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New A Level Subject Content Overview
Author : Martin Evans, Professor of Geomorphology, School of Environment, Education and Development, University of Manchester.
Professor Evans was chair of the ALCAB Geography panel.
1. Water and Carbon Cycles
Cycling of carbon and water are central to supporting life on earth and an understanding of these cycles underpins some of the most difficult international challenges of our times. Both these cycles are included in the core content elements of the specifications for A Level geography to be first taught from 2016^1. Whether we consider climate change, water security or flood risk hazard an understanding of physical process is central to analysis of the geographical consequences of environmental change. Both cycles are typically understood within the framework of a systems approach which is a central concept to much physical geographical enquiry. The concept of a global cycle integrates across scales. Systems theory allows us to conceptualise the main stores and pathways at a global scale. The systems framework also allows for more detailed (process detail) and local knowledge to be nested within the wider conceptual framework. Local studies on aspects of hydrology or carbon cycling can be understood as part of a broader attempt to understand in detail the nature of water and carbon cycling. Global environmental challenges frequently excite student interest in physical geography but it can be difficult for students to see how they can conduct relevant investigations of fieldwork given the large scale and complexity of the issues. By embedding their knowledge within systems framework students can understand how measurements and understanding derived from their own fieldwork and local studies contribute to the wider project of elucidating the cycling of water and carbon at the global scale.
This capacity to link scales means that the study of biogeochemical cycles is intrinsically geographical so that students can understand how processes operate in, and impact upon particular places and how they are distributed in space.
(^1) Geography GCE AS and A Level Subject Content. Department for Education (2014)
Water and carbon cycling
This overview starts with a discussion of the systems approach and then considers the key processes and geographical understanding of both the water and carbon cycles. Potential fieldwork activities under this theme are discussed and finally some starting points for relevant case study material are outlined.
A Systems Approach
Water and carbon cycles are understood through a systems approach. Systems are bounded and have inputs, outputs and throughputs.
The throughputs are mediated by processes internal to the system which are often understood grouped together as sub-systems (figure 1). For example, in the case of the water cycle we might consider a drainage basin sub-system which has inputs of rainfall from the atmosphere, outputs of river discharge (to the ocean) and evaporation (to the atmosphere) and includes water storage in surface waters and as soil moisture and groundwater.
There are three concepts key to understanding biogeochemical cycling within a systems framework;
Stores or stocks are the total amount of the material of interest held within a part of the system. This is effectively how much of the material there is and where it is. For example, soils are a major store of carbon within the terrestrial carbon system. Stocks are usually expressed in units of mass. e.g. total global soil carbon storage is estimated at 1500- PgC (PgC is petagrams of carbon, a unit equivalent to grams or one gigatonne.) Fluxes are measurements of the rate of flow of material between the stores. Because fluxes are a rate the units are mass per unit time, commonly for global cycles these are expressed as Pg per year. Processes are the physical mechanisms which drive the flux of material between stores. For example one of the key processes which drive the flux of carbon from the atmosphere to the vegetation store is photosynthesis.
2. The Water Cycle
Water is present in three phases on earth, as liquid water, as ice and as atmospheric moisture. At the global scale there are stores of water in all three phases. Liquid water dominates with about 98% of water in liquid form, predominantly in the oceans (table 1)
Water is cycled between these stores by a range of key processes as identified in figure 2. The mean residence times for various stores vary both with the size of the store (larger stores take longer to turn over) and with the rate of the processes which move water between stores (for example surface runoff is relatively rapid but groundwater flows are much slower so that groundwater residence times can be high). Brief introductions to key processes are given below.
Figure 2: The global water cycle. Fluxes of moisture indicated in units of 10^3 km^3 yr- Original Figure © Open University Modified with data from Trenberth et al. 2007.
