3.a. Human factors can disturb and enhance the natural processes and stores in the water and carbon cycles

Question 1

How do most natural systems exist?

Answer 1

• Most natural systems unaffected by human activity exist in a state of dynamic equilibrium
• Dynamic in the sense that they have continuous inputs, throughputs, outputs and stores of energy and materials

Question 2

What does dynamic equilibrium mean?

Answer 2

• Short term - inputs, outputs and stores of water or carbon fluctuate from year to year
• Long term - flows and stores usually maintain balance, allows a system to retain stability - negative feedback loops within systems restore balance

Question 3

Examples of negative feedback loops?

Answer 3

• In drainage basin, unusually heavy rainfall will increase amount of water stores in aquifers, this then raises the water table, increasing the flow from springs until the water table reverts to normal levels
• Carbon - burning fossil fuels increases atmospheric CO2, but also stimulates photosynthesis, negative feedback response should remove excess CO2 from the atmosphere and restore equilibrium

Question 4

What is urbanisation?

Answer 4

Conversion of land use from rural to urban, farmland and woodland replaced by housing, offices, factories and roads

Question 5

How does urbanisation affect the water cycle?

Answer 5

Natural surfaces such as vegetation and soil give way to concrete, brick, tarmac, etc - these artificial surfaces are largely impermeable and provide minimal water storage capacity to buffer run-off

Urban areas also have drainage systems designed to remove surface water rapidly (eg. pitched roofs, sewerage systems), which means that streams and rivers which drain urban areas have characteristically short lag times and high, but short-lived, peak flows

In addition to changing land use, urbanisation also encroaches on floodplains, which are natural storage areas for water - urban development on floodplains reduces water storage capacity in drainage basins, increasing river flow and flood risks

Question 6

How does farming impact stores in the water cycle?

Answer 6

Soil erosion by wind and water is most severe when crops have been lifted and soils have little protective cover

Question 7

How does farming impact flows in the water cycle?

Answer 7

Crop irrigation diverts surface water from rives 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

Interception of rainfall by annual crops, evaporation and transpiration from leaf surfaces are all less in farmland compared to forest and grassland ecosystems

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 under drainage increases the rate of water transfer to streams and rivers, surface runoff increases where heavy machinery compacts soil, therefore peak flows on streams draining farmland are generally higher than in natural ecosystems

Question 8

How does farming impact stores in the carbon cycle?

Answer 8

Clearance of forest for farming reduces carbon storage in both the above and below ground biomass

Soil carbon storage 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

Question 9

How does farming impact flows in the carbon cycle?

Answer 9

Where farming replaces natural grassland, carbon exchanges are generally lower than in natural ecosystems - this is in part explained by a lack of biodiversity in farmed systems and the growth cycle of crops are often compressed into just four or five months

Question 10

How does forestry impact the water cycle?

Answer 10

• Higher rates of rainfall interception in plantations than in natural forest east, in eastern England, interception rates for Sitka spruce are as high as 60%, in upland Britain where temperatures and evaporation are lower, interception is about 30% - in the UK preferred plantation species are conifers, the needle-like structure of consider leaves, their evergreen habit and high density of planting all contribute to high rates of interception
• Increased evaporation, a large proportion of intercepted rainfall is stored on leaf surfaces and is evaporated directly to the atmosphere
• Reduced run-off and stream discharge, with high interception and evaporation rates and the absorption of water by tree roots, drainage basin hydrology is altered, streams draining plantations typically have relatively long lag times, low peak flows and low total discharge, the effect of conifer plantations in upland catchments is often to reduce water yield for public supply
• Compared to farmland and moorland, transpiration rates are increased, typical transpiration rates for Sitka spruce in the Pennines are around 350mm/yr-1 of rainfall equivalent
• Clear felling to harvest timber creates sudden but temporary changes to the local water cycle, increasing run-off, reducing evapotranspiration and increasing stream discharge

Question 11

How does forestry impact the carbon cycle?

Answer 11

• In a typical plantation in the UK, mature forest trees contain on average 170-200 tC/ha^-1, which is 10x higher than grassland and 20x higher than heathland
• The soil represents an even larger carbon pool, in England measurements of forest soil carbon are around 500 tC/ha^-1
• Forest trees extract CO2 from the atmosphere and sequester it for hundreds of years, most of the carbon is stored in the wood of the tree stem, however forest trees only become an active carbon sink (i.e. absorbing more carbon than they release) for the first 100 years after planting
• After this, the amount of carbon captured levels off and is balanced by inputs of litter to the soil, the release of CO2 in respiration and by the activities of soil decomposers
• In consequence, forestry plantations usually have a rotation period of 80-100 years, after this time the trees are felled and reforestation begins afresh

Question 12

What is water extraction?

