4.3 How much change occurs over time in the water and carbon cycles? Flashcards
4.3 How much change occurs over time in the water and carbon cycles?
Key idea ➡ Human factors can disturb and enhance the natural processes and stores in the water and carbon cycles.
Dynamic equilibrium and the water and carbon cycles
-Land-use changes
-Water extraction
-Aquifers and artesian basins
-Fossil fuels and the carbon cycle
-Positive and negative feedback loops in the water and carbon cycles
Dynamic equilibrium and the water and carbon cycles definition
Most natural systems, unaffected by human activity, exist in a state of dynamic equilibrium. They are dynamic in the sense that they have continuous inputs, throughputs, outputs and variable stores of energy and materials. In the short term, inputs, outputs and stores of water or carbon will fluctuate from year to year.
In the long term, however, flows and stores usually maintain a balance, allowing a system to retain its stability.
Negative feedback loops within systems (Dynamic equilibrium and the water and carbon cycles)
In the long term, however, flows and stores usually maintain a balance, allowing a system to retain its stability.
Negative feedback loops within systems restore balance. In a drainage basin unusually heavy rainfall will increase the amount of water stored in aquifers. This in turn will raise the water table, increasing flow from springs until the water table reverts to normal levels.
In the carbon cycle, burning fossil fuels increases atmospheric CO₂ but at the same time stimulates photosynthesis. This negative feedback response should remove excess CO₂ from the atmosphere and restore equilibrium.
Land-use changes (Dynamic equilibrium and the water and carbon cycles)
-Urbanisation
-Farming
-Forestry
Urbanisation (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Urbanisation is the conversion of land use from rural to urban. Farmland and woodland are replaced by housing, offices, factories and roads; natural surfaces such as vegetation and soil give way to concrete, brick or tarmac.
These artificial surfaces are largely impermeable so they allow little or no infiltration and provide minimal water storage capacity to buffer run-off.
Drainage systems - Urbanisation (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Urban areas also have drainage systems designed to remove surface water rapidly (e.g. pitched roofs, gutters, sewage systems). As a result a high proportion of water from precipitation flows quickly into streams and rivers, leading to a rapid rise in water level.
Floodplains - Urbanisation (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
In addition to changing land use, urbanisation also encroaches on floodplains. Floodplains are natural storage areas for water. Urban development on floodplains reduces water storage capacity in drainage basins, increasing river flow and flood risks.
Floodplains
The flat area around a river that is covered with sediment as a result of frequent flooding.
Farming (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Farming brings changes to vegetation and soils which have implications for the carbon and water cycles.
The clearance of forests - Farming (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
The clearance of forest for farming reduces carbon storage in both the above- and below-ground biomass. Soil carbon storage is also reduced by ploughing and the exposure of soil organic matter to oxidation. Further losses occur through the harvesting of crops with only small amounts of organic matter returned to soils. Soil erosion invariably accompanies arable farming. Erosion by wind and water is most severe when crops have been lifted and soils have little protective cover.
Below-ground biomass
One of seven key agriculture, forestry, and land-use carbon pools. It includes all living biomass of live roots. Fine roots of less than 2 mm diameter are often excluded because these often cannot be distinguished empirically from soil organic matter or litter.
Above-ground biomass
One of seven key agriculture, forestry, and land-use carbon pools. It includes trees defined as generally 5 cm or greater in diameter (at breast height (1.31 m above ground)).
Arable farming
Growing crops on good land to be eaten directly, or to be fed to animals
Changes to the carbon cycle - Farming (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Changes to the carbon cycle are less apparent on pasture land or where farming replaces natural grasslands. For instance in North America, the net primary production of annual crops such as wheat on the Great Plains exceeds that of the original Prairie grasslands.
However, carbon exchanges through photosynthesis are generally lower than in natural ecosystems. In part this is explained by a lack of biodiversity in farmed systems, and the growth cycle of crops often compressed into just four or five months.
Interception of rainfall by crops - Farming (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Interception of rainfall by annual crops is less than in forest and grassland ecosystems. So too is evaporation and transpiration from leaf surfaces. Ploughing increases evaporation and soil moisture loss, and furrows ploughed downslope act as drainage channels, accelerating run-off and soil erosion.
Infiltration due to ploughing is usually greater in farming systems, while artificial underdrainage increases the rate of water transfer to streams and rivers. Surface run-off increases where heavy machinery compacts soils. Thus peak flows on streams draining farmland are generally higher than in natural ecosystems.
Farming modifies the natural water cycle - Farming (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Farming also modifies the natural water cycle. Crop irrigation diverts surface water from rivers and groundwater to cultivated land.
Some of this water is extracted by crops from soil storage and released by transpiration; but most is lost to evaporation and in soil drainage.