Store Volume 10^3 km^3 % Residence time Oceans 1335040 96.9 3600 years Icecaps 26350 1.9 15000 years Groundwater 15300 1.1 Up to 10000 years Rivers and Lakes
178 0.01 2 weeks to 10 years Soil Moisture 122 0.01 2 - 50 weeks Atmospheric Moisture
13 0.001 10 days
Table 1 : Scale of stores in the global water cycle and typical residence times. (Data from Trenberth et al. 2007; and Lockwood, 2012)
Key Water Cycle Processes
Evapo-transpiration
Evaporation can occur from open water or from wet surfaces. Total evaporative losses also include water vapour transpired by vegetation, taken up by root systems and released through the stomata of the leaves. Taken together these processes are often referred to as evapotranspiration.
Rates of evaporation are controlled by the surface energy balance, temperature, relative humidity and wind speed. Rates of transpiration are also affected by plant type and growth condition. Both evaporation and transpiration are maximised when water is not limited, this is known as Potential
Evapotranspiration , and values ( Actual Evapotranspiration ) may fall below this level due to
reduced soil moisture or due to closure of plant stomata under moisture stress.
Rates of evaporation over the ocean exceed terrestrial rates because over the land actual evapotranspiration is less than potential. This result in a net transfer of atmospheric moisture to the continents as moist air moves across the continents driven by global air mass circulation.
The Geography of the Water Cycle
The water cycle can be studied at scales from global to a small-scale hillslope plot. For any unit we can measure or estimate a water budget by quantifying the key stores and fluxes. The spatial variation of these local budgets and their aggregation to larger spatial units produces understanding of spatial variability in hydroclimate, water availability, and flood hazard. Water is essential for human populations and yet also poses significant risks. Understanding local, regional, and global transfers of water and the way in which these interact with and control physical and biological processes, are key parts of physical geography.
3. The Carbon Cycle
Cycling of the element carbon is intimately associated with life on earth. Carbon is present in carbon based molecules that are integral to all living creatures, as carbon dioxide and methane in the atmosphere, in carbonate rocks in the lithosphere and as organic molecules in soils and sediments which are derived from formerly living material. Major carbon stores include the ocean, ocean sediments, soils, bedrock, vegetation and the atmosphere. Atmospheric carbon has become a major policy focus because of the role of carbon dioxide and methane as greenhouse gasses. The magnitude of the major stores and the way in which they are connected by key processes is illustrated in figure 3 which is taken from the Inter-governmental Panel on Climate Change (IPCC). This diagram indicates significant anthropogenic perturbations of the carbon cycle since 1750AD.
About 90% of anthropogenic carbon release comes from combustion of fossil fuels with the remainder driven by land use change. Of the anthropogenic CO 2 released to the atmosphere about 24% is absorbed by the oceans and 26% is taken up by plants. Global CO2 concentrations have increased from less than 320 ppm in 1960 to around 400 ppm at present (see Earth System Research Laboratory website: http://www.esrl.noaa.gov/gmd/ccgg/trends/index.html)
Figure 3 : The global carbon cycle © IPCC. The boxes are stores of carbon and the arrows indicate fluxes and the processes which drive those fluxes. Figures in black are estimates of the natural stores and fluxes and figures in red indicate anthropogenic impacts on the carbon cycle in the period after 1750 AD. For full details of the estimates underlying this figure see: https://www.ipcc.ch/report/ar5/wg1/
The terrestrial carbon cycle
The terrestrial carbon cycle is dominated by uptake of CO 2 from the atmosphere by plant photosynthesis. CO 2 is released back to the atmosphere due to respiration of plants and animals and CO 2 and methane are released due to decomposition of dead organic matter. Carbon is cycled relatively rapidly between soil and vegetation and the atmosphere. This cycling of carbon through living systems is sometimes called the fast carbon cycle as distinct from the slow carbon cycle (see below). Terrestrial carbon cycling occurs within ecosystems which, in the modern world, are almost all subject to intensive human impacts. Land use change and other human impacts on ecosystems have the potential to change the balance of carbon uptake and release in the terrestrial system.
