4.1 How important are water and carbon to life on Earth? Flashcards
4.1 How important are water and carbon to life on Earth?
Key idea ➡ Water and carbon support life on Earth and move between the land, oceans and atmosphere.
The importance of water in supporting life on the planet
Scientists believe that water is the key to understanding the evolution of life on Earth as it provides a medium that allows organic molecules to mix and form more complex structures.
The ubiquity of liquid water on Earth is due ti the distance of Earth from the Sun (149.89 million km): it lies in the so-called ‘Goldilocks zone’, which is ‘just right’ for water to exist in its liquid form (H₂O).
Distance of Earth from the Sun
149.89 million km, 93.14 million miles
Water and the creation of benign thermal conditions
Water helps to create benign thermal conditions on Earth. For example, oceans, which occupy 71 per cent of Earth’s surface, moderate temperatures by absorbing heat, storing it and releasing it slowly.
Water also moderates the environment in other ways. Clouds made up of tiny water droplets and ice crystals reflect around a fifth of incoming solar radiation and lower surface temperatures.
At the same time water vapour, a potent greenhouse gas, absorbs long-wave radiation from the Earth helping to maintain average global temperatures almost 15°C higher than they would be otherwise.
The uses of water for flora, fauna and people
Water comprises up to 65-95% of all living organisms and is crucial to their growth, reproduction and other metabolic functions.
Examples include photosynthesis (in plants/flora) and economic activity.
Flora
Plant life
Fauna
Animal life
Photosynthesis (The uses of water for flora, fauna and people)
Plants, which manufacture their own food, need water for photosynthesis, respiration and transpiration. Photosynthesis takes place in the leaves of plants combining CO₂ sunlight and water to make glucose and starches. Respiration in plants and animals converts glucose to energy through its reaction with oxygen, releasing water and CO₂ in the process.
Plants also require water to maintain their rigidity (plants wilt when they run out of water) and to transport mineral nutrients from the soil. In people and animals water is the medium used for all chemical reactions in the body including the circulation of and nutrients. Transpiration of water from leaf surfaces cools plants by evaporation.
Sweating is a similar cooling process in humans. In fur-covered mammals, reptiles and birds, evaporative cooling is achieved by panting.
Economic activity (The uses of water for flora, fauna and people)
Water is an essential resource for economic activity. It is used to generate electricity, irrigate crops, provide recreational facilities and satisfy public demand (drinking water, sewage disposal), as well as in a huge range of industries including food manufacturing, brewing, paper making and steel making.
The importance of carbon to life on Earth
Carbon is a common chemical element. It is stored in carbonate rocks such as limestone, sea floor sediments, ocean water (as dissolved CO₂), the atmosphere (as CO₂ gas), and in the biosphere.
Life as we know it is carbon based: built on large molecules of carbon atoms such as proteins, carbohydrates and nucleic acids.
Apart from its biological significance, carbon is used as an economic resource. Fossil fuels such as coal, oil (hydrocarbons) and natural gas power the global economy.
One example is oil’s usage as a raw material in the manufacture of products ranging from plastics to paint and synthetic fabrics. Agricultural crops and forest trees (a.k.a. carbon sinks) also store large amounts of carbon available for human use as food, timber, paper, textiles and many other products.
Carbon sinks
A forest, ocean, or other natural environment viewed in terms of its ability to absorb carbon dioxide from the atmosphere.
Biosphere
Consists of all life on Earth and all parts of the Earth in which life exists, including land, water, and the atmosphere.
The water and carbon cycle
At the global scale, water and carbon flow in closed systems between the atmosphere, the oceans, land and the biosphere. The cycling of individual water molecules and carbon atoms occurs on time scales varying from days to millions of years.
The water cycle inputs/outputs/stores/transfers
Inputs - precipitation including rain and snow, and solar energy for evaporation.
Outputs - evaporation and transpiration from plants (evapotranspiration), runoff into the sea, percolation of water into underlying rock strata.
Stores - puddles, rivers, lakes (surface storage), soil, groundwater storage and water stored in vegetation.