Forestry (Land-use changes (Dynamic equilibrium and the water and carbon cycles))
Forest management in plantations modifies the local water and carbon cycles. Changes to the water cycle are shown on pages 120-121.
Changing land use from farmland, moorland and heath to forestry increases carbon stores. In a typical plantation in the UK, mature forest trees contain on average 170-200 tonnes C/ha. This is ten times higher than grassland, and 20 times higher than heathland.
Water extraction (Dynamic equilibrium and the water and carbon cycles)
-Water extraction on the River Kennet catchment
Water extraction definition (Dynamic equilibrium and the water and carbon cycles)
Water is extracted from surface and groundwater to meet public, industrial and agricultural demand. Direct human intervention in the water cycle changes the dynamics of river flow and groundwater storage.
Water extraction on the River Kennet catchment (Water extraction (Dynamic equilibrium and the water and carbon cycles))
The River Kennet in southern England drains an area of around 1,200 km² in Wiltshire and Berkshire. The upper catchment mainly comprises chalk which is highly permeable. This groundwater contributes most of the Kennet’s flow.
As a chalk stream, the river supports a diverse range of habitats and wildlife. Its water, filtered through the chalk, has exceptional clarity, high oxygen levels and is fast-flowing. Among the native fauna are Atlantic salmon, brown trout, water voles, otters and white-clawed crayfish.
Impacts on the regional water cycle - Water extraction on the River Kennet catchment (Water extraction (Dynamic equilibrium and the water and carbon cycles))
-Rates of groundwater extraction have exceeded rates of recharge, and the falling water table has reduced flows in the River Kennet by 10-14%.
-During the 2003 drought flows fell by 20%, and in the dry conditions of the early 1990s by up to 40%.
-Lower flows have reduced flooding and temporary areas of standing water and wetlands on the Kennet’s floodplain.
-Lower groundwater levels have caused springs and seepages to dry up and reduced the incidence of saturated overland flow on the chalk.
Aquifers and artesian basins (Dynamic equilibrium and the water and carbon cycles)
-Aquifers
-Artesian basins
Aquifers (Aquifers and artesian basins (Dynamic equilibrium and the water and carbon cycles))
Aquifers are permeable or porous water-bearing rocks such as chalk and New Red Sandstone, Groundwater is abstracted for public supply from aquifers by wells and boreholes.
Emerging in springs and seepages, groundwater feeds rivers and makes a major contribution to their base flow.
Water table
The upper level of the saturated zone of groundwater.
Abstraction
Pulling out specific differences to make one solution work for multiple problems.
Artesian basins (Aquifers and artesian basins (Dynamic equilibrium and the water and carbon cycles))
When sedimentary rocks form a syncline or basin-like structure, an aquifer confined between impermeable rock layers may contain groundwater which is under artesian pressure. If this groundwater is tapped by a well or borehole, water will flow to the surface under its own pressure.
This is known as an artesian aquifer. The level to which the water will rise - the potentiometric surface - is determined by the height of the water table in areas of recharge on the edges of the basin.
Artesian aquifer
A confined aquifer containing groundwater under pressure.
Artesian pressure
The hydrostatic pressure exerted on groundwater in a confined aquifer occupying a synclinal structure.
Hydrostatic pressure
The pressure of water against the walls of its container.
Fossil fuels and the carbon cycle (Dynamic equilibrium and the water and carbon cycles)
-Use of fossil fuels and impacts on the carbon cycle
-Sequestration of waste carbon
Use of fossil fuels and impacts on the carbon cycle (Fossil fuels and the carbon cycle (Dynamic equilibrium and the water and carbon cycles))
For the past two centuries, fossil fuels - coal, oil and natural gas - have driven global industrialisation and urbanisation.
Despite the development of nuclear power and renewable energy, the global economy remains overwhelmingly dependent on fossil fuels. In 2013 they accounted for 87% of global energy consumption.
Sequestration of waste carbon (Fossil fuels and the carbon cycle (Dynamic equilibrium and the water and carbon cycles))
The combustion of fossil fuels and the transfer of carbon from geological store to the atmosphere and oceans in the main driver of present-day global warming.
One possible solution to this problem is to capture and store CO₂ released by power plants and industry. This new technology of carbon sequestration is known as carbon capture and storage (CCS)
Positive and negative feedback loops in the water and carbon cycles (Dynamic equilibrium and the water and carbon cycles)
-Feedback in the water cycle
-Feedback in the carbon cycle
Positive and negative feedback loops in the water and carbon cycles definition (Dynamic equilibrium and the water and carbon cycles)
Feedback is an automatic response to changes which disturb a system’s balance or equilibrium. Change in natural systems can produce either positive or negative feedback responses.
