacial deposits impede drainage and contribute to waterlogging during summer months.

physical factors, seasonal changes affects on stores and flows of carbon

carbon is mainly stored as partly decomposed plant remains frozen in the permafrost. most of this carbon has been locked away for at least the past 500,000 years

low temps, the unavailability of liquid water for most of the year and parent rocks containing few nutrients, limit plant growth. thus the total carbon store of the biomass is relatively small. averaged over the year, photosynthesis and NPP are low, with the growing season lasting barely 3 months. however there is some compensation for the short growing season in the long hours of daylight in summer.

low temps and waterlogging slow decomposition and respiration and the flow of CO2 to the atmosphere

owing to the impermeability of the permafrost, rock permeability, porosity and the mineral composition of rocks exert little influence on the water and carbon cycle

oil and gas production and carbon/water cycles in Alaska

north slope alaska- between brooks range in south and arctic ocean in north= vast wilderness of tundra. oil and gas were discovered at Prudhoe bay 1968. major challenges presented in oil and gas development since start- harsh climate with extreme cold and long periods of darkness in winter; permafrost and the melting of the active layer in summer; remoteness and poor accessibility; a fragile wilderness of great ecological value

despite challenges production went ahead- driven by high global energy prices and UA gov’s policy to reduce dependence on oil imports. massive fixed investments: pipelines, roads, oil production plants, gas processing facilities, power lines, power generators and gravel quarries were completed in 1970/80s. 1990s= north slope accounted for ¼ of oil production USA. today only 6%. decline in recent years reflects 2 things: high production costs on north slope and massive growth of the oil shale industry in USA. decrease for north slope with decrease as a trend until 2020 (covid skewed figures and trade etc)

impact of oil and gas exploitation

significant impacts on permafrost and on local water and carbon cycles. permafrost the major carbon store in the tundra is highly sensitive to changes in thermal balance. in many areas this balance has been disrupted by the activities of oil and gas companies which have caused localised melting of the permafrost. melting is associated with: construction and operation of soil and gas installations, settlements and infrastructure diffusing heat directly to the environment; dust deposition along roadsides creating darkened snow surfaces, thus increasing absorption of sunlight; removal of the vegetation cover which insulates the permafrost

permafrost melting increases CO2 and methane. north slope estimated CO2 losses from the permafrost vary from 20 to 40 mil/tonnes/year, while methane losses around 100,000 tonnes/year. gas flaring and oil spillages also input CO2 to the atmosphere. other changes to the local carbon cycle are linked to industrial development. eg- destruction/degrading of tundra vegetation reduces photosynthesis and uptake of CO2 from the atmosphere; and the thawing of soil increases microbial activity, decomposition and emissions of CO2. moreover the slow growing nature of tundra vegetation means that regeneration and recovery from damages takes decades

similar changes have occurred to wate cycle- melting permafrost and snow cover increases run off and river discharge making floods more likely. means in summer, weltlands, ponds and lakes have become more extensive, increasing evaporation. strip mining of aggregates for construction creates artificial lakes which disrupt drainage and also expose permafrost to further melting. also drainage networks are disrupted by road construction and by seismic explosions used to prospect for oil and gas. finally, water abstracted from creeks and rivers for industrial use and for the building of ice roads in winter, reduce localised run off

management strategies to moderate the impact on water and carbon cycles

development on north slope has often involved deliberate destruction of the permafrost. now= emphasis on protecting the permafrost, thus minimising disruption to the water and carbon cycle and wildlife. however the purpose of these strategies is also pragmatic: melting permafrost causes widespread damage to buildings and roads as well as increased maintenance costs for pipelines and other infrastructure

insulated ice and gravel pads: roads and other infrastructure features can be constructed on insulating ice or gravel pads thus protecting the permafrost from melting

buildings and pipelines elevated on piles: constructing buildings, oil/gas pipelines and other infrastructure on piles allows cold air to circulate beneath these structures. this provides insulation against heat generating buildings, pipework etc which would otherwise melt the permafrost

drilling laterally beyond drilling platforms: new drilling techniques allow oil and gas to be accessed several km from the drilling site. with fewer sites needed for drilling rigs, the impact on vegetation and the permafrost due to construction is greatly reduced

more powerful computers can detect oil and gas bearing geological structures remotely: fewer exploration wells are needed thus reducing the impact on the environment

refrigerated supports: refrigerated supports are used on the Trans Alaksa pipeline to stabilise the temps of the permafrost beneath buildings and infrastructure