Transfers - infiltration, percolation, overland flow, through flow, groundwater flow.
Evapotranspiration
The combined amount of evaporation of water from soil and transpiration of water from plants. Correlates with species richness.
Evaporation
The change of a substance from a liquid to a gas, particularly in soil.
Transpiration
Evaporation (the change of a substance from a liquid to a gas) of water from the leaves of a plant.
The carbon cycle inputs/outputs/stores/transfers
Inputs - carbon dioxide added to the atmosphere by human activities.
Outputs - dissolved carbon carried from the land in solution by rivers.
Stores - marine sediments and sedimentary rocks, oceans, fossil fuel deposits, soil organic matter, atmosphere, terrestrial plants,
Transfers - photosynthesis, respiration, combustion, decomposition, diffusion, weathering and erosion, burial and compaction, carbon sequestration,
The global carbon cycle - Principal carbon stores
Atmosphere: 600 (billion tonnes of carbon in store)
-Oceans: 38,700 (billion tonnes of carbon in store)
-Sedimentary (carbonate) rocks: 60,000-100,000,000 (billion tonnes of carbon in store)
-Sea floors sediments 6,000 (billion tonnes of carbon in store)
-Fossil fuels: 4,130 (billion tonnes of carbon in store)
-Land plants: 560 (billion tonnes of carbon in store)
-Soils/peat: 2,300 (billion tonnes of carbon in store)
The slow carbon cycle
Through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere.
(Transfers of carbon compounds over extensive timescales (possibly millions of years).) Page 101.
The fast carbon cycle
Capture of atmospheric carbon dioxide by plants to create glucose and other sugars, which fuels the ecosystem. As these sugars are used in cellular respiration, carbon dioxide is produced.
(Relatively rapid transfers of carbon compounds over years, decades and centuries.) Page 101.
The processes of the water cycle
-The water balance
-Flows
-Catchment hydrology
The water balance (The processes of the water cycle)
`The water balance equation summarises the flows of water in a drainage basin over time. It states that precipitation is equal to evapotranspiration and streamflow, plus or minus water entering or leaving storage: P = R + ET + ΔS
Where P is precipitation, R is streamflow, ET is evapotranspiration, ΔS is the change in storage (in soil or the bedrock / groundwater)
Where Δ
Uppercase delta (Δ) at most times means “change” or “the change” in maths. Consider an example, in which a variable x stands for the movement of an object.
So, “Δx” means “the change in movement.” Scientists make use of this mathematical meaning of delta in various branches of science.
Flows (The processes of the water cycle)
-Precipitation
-Transpiration
-Condensation (Cumuliform clouds, Stratiform, Wispy, cirrus clouds, Lapse rates, Cloud formations and lapse rates)
Flows definition (The processes of the water cycle)
The principal flows in the water cycle that link the various stores are: precipitation, evaporation, transpiration, run-off, infiltration, percolation and throughflow.
Precipitation (Flows (The processes of the water cycle))
Precipitation is water and ice that falls from clouds towards the ground. It takes several forms: most commonly rain and snow, but also hail, sleet and drizzle.
Precipitation forms when vapour in the atmosphere cools to its dew point and condenses into tiny water droplets or ice particles to form clouds. Eventually these droplets or ice particles aggregate, reach a critical size and leave the cloud as precipitation.
Dew point
(Condensation begins) The temperature at which the water vapour in the air becomes saturated.
Transpiration (Flows (The processes of the water cycle))
Transpiration is the diffusion of water vapour to the atmosphere from the leaf pores (stomata) of plants. It is responsible for around 10 per cent of moisture in the atmosphere.
Like evaporation, transpiration is influenced by temperature and wind speed. It is also influenced by water availability to plants.
Transpiration example (Flows (The processes of the water cycle))
Deciduous trees shed their leaves in climates with either dry or cold seasons to reduce moisture loss through transpiration.