Positive feedback occurs when an initial change causes further change (a kind of ‘snowball’ effect). Negative feedback is the opposite: it counters system change and restores equilibrium.
Feedback in the water cycle (Positive and negative feedback loops in the water and carbon cycles (Dynamic equilibrium and the water and carbon cycles))
Rising temperatures affect the water cycle at the global scale. In a warmer world, evaporation increases and the atmosphere holds more vapour. The result is greater cloud cover and more precipitation. These changes create a positive feedback effect.
Because water vapour is a greenhouse gas, more vapour in the atmosphere increases absorption of long-wave radiation from the Earth causing further rises in temperature.
Pages 125-126.
Feedback in the carbon cycle (Positive and negative feedback loops in the water and carbon cycles (Dynamic equilibrium and the water and carbon cycles))
The global carbon cycle is currently in a state of disequilibrium. Human activity, primarily through burning fossil fuels, has increased the concentration of CO₂ in the atmosphere, the acidity of the oceans and the flux of carbon between the major stores.
Within the carbon cycle there are feedback loops which could either restore equilibrium or induce further disequilibrium.
Monitoring changes to the global water and carbon cycles
-Diurnal changes
-Seasonal changes
-Long-term changes
Monitoring changes to the global water and carbon cycles definition
Given the potentially damaging impact of climate change, accurate monitoring of changes in global air temperatures, sea surface temperatures (SST), sea ice thickness and rates of deforestation is essential. Because ground-based measurements of environmental change at the global scale are impractical, monitoring relies heavily on satellite technology and remote sensing.
Monitoring changes to the global water and carbon cycles and time scales
Continuous monitoring by satellite on day-to-day, month-to-month or year-to-year basis allows changes to be observed on various time scales. Using Geographic Information Systems (GIS) techniques these data can then be mapped and analysed to show areas of anomalies and trends, and regions of greatest change.
Diurnal changes (Monitoring changes to the global water and carbon cycles)
Significant changes occur within a 24-hour period in the water cycle.
For instance, lower temperatures at night reduce evaporation and transpiration. Convectional precipitation, dependent on direct heating of the ground surface by the Sun, is a daytime phenomenon often falling in the afternoon when temperatures reach a maximum.
This is particularly significant in climatic regions in the tropics where the bulk of precipitation is from convectional storms.
Convectional precipitation
The formation of precipitation due to surface heating of the air at the ground surface.
Seasonal changes (Monitoring changes to the global water and carbon cycles)
Ultimately the seasons are controlled by variations in the intensity of solar radiation.
In the UK, solar radiation intensity pursed in mid-June. A typical solar input in June in southern England is around 800 W/m²; in December the input falls to little more than 150 W/m².
As a result, evapotranspiration is highest in the summer months and lowest in winter. In the driest parts of lowland England up to 80% of precipitation may be lost to evapotranspiration.
Irradiance W/m²
Watt per square metre W/m² - referring to irradiance (flux density). Radiant flux received by a surface per unit area. This is sometimes also confusingly called “intensity”.
Long-term changes (Monitoring changes to the global water and carbon cycles)
-Water cycle
-Carbon cycle
Long-term changes definition (Monitoring changes to the global water and carbon cycles)
The climate record over the last million years shows the Earth’s climate has been highly unstable, with large fluctuations in global temperatures occurring at regular intervals. In the past 400,000 years there have been four major glacial cycles with cold glacials followed by warmer inter-glacials. Each cycle lasted around 100,000 years.
At the height of the last glacial, 20,000 years ago, average annual temperatures in the British Isles were 5ºC lower than today, and Scitkabdm Wakes and most of northern England and Ireland were submerged by ice up to 1 km thick.
Water cycle (Long-term changes definition (Monitoring changes to the global water and carbon cycles))
During glacial periods the water cycle undergoes a number of changes. The most obvious is the net transfer of water from the ocean reservoir to storage in ice sheets, glaciers and permafrost. As a result, in glacials the sea level worldwide falls by 100-130 m; and ice sheets and glaciers expand to cover around one-third of the continental land mass.
As ice sheets advance equatorwards they destroy extensive tracts of forest and grassland.
Carbon cycle (Long-term changes definition (Monitoring changes to the global water and carbon cycles))
MostThe most striking feature of the carbon cycle during glacial periods is the dramatic reduction CO₂ in the atmosphere. There has been a correlation between temperature and atmospheric CO₂ over the past 400,000 years (Page 129).
At times of glacial maxima CO₂ concentrations fall to around 180 ppm, while in warmer inter-glacial periods they are 100 ppm higher.
Glacial maxima
Peaks of glacial advance, lowest temperatures and the highest ice volume.
