The carbon cycle Flashcards
what are the key features of carbon?
-it is a solid, black element which bonds easily with other molecules and so is found in many forms
-the main compounds of carbon are carbon dioxide, methane, calcium carbonate, hydrocarbons ( fossil fuels ) and biomolecules ( organic molecules including carbohydrates, proteins, DNA etc. )
-it plays a major role in regulating global climate, particularly temperature and the acidity of rain, rivers and oceans
how is carbon stored in the lithosphere?
-over 99.9% of the carbon on earth is stored in sedimentary rocks, such as limestone ( CaCO3 ), and marine sediments
-about 0.004% is stored in fossil fuels, such as coal and oil, in the lithosphere
-organic forms of carbon include litter, organic matter and humic substances found in soils
how is carbon stored in the hydrosphere?
-CO2 is dissolved in rivers, lakes and oceans
-the oceans are the second largest carbon store on earth, containing approximately 0.04%, with the majority found in the deep layer of water in the form of dissolved inorganic carbon
-a small amount is found at the ocean surface where it is exchanged with the atmosphere
how is carbon stored in the atmosphere?
-contains about 0.001% of the earth’s carbon, mainly as CO2 and CH4 ( greenhouse gases )
-human activity has caused CO2 levels in the atmosphere to increase by around 40% since the industrial revolution
how is carbon stored in the biosphere?
-contains about 0.004% of the earth’s total carbon, 19% of which is stored in the tissues of plants which is transferred to the soil ( pedosphere ) when they die and decay
-others stores include plant litter, soil humus, peat and animals
-global stores are unevenly distributed, e.g. the oceans are larger in the southern hemisphere and storage in the biosphere mostly occurs on land, so terrestrial plant storage is focussed in the tropics and the northern hemisphere
how is carbon stored in the cryosphere?
-contains less than 0.01% of the earth’s carbon, most of which is in the soil in areas of permafrost ( permanently frozen ground ) where decomposing plants and animals have frozen into the ground
how much carbon is stored in each of the earth’s subsystems?
-lithosphere = 60-100 million Gt
-hydrosphere = 38,000 Gt
-atmosphere = 750 Gt
-biosphere = 3170 Gt
-pedosphere = 2300 Gt
-cryosphere = 1700 Gt
how long is carbon stored in each of the earth’s subsystems?
-lithosphere = 240-300 million years
-hydrosphere = surface 25 years, deep 1250 years
-atmosphere = 6 years
-biosphere = 18 years
-pedosphere = days to 1000s of years
-cryosphere = 1000s of years
what is the carbon cycle?
-the process by which carbon is stored and transferred
-it is a closed system as there are inputs and outputs of energy, but the amount of carbon in the system remains the same
how does carbon flow between the major stores in the carbon cycle?
-long-term / slow carbon cycle = the movement of carbon between the atmospheric, oceanic and lithospheric stores, which takes between 100 and 200 million years
-short-term / fast carbon cycle = the movement of carbon from living things to the atmosphere and oceans, which moves up to 1000 times more carbon in a much shorter space of time
what is the difference between a net carbon source and sink?
-net carbon source = releases more carbon into the atmosphere than it absorbs, e.g. the burning of fossil fuels or volcanic eruptions
-net carbon sink = absorbs more carbon from the atmosphere than it releases, e.g. plants, the ocean and soil
what factors ( i.e. flows ) drive the change in magnitude of carbon stores?
-photosynthesis
-respiration
-decomposition
-combustion
-ocean uptake and loss / oceanic carbon pumps
-carbon sequestration
-weathering
what is photosynthesis?
-the process by which plants and phytoplankton use energy from the sun to react CO2 and water to produce glucose and oxygen, enabling them to grow
-this transfers carbon stored in the atmosphere to biomass, which is passed through the food chain and released through respiration and decomposition, helping to maintain the balance between oxygen and CO2 in the atmosphere
what is respiration?
-the process by which plants and animals break down glucose for energy, releasing CO2 and CH4
-this transfers carbon from living organisms in the biosphere to the atmosphere
-over geologic time, there has been more O2 put into the atmosphere and CO2 removed by photosynthesis than the reverse by respiration as not all organic matter is oxidised
what is decomposition?
-the process by which decomposers ( e.g. bacteria and fungi ) break down the cells and tissues in dead organisms into smaller organic or inorganic matter, releasing CO2 and CH4
-this transfers carbon from dead biomass to the atmosphere and the soil in the form of humus
why is decomposition important?