Condensation (Flows (The processes of the water cycle))
-Cumuliform clouds
-Stratiform
-Wispy, cirrus clouds
-Lapse rates
-Cloud formations and lapse rates
Condensation definition (Flows (The processes of the water cycle))
Condensation is the phase change of vapour to liquid water. It occurs when air is cooled to its dew point. At this critical temperature air becomes saturated with vapour resulting in condensation.
Clouds form through condensation in the atmosphere.
Condensation at or near the ground (Flows (The processes of the water cycle))
Condensation at or near the ground produces dew and fog. Both types of condensation deposit large amounts of moisture on vegetation and other surfaces.
Dew
Tiny drops of water that form on cool surfaces at night, when atmospheric vapor condenses.
Cumuliform clouds (Condensation (Flows (The processes of the water cycle)))
Cumuliform clouds, with flat bases and considerable vertical development most often form when air is heated locally through contact with the Earth’s surface.
This causes heated air parcels to rise freely through the atmosphere (convection), expand (due to the fall in pressure with altitude) and cool. As cooling reaches the dew point, condensation begins and clouds form.
Wispy, cirrus clouds (Condensation (Flows (The processes of the water cycle)))
Wispy, cirrus clouds, which form at high altitude, consist of tiny ice crystals. Unlike cumuliform and stratiform clouds they do not produce precipitation and therefore have little influence on the water cycle.
Stratiform (Condensation (Flows (The processes of the water cycle)))
Stratiform or layer clouds develop where an air mass moves horizontally across a cooler surface (often the ocean). This process, together with some mixing and turbulence, is known as advection.
Lapse rates (Condensation (Flows (The processes of the water cycle)))
-Environmental lapse rate (ELR):
The ELR is the vertical temperature profile of the lower atmosphere at any given time. On average the temperature falls by 6.5°C for every kilometre of height gained.
-Dry adiabatic lapse rate (DALR):
The DALR is the rate at which a parcel of dry air (I.e. less than 100 per cent humidity so that condensation is not taking place) cools. Cooling, caused by adiabatic expansion, is approximately 10°C/km.
-Saturated adiabatic lapse rate (SALR):
The SALR is the rate at which a saturated parcel of air (i.e. one in which condensation is occurring) cools as it rises through the atmosphere. Because condensation releases latent heat, the SALR, at around 7°C/km, is lower than the DALR.
Cloud formations and lapse rates (Condensation (Flows (The processes of the water cycle)))
Clouds are visible aggregates of water or ice or both that float in the free air. We have seen that they form when water vapour is cooled to its dew point. Cooling occurs when:
-Air warmed by contact with the ground or sea surface, rises freely through the atmosphere. As the air rises and pressure falls it cools by expansion (adiabatic expansion). This vertical movement of air is known as convection.
-Air masses move horizontally across a relatively cooler surface a process known as advection.
-Air masses rise as they cross a mountain barrier or as turbulence forces their ascent. a relatively warm air mass mixes with a cooler one.
Adiabatic expansion
The expansion of a parcel of air due to a decrease in pressure. Expansion causes cooling.
Atmospheric instability
-Lighter, warm or moist air is overlain by
denser cold or dry air.
-Some air sinks, some air rises.
-Condition that promotes vertical motions.
Catchment hydrology (The processes of the water cycle)
-Evaporation
-Interception
-Infiltration, throughflow, groundwater flow and run-off
-Cryospheric processes
Evaporation (Catchment hydrology (The processes of the water cycle))
Evaporation is the phase change of liquid water to vapour and is the main pathway by which water enters the atmosphere. Heat is needed to bring about evaporation and break the molecular bonds of water. But this energy input does not produce a rise of temperature in the water.
Instead the energy is absorbed as latent heat and released later in condensation. This process allows huge quantities of heat to be transferred around the planet: from the oceans to the continents; and from the tropics to the poles.
Interception (Catchment hydrology (The processes of the water cycle))
Vegetation intercepts a proportion of precipitation, storing it temporarily on branches, leaves and stems. Eventually this moisture either evaporates (interception loss) or falls to the ground.