Key Processes controlling carbon cycling
Figure 4 : Some key processes which drive the flux of carbon between the main stores © Candace Dunlap. Source: http://serc.carleton.edu/eslabs/carbon/index.html
Photosynthesis and respiration
Key to terrestrial carbon cycling are the processes of photosynthesis and respiration. Photosynthesis is the process of the production of carbohydrate molecules from carbon dioxide and water using energy from light. Plants and some algae and bacteria photosynthesise and so fix gaseous carbon dioxide from the atmosphere into solid form in their tissues. CO 2 is released to the atmosphere by living things through the process of respiration. Life derives energy from the combination of sugars and oxygen and CO 2 is a by-product of this reaction.
Decomposition
CO 2 from plants and animals is also returned to the atmosphere through processes of decomposition of dead tissue. These decomposition processes occur through the action of fungi
and bacteria. Carbon is released in gaseous form but decomposition may also produce soluble organic compounds so that carbon can also be mobile dissolved in runoff from the land surface.
Methanogenesis
Methane is a by-product of respiration by methanogenic bacteria which are found in anaerobic (low oxygen) environments. Methane emissions from wetland environments such as peatlands or rice paddy are significant because of the high global warming potential of methane.
Carbon sequestration in oceans
Carbon dioxide moves from the atmosphere to the ocean by diffusion. CO 2 dissolved in the surface of the ocean can be transferred to the deep ocean in areas where cold dense surface waters sink. This is a physical process sometimes called the physical pump. Phytoplankton in the ocean also fixes carbon dioxide through photosynthesis and these organisms form the bottom of the marine food web. Carbon from this source may be transferred to the deep ocean either as dead organisms sink or transported with downwelling waters. Removal of carbonate from sea water by shell building organisms is another important mechanism controlling transfer of carbon to deep ocean sediments.
Fossil fuels
Fossil fuel reserves are significant stores of fossil carbon. Coal for example is lithified peaty deposits. Burning of fossil fuels such as coal and gas releases carbon dioxide to the atmosphere. This is carbon released from long term storage deep in the earth and is a human induced acceleration of the cycling of this carbon.
The geography of the carbon cycle and carbon budgets
The carbon cycle like the water cycle can be studied at a range of spatial scales. For example, the key processes of the terrestrial carbon cycle can be considered through study of the carbon budget of a field or catchment or local ecosystem. Figure 5 shows an example of a carbon budget for an area of tropical forest showing that deforestation leads to a shift of the ecosystem from a carbon sink to a carbon source. Carbon cycling is strongly controlled by biological factors so variation in space is closely linked to biogeography and the distribution of major ecosystem types. In the modern world human activity is a major control on species distribution and ecosystem function.
carbon cycling. Two contexts of particular interest for exploring the linkages between the two cycles are climate change and land use change. Both climate change and land use change may lead to significant perturbations in terrestrial ecosystems which impact on both water and carbon cycling (see for example material on desertification below).
4. Fieldwork opportunities in relation to water and carbon cycling
As noted one of the characteristics of these cycles is the way in which they can be applied at a range of scales to integrate local investigations into wider understanding. Consequently the carbon and water cycling topics provide excellent opportunities for a range of local fieldwork investigations.
Potential topics include:
Investigations of rates of infiltration
Infiltration is a key process responsible for the partitioning of rainfall between overland flow (runoff) and soil storage or subsurface flow. Infiltration is easy to measure using simple infiltration rings which can be made from plastic pipe. Infiltration rates may be affected by a range of factors such as surface cover, soil moisture, soil texture, slope, and soil compaction allowing groups to conduct a range of related but distinct investigations in a constrained area.
Measurement of water balance
Catchment discharge is a fundamental parameter in the drainage basin water balance. Use of secondary rainfall and runoff data will allow students to construct simple water balance for catchments. Practice measuring stream discharge and rainfall in the field will help students to understand the potential errors associated with these estimates. Understanding errors is central to any budgeting exercise. Measurement of rainfall in multiple simple rain gauges (for example around school grounds) will allow students to examine spatial variation in rainfall and its potential impact on creating good estimates of rainfall inputs.
Estimation of carbon stocks in woodland.