-it ensures that the important elements of life ( C, H, O, N, P, S, Mg ) can be continually recycled into the soil and made available for life
-e.g. plants need a supply of N, P and S atoms from the soil in addition to the C, H and O atoms from photosynthesis to make its DNA molecules
-therefore, plant growth is limited by the availability of mineral ions in the soil and the reactants in photosynthesis, which impacts the carbon cycle because decomposition enables plants to grow and store carbon in the biosphere before returning it back to the atmosphere by respiration and decomposition to be taken up again by photosynthesis
what is combustion?
-the process of burning fossil fuels or organic matter in the presence of oxygen to release CO2, H2O and energy
-this transfers carbon stored in living, dead or decomposed biomass ( including peaty soils ) to the atmosphere
what is ocean uptake and loss / oceanic carbon pumps?
-CO2 is directly dissolved from the atmosphere into the ocean, and transferred to the oceans when it is taken up by organisms that live in them ( e.g. plankton )
-carbon is also transferred from the ocean to the atmosphere when carbon-rich water from deep in the oceans rises to the surface and releases CO2
what is the process of vertical deep mixing?
-CO2 is more soluble as the temperature of the water decreases, so more CO2 can dissolve into colder water, which occurs at the ocean surface due to the close proximity to the atmosphere
-warm water in oceanic surface currents is carried from the warm tropics to the cold polar regions where the water is cooled, making it dense enough to sink below the surface layer
-when cold water returns to the surface and warms up again, it loses CO2 to the atmosphere
-this ensures that CO2 is constantly being exchanged between the ocean and the atmosphere by acting as an enormous carbon pump
what is meant by the biological pump?
-the process by which inorganic carbon ( CO2 ) is fixed into organic carbon by photosynthesis and then sequestered away from the atmosphere by transport into the deep ocean
what is the process of the biological pump?
-marine organisms sequester carbon when they photosynthesise and so it gets incorporated into them as organic matter or structural CaCO3
-when organisms die, their dead cells, shells and other parts sink into deep water where they decay and decompose, releasing CO2
-some material sinks right to the bottom of the ocean where it forms layers of carbon-rich sediments, and burial by overlying layers can eventually turn these sediments into sedimentary limestone
what is carbon sequestration?
-the capture of CO2 from the atmosphere, or anthropogenic ( human ) CO2 from large-scale stationary sources like power plants before it is released to the atmosphere, before it is put into long-term storage
-carbon from the atmosphere can be sequestered ( captured and held ) in sedimentary rocks or as fossil fuels ( until we burn them ) which form over millions of years when dead animal and plant material in the ocean falls to the floor and is compacted
what is weathering?
-the breakdown of rocks in situ by a combination of weather, plants and animals, which transfers carbon from the atmosphere to the hydrosphere and biosphere
what is the geological component of the carbon cycle?
-where the carbon cycle interacts with the rock cycle in the processes of weathering, burial, subduction and volcanic eruptions
-atmospheric carbon reacts with water vapour to form carbonic acid, which reaches the surface as rain and dissolves rocks through chemical weathering
-mineral ions are carried in surface waters like streams and rivers to the ocean where they settle out as minerals ( e.g. calcite ) and react with dissolved CO2 to form CaCO3, which is used by sea creatures ( e.g. to make shells )
-when marine organisms die, their skeletons sink to the bottom of the oceans where they collect as sediment, and burial by overlying layers can eventually turn these sediments into sedimentary limestone
-subduction causes the sea-floor deposits to heat up in the mantle, melt and then rise back up to the surface through volcanic eruptions, returning CO2 to the atmosphere
how do carbon flows happen over different time and spatial scales?
-fast carbon flows ( e.g. photosynthesis, respiration, decomposition and combustion ) quickly transfer carbon between sources, which only takes a matter of minutes, hours or days
-in contrast, sequestration is a slow carbon flow, which takes millions of years for carbon to be sequestered in sedimentary rocks
-they also depend on spatial scale, e.g. at a plant scale, respiration and photosynthesis are the main flows, at an ecosystem scale, carbon flows such as decomposition and combustion also occur, whereas at a continental scale, all of the carbon flows including sequestration occur
what are the natural drivers of combustion / change in the carbon cycle over time?
-wildfires
-volcanic activity
how do wildfires drive change in the carbon cycle over time?