Rainwater that is briefly intercepted before dripping to the ground is known as throughfall. During periods of prolonged or intense rainfall, intercepted rainwater may flow to the ground along branches and stems stemflow.
Infiltration, throughflow, groundwater flow and run-off (Catchment hydrology (The processes of the water cycle))
Rain falling to the ground and not entering storage follows one of two flowpaths to streams and rivers.
-Infiltration by gravity into the soil and lateral movement or throughflow to stream and river channels.
-Overland flow across the ground surface either as a sheet or as trickles and rivulets to stream and river.
Conflicting ideas about Infiltration, throughflow, groundwater flow and run-off (Catchment hydrology (The processes of the water cycle))
Two conflicting ideas explain the flowpaths followed by rainwater. One relates overland flow to the soil’s infiltration capacity or the maximum rate it can absorb rain. Thus, it is argued that when rainfall intensity exceeds infiltration capacity overland flow occurs. The second idea states that rainfall, regardless of its intensity, always infiltrates the soil.
Overland flow only occurs when soil becomes saturated and the water table rises to the surface. This process is known as saturated overland flow.
Where soils are underlain by permeable rocks - Infiltration, throughflow, groundwater flow and run-off (Catchment hydrology (The processes of the water cycle))
Where soils are underlain by permeable rocks, water seeps or percolates deep underground. This water then migrates slowly through the rock pores and joints as groundwater flow, eventually emerging at the surface as springs or seepages. Groundwater levels on the chalk in southern England follow a distinct seasonal pattern.
By late October the water table is beginning to rise as temperatures and evapotranspiration fall. This recharge continues until late January. Groundwater levels then decline throughout the late winter, spring and summer, reaching their lowest point in early autumn.
Cryospheric processes (Catchment hydrology (The processes of the water cycle))
Ablation is loss of ice from snow, ice sheets and glaciers due to a combination of melting, evaporation and sublimation. Meltwater is an important component of river flow in high latitudes and mountain catchments in spring and summer.
Rapid thawing of snow in upland Britain in winter is a common cause of flooding in adjacent lowlands (e.g. Welsh uplands and the Lower Severn Valley, Pennines and the Vale of York).
Sublimation
A change directly from the solid to the gaseous state without becoming liquid.
The processes of the carbon cycle
-Carbon exchanges
-Carbon sequestration in the ocean
Carbon exchanges (The processes of the carbon cycle)
-Precipitation
-Photosynthesis
-Weathering
-Respiration
-Decomposition
-Combustion
Precipitation (Carbon exchanges (The processes of the carbon cycle))
Atmospheric CO₂, dissolves in rainwater to form weak carbonic acid. This is a natural process. However, rising concentrations of CO₂, in the atmosphere, due to anthropogenic emissions, have increased the acidity of rainfall.
This has contributed to increased acidity of ocean surface waters with potentially harmful effects on marine life.
Photosynthesis (Carbon exchanges (The processes of the carbon cycle))
The flux of carbon from the atmosphere to land plants and phytoplankton via photosynthesis averages around 120 gigatonnes (GT) a year. Using the Sun’s energy, CO₂, from the atmosphere and water, green plants and marine phytoplankton convert light energy to chemical energy (glucose) through the process of photosynthesis.
6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂
(Carbon dioxide + water + energy from light produces glucose and oxygen)
Plants use energy in the form of glucose to maintain growth, reproduction and other life processes. In doing so they release CO₂ to the atmosphere in respiration.
Weathering (Carbon exchanges (The processes of the carbon cycle))
Weathering is the in situ breakdown of rocks at or near the Earth’s surface by chemical, physical and biological processes. Most weathering involves rainwater which contains dissolved CO₂, derived from the soil as well as the atmosphere. As we have seen, rainwater is a weak carbonic acid, which slowly dissolves limestone and chalk in a process known as carbonation.
carbonation = CaCO₃ + H₂CO₃ → Ca(HCO₃)₂
(Calcium carbonate + carbonic acid produces calcium bicarbonate)
Respiration (Carbon exchanges (The processes of the carbon cycle))
Respiration is the process in which carbohydrates (e.g. glucose) fixed in photosynthesis are converted to CO₂ and water.