Answer 12

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

Question 13

What is the significance of the River Kennet?

Answer 13

• River Kennet in southern England drains an area of around 1200km^2 in Wiltshire and Berkshire
• Upper catchment mainly comprises chalk which is highly permeable, so groundwater contributes most of the Kennet’ 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, water voles and white-clawed crayfish
• Within and close to the catchment, several urban areas rely on water from the Kennet basin to meet public supply - Swindon, the largest, has a population of over 200,000
• The Kennet also supplies water for local industries, agriculture and public use, Thames Water abstracts groundwater from the upper catchment from boreholes - none of this water is returned to the river as waste water

Question 14

What effects has water extraction had on the river and regional water cycle?

Answer 14

• Rates of groundwater extraction have exceeded rates of recharge, and the falling water table has reduced lows 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

Question 15

What are aquifers?

Answer 15

• 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
• Within an aquifer the upper surface of saturated is known as the water table, its height fluctuates seasonally and is also affected by periods of exceptional rainfall, drought and abstraction
• In normal years in southern England, the water table falls between March and September as rising temperatures increase evapotranspiration losses - recharge resumes in the late autumn

Question 16

What are artesian basins?

Answer 16

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

London is located at the centre of a synclinal structure which forms an artesian basin, groundwater in the chalk aquifer is trapped between impermeable London Clay and Gault Clay

Rainwater enters the chalk aquifer where it outcrops on the edge of the basin in the North Downs and Chilterns, groundwater then flows by gravity through the chalk towards the centre of the basin, thus under natural conditions the wells and boreholes in the London area are under artesian pressure

Groundwater from the chalk is an important source of water for the capital, however overexploitation in the 19th century and in the first half of the 20th century caused a drastic fall in the water table - in central london it well by nearly 90m

In the past 50 years declining demand for water by industry in London and reduced rates of abstraction have allowed the water table to recover - by the early 1990s it was rising at a rate of 3m/yr^-1 and began to threaten buildings and underground tunnels

Since 1992, Thames Water has been granted abstraction licenses to slow the rise of the water table which is now stable

Question 17

How is the present global economy powered? (energy mix)

Answer 17

• 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 overwhelming dependant on fossil fuels - in 2013 they accounted for 87% of global energy consumption

Question 18

How does the use of fossil fuels impact on the carbon cycle?

Answer 18

• Fossil fuel consumption releases 10 billion tonnes of CO2 to the atmosphere annually, increasing atmospheric CO2 concentration by over 1ppm
• It is estimated that since 1750, cumulative anthropogenic CO2 emissions total nearly 2000GT, 3/4 of these emissions are from the burning of fossil fuels
• Since 1750, 879GT of anthropogenic CO2 emissions have remained in the atmosphere, raising CO2 concentrations from 280ppm to 400ppm
• Today CO2 levels in the atmosphere are the highest they have been for at least 800,000 years
• Although anthropogenic emissions comprise less than 10% of the natural influx from the biosphere and oceans to the atmosphere, they impact significantly on the size of the atmosphere, ocean and biosphere carbon stores
• Despite international efforts to limit human carbon emissions, in the period 2000-09, they grew faster than in any previous decade
• Without increased absorption of anthropogenic carbon by the oceans and biosphere, today’s atmospheric CO2 concentrations would exceed 500ppm

Question 19

What is CCS?

Answer 19

• Combustion of fossil fuels and the transfer of carbon from the geological store to the atmosphere and oceans is 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, process know as carbon capture and storage (CCS)

Question 20

How does CCS work?

Answer 20

• So far, technology has been piloted at a handful of coal-fired power stations
• First, CO2 is separated from power station emissions
• CO2 is then compressed and transported by pipeline to storage areas
• Then is injected into porous rocks deep underground where it is stored permanently

Question 21

How could this technology be beneficial?

Answer 21

• Could eventually play an important part in reducing CO2 and other greenhouse has emissions
• For instance, in the USA 40% of all CO2 emissions are from coal and gas fired power stations and CCS has the potential to reduce these emissions by 80-90%
• In the UK, two CCS pilot projects are already underway: North Yorkshire and north-east Scotland
• The Drax project in Yorkshire will capture 2 million tonnes of CO2 per year, this carbon will then be transported by pipeline to the North Sea and stored in depleted gas reservoirs, CO2 gas can also be pumped into ‘mature’ oilfields to extract oil that would otherwise be uneconomic to recover

Question 22

What are the limitations of CCS?