-burning rapidly transfers large quantities of carbon from biomass or soil to the atmosphere as CO2, and loss of vegetation decreases photosynthesis so less carbon is removed from the atmosphere
-in the longer term, fires can encourage the growth of new plants, which take in carbon from the atmosphere for photosynthesis, and some plants need wildfires to grow as it reduces competition for space
-therefore, depending on the amount and type of regrowth, fires can have a neutral effect on the amount of atmospheric carbon
-however, increasingly large and more frequent fires ( possibly made worse by warming temperatures and changing precipitation levels ) can change the carbon balance
how does volcanic activity drive change in the carbon cycle over time?
-carbon ( stored in rocks within the earth for millions of years ) is released to the atmosphere, mainly as CO2, during volcanic eruptions
-today, between 130 and 380 million tonnes of CO2 is released annually through volcanic eruptions, which is relatively low compared to millions of years ago and the 30 billion released by human activities, so they contribute a fairly low proportion of CO2 to the overall carbon cycle
-however, there is the potential for a very large eruption to significantly disrupt the carbon cycle by reducing photosynthesis rates ( thus also affecting the water cycle ), e.g. the 1815 Mt Tambora eruption in Indonesia emitted SO2 which entered the atmosphere, reducing insolation and lowering global temperatures by 0.4-0.7°C in 1816
what are the human / anthropogenic drivers of change in the carbon cycle over time?
-hydrocarbon ( fossil fuel ) extraction and burning
-deforestation
-farming practices
-land use change
how does hydrocarbon extraction and burning drive change in the carbon cycle over time?
-dead plants or animals that were unable to decompose millions of years ago ( due to the lack of oxygen ) turned into fossil fuels following burial due to the pressure from multiple layers of sediment
-extracting and burning / combusting fossil fuels to produce energy releases CO2 into the atmosphere, but without human intervention, the carbon would remain sequestered in the lithosphere for thousands or millions of years to come
-also, extraction processes destroy the environment and biodiversity is lost, which reduces the availability of vegetation to photosynthesise the CO2 from the atmosphere
-since the industrial revolution, carbon flows from the lithosphere to the atmosphere much faster than it would do so naturally, e.g. global energy-related CO2 emissions rose to a historic high of 33.1 Gt in 2018, with electricity generation accounting for just over 60% of the 1.7% growth since 2017 ( mostly in China, India and the USA due to coal burning )
how does deforestation drive change in the carbon cycle over time?
-forests may be cleared for agriculture, logging, or to make way for developments which reduces the size of the carbon store, and if this involves the burning of above-ground biomass, there is a rapid flow of carbon from the biosphere to the atmosphere
-also, the removal of vegetation decreases photosynthesis and respiration, increasing the amount of atmospheric carbon
how do farming practices drive change in the carbon cycle over time?
-when soil is ploughed, the layers invert, air mixes in, and soil microbial activity increases so organic matter is broken down much more rapidly, releasing carbon stored in the pedosphere into the atmosphere
-livestock ( pastoral farming ) accounts for 14.5% of all anthropogenic carbon emissions as CH4 is released when they digest food ( enteric fermentation ) and CO2 is released when they respire
-arable farming also increases the concentration of greenhouse gases in the atmosphere as growing rice in rice paddies releases a lot of methane ( making up 10% of total agricultural emissions )
-as the world’s population has risen, so has food production and the use of agricultural machinery, meaning that carbon emissions from farming practices have increased
how does land use change drive change in the carbon cycle over time?
-as well as deforestation, the change of land use from natural or agricultural to urban is a major source of carbon
-this is because vegetation is removed to make way for buildings, reducing carbon storage in the biosphere, and the cement manufacture required for all the buildings and infrastructure releases lots of CO2
what is meant by the carbon balance / budget?
-the difference between the inputs of carbon into a subsystem and outputs of carbon from it
-the balance of the inputs and outputs of a subsystem determines whether it acts as a carbon source ( outputs outweigh inputs ) or a carbon sink ( inputs outweigh outputs )
what are the two main types of carbon sequestration?
-geologic = CO2 is captured at its source ( e.g. power plants or industrial processes ) and then injected in liquid form into stores underground ( e.g. depleted oil and gas reserves or the deep ocean )
-terrestrial / biologic = plants capture CO2 from the atmosphere and store it as carbon in their stems and roots, as well as in the soil
what are the advantages and disadvantages of terrestrial sequestration?