The stock of carbon within woodland can be simply estimated. There are standard equations to estimate living biomass of trees from the diameter of the tree measured at 1.3m height (a simple guide from the field studies council is here: http://tinyurl.com/q5mgxru).
Tree biomass is 50% carbon so it is a simple conversion to work out how much carbon is stored in the tree. Where tree age can also be estimated, either from the girth of the tree, knowledge of the site, or from tree ring evidence on similar felled trees then the rate of carbon sequestration as mass of carbon per year can be calculated, in this case students can estimate both the stock of carbon and the flux.
Estimation of carbon stocks in peatlands
Peatland depth is easily measured by probing the peatland either with a commercially available peat probe or with sections of narrow threaded rod which can be connected to make a portable probe. Multiple probings of depth in an area can be used to estimate peat volume at a site. This is a good opportunity to introduce concepts of averaging or simple geospatial techniques such as the use of Thiessen polygons. Peat volume can be converted to organic carbon stocks by knowing typical peat densities (circa 0.1-0.2 gC cm-3). Where the age of local peatlands is known for example from published radiocarbon dates on basal peats the total carbon stock can be converted to an average flux over this time period by dividing the stock by the age to give flux in units of grammes of carbon per metre squared per year (gC m-2^ a-1)
Estimation of fluvial carbon flux
Estimating fluxes of materials in rivers involves measurement of discharge and of the concentration of the material of interest. Flux is calculated as the product of discharge and concentration. This is a good place to practice the use of commensurate units to produce flux estimates in sensible units. For example students might measure river discharge either from stage at a site with known stage- discharge relationships (e.g. a weir) or by measurement of velocity and river cross section. Sediment concentration could be measured by filtering water samples. The organic component of sediment can be estimated as the fraction lost after an hour in a furnace at 550 degrees C. Carbon content of sediment is typically 50% of organic content. The fluvial particulate carbon concentration (the amount of carbon being transported in the sediment) in g m-3^ multiplied by discharge in m^3 s- will give a carbon flux in grams per second (g s-1). The same calculation can be applied to dissolved carbon concentrations estimated by colourimetry. This approach is particularly relevant to peatland streams where ‘brown water’ is indicative of high dissolved carbon concentrations.
evaporation is also important in sustaining regional rainfall in areas marginal to the forest so that for example Amazonian rainforest supports rainfall totals over key agricultural regions of Brazil
Deforestation leads to higher albedo and higher surface temperatures lead to greater loss of sensible heat to the atmosphere but reduced biomass and lower interception means that there is less evapotranspiration. This leads to lower atmospheric humidity and reduced precipitation (Marengo, 2006).
Forest land use change and carbon cycling.
Forests are significant stores of carbon. Carbon is stored primarily in the biomass of the trees but thick litter layers on the forest floor can also be significant. Forest landscapes are important resources for human populations so that throughout human history deforestation for timber resources, or to clear land for agriculture has been a major human impact on ecosystems. Similarly in some contexts afforestation associated with timber cropping is significant. Historical and geographical changes in forest clearance and forest management have a significant impact on forest carbon storage. Forest growth leads to sequestration of carbon from the atmosphere whilst forest clearance can lead to carbon emissions to the atmosphere through burning or decomposition of biomass and through changes in soil organic carbon storage associated with forest clearances e.g. figure 5.
Figure 7 illustrates how these impacts on carbon balance have been variable in time and space. Tropical forests are an important carbon store sequestering up to 1 Pg of carbon per year. Clearance of tropical forests for settlement and agriculture has led to significant increases in carbon emissions from the tropics. In contrast in N. America (and in Europe to a lesser extent) forest regrowth due to agricultural abandonment after the great depression of the 1930’s has led to increases in carbon sequestration by forest land
Figure 7 : The geography of carbon emissions from landuse change. Reproduced from UNEP (2009) Vital Forest Graphics. Source: http://www.unep.org/vitalforest/
Carbon cycling in eroding and restored peatlands
Peatlands are thick organic soils which develop in areas where water table is high. Low oxygen conditions below water table inhibit microbial decomposition of plant litter so that thick organic layers accumulate. In the UK upland peatlands which have accumulated over approximately the last 7000 years are often 2-4 m deep. The organic matter in peat is 50% carbon so these peatlands are major soil carbon stores which lock up carbon which has been removed from the atmosphere as peatland plants photosynthesise. The northern peatlands which include the large peatlands of Siberia and the Canadian Shield as well as European peatlands store 20-30% of global soil carbon. The amount of carbon in the northern peatlands is equivalent to around 60% of the atmospheric carbon pool. Threats to the integrity of this carbon store therefore pose the risk of releasing significant amounts of carbon to the atmosphere.