Answer 22

• Involves big capital costs - The Drax and Peterhead (Scotland) projects will cost at least £1 billion
• Uses large amounts of energy, typically 20% of a power plant’s output is needed to separate the CO2 and compress it
• Requires storage reservoirs with specific geological conditions, i.e. porous rocks overlain by impermeable strata

Question 23

What is positive and negative feedback?

Answer 23

Feedback is an automatic response to changes which disturb a system’s equilibrium - change in natural systems can produce either positive or negative feedback responses
Positive feedback occurs when an initial change causes further change (intensifies original change), negative feedback counters system change and restores equilibrium

Question 24

Examples of positive feedback in the water cycle

Answer 24

Temperatures increase, leads to increased evapotranspiration which increases the amount of water in the atmospheric store, leading to greater cloud cover and more precipitation, this is a positive feedback loop as water vapour is a greenhouse gas, so more vapour in the atmosphere increases absorption of long-wave radiation from the Earth causing further rises in temperature

Question 25

Examples of negative feedback loops in the water cycle

Answer 25

More atmospheric vapour creates greater cloud cover which reflects more solar radiation back into space (albedo effect) - as smaller amounts of solar radiation are absorbed by the atmosphere, oceans and land, average global temperatures fall

Question 26

Examples of positive feedback loops in the carbon cycle

Answer 26

Higher global temperatures speeds up decomposition, releasing more CO2 to the atmosphere, therefore amplifying the greenhouse effect
Arctic Tundra - higher temperatures leads to Arctic sea ice and snow cover shrinking, exposing large expanses of sea and land, this means that more sunlight is absorbed, warming the tundra and melting the permafrost, this is significant as the tundra stores an estimated 1600GT of organic carbon in the permafrost

Question 27

Examples of negative feedback loops in the carbon cycle

Answer 27

Negative feedback could neutralise rising levels of atmospheric CO2 by stimulating photosynthesis in a press called carbon fertilisation
in this way excess CO2 is extracted from the atmosphere and stored in the biosphere, eventually much of this carbon would find its way into long-term storage in soils and ocean sediments, allowing the system to return to a steady state
Increased global temperatures lead to increased evapotranspiration which increases the amount of water in the atmospheric store, leading to condensation and increased cloud cover, contributing to the albedo effect therefore reflecting more solar radiation back into space, less absorbed by the atmosphere, oceans and land, average global temperatures fall

Question 28

How can changes to the global water and carbon cycles be monitored?

Answer 28

• 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
• Ground-based measurements of environmental change at the global scale are impractical, therefore monitoring relies heavily on satellite technology and remote sensing, continuous monitoring by satellite on a daily, monthly or yearly basis allows changes to be observed on various time scales
• Using GIS techniques, this data can then be mapped and analysed to show areas of anomalies and trends, and regions of greatest change

Question 29

How is arctic sea ice monitored?

Answer 29

NASA’s Earth Observing System (EOS) satellites have monitored sea ice growth and retreat since 1978
Measures microwave energy radiated from Earth’s surface, comparison of time series images to show changes

Question 30

How are ice caps/ glaciers monitored?

Answer 30

Ground-based estimates of mass balance and satellite technology such as ICESat-2
Measures surface height of ice sheet and glaciers using laser technology, shows extent and volume of ice and changes

Question 31

How are sea surface temperatures monitored?

Answer 31

NOAA satellites
Radiometers measure the wave band of radiation emitted from the ocean surface, hangers in global SSTs and areas of upwelling and downwelling

Question 32

How is water vapour monitored?

Answer 32

NOAA polar orbiters
Measures cloud liquid water, total precipitable water and long term trends in cloud cover and water vapour in the atmosphere

Question 33

How is deforestation monitored?

Answer 33

ESA albedo (reflectivity) images from various satellites
Measurements of reflectivity of Earth’s surface and land use changes

Question 34

How are atmospheric CO2 levels monitored?

Answer 34

NASA’s Orbiting Carbon Observatory-2 + Ground-based measurements at Mauna Loa, Hawaii, since 1958
Satellites measure amounts of global atmospheric CO2 as well as the effectiveness of absorption of CO2 by plants

Question 35

How is primary production in oceans monitored?

Answer 35

NASA’s MODIS/AQUA
Measures net primary production in oceans and on land

Question 36

What diurnal changes occur within the water cycle?

Answer 36

Lower temperatures at night reduce evaporation and transpiration
Conventional precipitation, dependant on direct heating of the ground surface by the Sun, is a daytime phenomenon often falling in the afternoon when temperatures reach a maximum of
This is particularly significant in climatic regions in the tropics where the bulk of precipitation is from conventional storms

Question 37

What diurnal changes occur within the carbon cycle?