Respiration = C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Respiration in Plants and Animals (Carbon exchanges (The processes of the carbon cycle))
Plants and animals absorb oxygen which ‘burns’ these carbohydrates and provides the energy needed for metabolism and growth. Respiration is the reverse of photosynthesis. Whereas photosynthesis absorbs CO₂ and emits oxygen, respiration absorbs oxygen and releases CO₂.
Respiration and photosynthesis are the two most important processes in the fast carbon cycle. The volume of carbon exchanged by respiration and photosynthesis each year is one thousand times greater than that moving through the slow carbon cycle.
Decomposition (Carbon exchanges (The processes of the carbon cycle))
Decomposer organisms such as bacteria and fungi breakdown dead organic matter, extracting energy and releasing CO₂, to the atmosphere and mineral nutrients to the soil. Rates of decomposition depend on climatic conditions.
The fastest rates occur in warm, humid environments such as the tropical rainforest. In contrast, decomposition is slow in cold environments like the tundra or drylands such as tropical deserts.
Combustion (Carbon exchanges (The processes of the carbon cycle))
Combustion occurs when organic material reacts or burns in the presence of oxygen. The combustion process releases CO₂ as well as other gases such as sulphur dioxide and nitrogen oxides.
Combustion is a natural fuel use in many ecosystems. Wildfires caused by lightning strikes are essential to the health of some ecosystems such as the coniferous forests of the Rocky Mountains.
Long, cold winters slow the decomposition of forest litter which builds up on the forest floor. Fire shifts this log jam, freeing carbon and nutrients previously inaccessible to forest trees. It also opens up the forest canopy, creating new habitats and increasing biodiversity.
Combustion sources (Carbon exchanges (The processes of the carbon cycle))
Combustion also results from human activities such as the deliberate firing of forest and grassland in order to clear land for cultivation or improve the quality of grazing. More important is the combustion of fossil fuels.
Despite international efforts to curb CO₂ emissions, oil, coal and natural gas power the global economy and their consumption continues to grow. Currently the burning of fossil fuels transfers nearly 10 GT of CO₂ a year from geological store to the atmosphere, oceans and biosphere.
Carbon sequestration in the ocean (The processes of the carbon cycle)
-Physical (inorganic) pump
-Biological (inorganic) pump
-Vegetation
Physical (inorganic) pump (Carbon sequestration in the ocean (The processes of the carbon cycle))
The physical carbon pump involves the mixing of surface and deep ocean waters by vertical currents, creating a more even distribution of carbon - both geographically and vertically - in the oceans. Initially CO₂ enters the oceans from the atmosphere by diffusion.
Surface ocean currents then transport the water and its dissolved CO₂ polewards where it cools, becomes more dense and sinks.
Downwelling - Physical (inorganic) pump (Carbon sequestration in the ocean (The processes of the carbon cycle))
Downwelling occurs in only a handful of places in the oceans. One of these places is in the North Atlantic between Greenland and Iceland.
Downwelling carries dissolved carbon to the ocean depths where individual carbon molecules may remain for centuries. Eventually deep ocean currents transport the carbon to areas of upwelling. There cold, carbon-rich water rises to the surface and CO₂ diffuses back into the atmosphere.
Downwelling
The downward movement of fluid, especially in the sea, the atmosphere, or deep in the earth.
Biological (organic) pump (Carbon sequestration in the ocean (The processes of the carbon cycle))
Carbon is also exchanged between the oceans and atmosphere through the actions of marine organisms. Globally nearly half of all carbon fixation by photosynthesis takes place in the oceans. Around 50 GT of carbon is drawn from the atmosphere by the biological pump every year.
Vegetation (Carbon sequestration in the ocean (The processes of the carbon cycle))
Land plants, especially trees in the rainforests and boreal forests, contain huge stores of carbon. Most of this carbon, extracted from atmospheric CO₂ through photosynthesis, is locked away for decades.