Upland peatlands in the UK have been severely affected by air pollution which has caused loss of key peat forming species such as sphagnum. Historic overgrazing has further impacted some peatlands and they have been subject to climate stress during the Little Ice Age. Together these factors have led to massive erosion (Figure 8). Peatlands have also been subject to drainage during the 20th^ century. Only 20% of UK upland peatlands are not degraded. Organic matter eroded from peatlands may be oxidised in river systems releasing CO2. Erosion of gullies and drainage also leads to reduced water tables in the peatland. This enhances decomposition in the upper layers of the peat and releases more dissolved carbon to waters and more CO 2 to the atmosphere. Consequently rates of carbon sequestration in degraded peatlands are reduced or the peatlands may even become carbon sources. Across the uplands of the UK there are now
surface and the Millennium ecosystem assessment (http://www.millenniumassessment.org/en/index.html) estimates that 10-20% of drylands suffer from land degradation. Desertification driven partly by climate change and also by land use change leads to reductions in soil moisture. Under conditions of desertification, accelerated soil erosion and reduced vegetation cover lead to reduced carbon sequestration. This results from reduced vegetation cover (lower photosynthetic fixation of carbon) and from oxiation of soil carbon to CO 2. This leads to reduced soil carbon. Global carbon emissions from dryland degradation are estimated at 0.23-0.29 PgC a-1^ (Lal 2001) which is around a quarter of total global emissions from land use change.
Desertification also impacts on the water cycle since reduced vegetation cover reduces infiltration and increases runoff which can lead to reductions in soil moisture. Where erosion control measures limit desertification, enhanced storage of organic matter in the soil helps to retain moisture and promote vegetation cover. Therefore, in dryland systems the water and carbon cycles are closely coupled since moisture availability is a major control on plant growth and hence on the terrestrial carbon cycle.
1. References and web resources
Earth System Science in a Nutshell: http://serc.carleton.edu/introgeo/earthsystem/nutshell/index.html
Lal, R. (2001) Potential of Desertification Control to Sequester Carbon and Mitigate the Greenhouse Effect. Climatic Change 51(1) 35-
Lockwood, J. Atmospheric Moisture. In Holden, J. (ed) 2012 An introduction to Physical Geography and the Environment Pearson, Harlow. 875p.
Marengo, J. (2006) On the hydrological cycle of the Amazon Basin: a historical review and current state of the art Revista Brasileira de Meteorologia , 21(3) 1-19. [Available at: http://www.researchgate.net/profile/Jose_Marengo/publication/228647135_On_the_hydrological_c ycle_of_the_Amazon_Basin_A_historical_review_and_current_state-of-the- art/links/00b7d51c60a76d339e000000.pdf ]
Riebeek, H. (2011) The carbon Cycle Article on the NASA Earth Observatory website: http://earthobservatory.nasa.gov/Features/CarbonCycle/
Shaw, E.M., Beven, K.J., Chappell, N., and Lamb, R. (2010) Hydrology in Practice. CRC Press London.
Trenberth, K.E., Smith, L., Qian, T., Dai, A., and Fasullo, J. (2007) Estimates of the Global Water Budget and its Annual Cycle Using Observational and Model Data. J. Hydrometeor , 8 , 758–769.
Visualising carbon pathways: http://serc.carleton.edu/eet/carbon/index.html [Interesting resource which allows students to create animations of parts of the carbon cycle]