Answer 37

During the daytime, CO2 flows from the atmosphere to vegetation, at night the flux is reversed
Without sunlight, photosynthesis cannot occur and vegetation loses CO2 to the atmosphere through respiration
The same diurnal pattern is observed with phytoplankton in the oceans

Question 38

What seasonal changes occur within the water cycle?

Answer 38

Seasons controlled by variation in intensity of solar radiation - in UK, solar radiation intensity peaks in mid-June
Typical solar output in June is around 800 W/m^2, in December, input falls to little more than 150 W/m^2
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
With large losses of precipitation to evapotranspiration and the exhaustion of soil moisture, river flows in England are normally at their lowest in late summer

Question 39

What seasonal changes occur within the carbon cycle?

Answer 39

• Seasonal variations in the carbon cycle are shown by monthly changes in the net primary productivity of vegetation - middle and high latitudes, day length (photoperiod) and temperature drive seasonal changes in NPP, similar seasonal variations occur in the tropics, though there the main cause is water availability
• During the northern hemisphere summer, there is a net global flow of CO2 from the atmosphere to the biosphere, this causes atmospheric CO2 levels to fall by 2ppm
• At the end of summer, as photosynthesis ends, the flow is reversed with natural decomposition releasing CO2 back to the atmosphere
• Seasonal fluctuations in the global CO2 flux is explained by the concentration of continental land masses in the northern hemisphere - during the growing season, ecosystems such as the boreal and temperate forests extract huge amounts of CO2 from the atmosphere which has a global impact
• In the oceans, phytoplankton are stimulated into photosynthetic activity by rising water temperature, more intense sunlight and the lengthening photoperiod
• Every year in the North Atlantic there is an explosion of microscopic oceanic plant life, which starts in March and peaks in mid-summer, the resulting algal blooms are so extensive, they are visible from space

Question 40

Long-term changes to climate - overview

Answer 40

• 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 5 degrees lower than today, and Scotland, Wales and most of northern England and Ireland were submerged by ice up to 1km thick
• During the warm interglacial periods, temperatures were similar to those of today, however on much longer time scales global temperatures have been even more extreme
• For example, 250 million years ago average global temperatures reaches 22 degrees, at least 7-8 degrees higher than todays - these climatic shifts has a major impact on the water and carbon cycles

Question 41

Long term changes to the water cycle

Answer 41

• During glacial periods, the water cycle undergoes a number of changes, most obvious is the net transfer of after from the ocean reservoir to storage in ice sheets, glaciers and permafrost
• As a result, in glacials the sea level worldwide falls by 100-130m and ice sheets and glaciers expand to cover around 1/3 of the continental land mass
• As ice sheets advance equator-wards they destroy extensive tracts of forest and grassland, the area covered by vegetation and water stored in the biosphere shrinks
• Meanwhile, in the tropics, the climate becomes drier and deserts and grasslands displace large areas of rainforest
• Lower rates of evapotranspiration during glacial phases reduce exchanges of water between the atmosphere and the oceans, biosphere and soils - this, together with so much freshwater stored as snow and ice, slows the water cycle

Question 42

Long term changes to the carbon cycle

Answer 42

• Most striking feature of the carbon cycle during glacial periods is the dramatic reduction in CO2 in the atmosphere
• At times of glacial maxima, CO2 concentrations fall to around 180ppm, while in warmer interglacial periods they are 100ppm higher
• No clear explanation exists for this drop, however it is possible that excess CO2 finds its way from the atmosphere to the deep ocean
• One mechanism is changes in ocean circulation during glacials that bring nutrients to the surface and stimulate phytoplankton growth, phytoplankton fix large amounts of CO2 by photosynthesis before dying and sinking to the deep ocean where the carbon is stored, lower ocean temperatures also make CO2 more soluble in surface waters
• Other changes occur in the terrestrial biosphere, the carbon pool in vegetation shrinks during glacials as ice sheets advance and occupy large areas of the continents - in this process deserts expand, tundra replaces temperate forests and grasslands encroach on tropical rainforests
• With so much of the land surface buried by ice, carbon stored in soils will no longer be exchanged with the atmosphere
• Meanwhile, expanses of tundra beyond the ice limit sequester huge amounts of carbon in permafrost
• With less vegetation cover, fewer forests, lower temperatures and lower precipitation, NPP and the total volume of carbon fixed in photosynthesis will decline
• The implications are an overall slowing of the carbon flux and smaller amounts of CO2 returned to the atmosphere through decomposition