-✓ = has the potential to increase the world’s forest cover by 0.9 billion ha without affecting existing cities or agriculture, protection of existing forests will preserve current carbon sinks, restoring degraded forests increases biomass density and thus carbon uptake by the biosphere, trees in croplands ( silviculture ) and orchards can store carbon both above and below ground and reduce CO2 emissions if they are grown as a renewable source of fuel
-✘ = a forest might lose the captured carbon back to the air in a catastrophic forest fire or if it suffers disease or infestation, land-based sequestration plantations are slow growing and require active monitoring and management for many decades, carbon is never removed permanently from the atmospheric system as plants respire and return some to the atmosphere, trees lower the albedo in deserts ( e.g. Israel ) which causes warming, planting monocultures decreases biodiversity and spreads disease
what is the impact of the carbon cycle on the land?
-it allows plants to grow as if there was no carbon in the atmosphere, plants couldn’t photosynthesise, and if there was no decomposition, dead plants would remain where they fell and their nutrients would never be recycled
-changes in the carbon cycle can reduce the amount of carbon stored in the land, e.g. warmer temperatures caused by global warming are causing permafrost to melt, releasing carbon previously stored in the ground into the atmosphere
-an increase in global temperatures could also increase the frequency of wildfires
what is the impact of the carbon cycle on the oceans?
-CO2 is dissolved directly into the oceans from the atmosphere where it is used by organisms such as phytoplankton and seaweed during photosynthesis and by other marine organisms to form calcium carbonate shells and skeletons
-increased levels of CO2 in the atmosphere can increase the acidity of the oceans because the oceans initially absorb more CO2 ( leaving as much as 20% in the atmosphere ), e.g. since 1750, the pH of the ocean’s surface has dropped by 0.1 / 30%
-carbonic acid reacts with carbonate ions in the water to form bicarbonate, which means that there are less carbonate ions available for marine organisms like coral to build their shells, resulting in them being thinner and more fragile and a reduction in biodiversity
-this threatens the survival of coastal communities through e.g. reduced food availability and increased storm surges
-also, global warming can affect oceans, e.g. organisms that are sensitive to temperature ( such as phytoplankton ) may not be able to survive at higher temperatures so their numbers decrease, meaning that less CO2 is used by them for photosynthesis and removed from the atmosphere
-warmer water is also less able to absorb CO2, so as temperatures rise the amount of CO2 that could potentially be dissolved in the sea decreases along with the effectiveness of the oceans as a carbon sink
what is the impact of the carbon cycle on the atmosphere and climate?
-the carbon cycle affects the amount of gases containing carbon ( e.g. CO2 and CH4 ) in the atmosphere, which are greenhouse gases and so trap some of the sun’s energy before re-emitting it in all directions, keeping the planet warm ( greenhouse effect )
-as the concentrations of GHGs in the atmosphere increase ( e.g. due to human activities ), temperatures are expected to rise ( i.e. global warming )
-changes in temperature across the globe will affect other aspects of climate, e.g. more intense storms are predicted
what are the relative impacts of the main GHGs?
-most abundant = CO2 ( 394ppm )
-strongest effect = halocarbons ( 1000x more warming than CO2 )
-longest atmospheric lifetime = halocarbons, nitrous oxide and CO2 ( around 100 years, compared to 12 years for CH4 )
what is the greenhouse effect?
-insolation passes through the atmosphere and is either reflected back to space or absorbed by the earth’s surface
-the absorbed solar radiation is re-emitted as IR, and in the atmosphere, some IR is absorbed by GHGs and then re-emitted in all directions, warming the earth’s surface
what is meant by radiative forcing?
-the amount that the earth’s energy budget is out of balance / the difference between the incoming solar energy absorbed by the earth and energy radiated back to space
-radiative forcing has increased from negligible in 1750 to around 2.8 watts/m2 in 2023 due to increased GHG emissions and changing albedos as a result of land use changes ( enhanced greenhouse effect )
what is the impact of the enhanced greenhouse effect on the ocean?
-ocean warming causes sea ice to melt, resulting in a positive feedback loop as the highly reflective ice is replaced by water ( lower albedo ) so the ocean is able to absorb more sunlight, which in turn amplifies the warming that caused it to melt in the first place
-sea ice provides a habitat for algae and so the loss of it affects marine life all the way up the food chain ( e.g. from fish to polar bears ), leading to population declines due to the increased competition for resources
-a decrease in salinity in the deep North Atlantic has been observed due to the melting of freshwater ice ( e.g. in Greenland ), which has been linked to a possible slowing down of the large-scale oceanic circulation in the North-East Atlantic
by how much are sea levels rising and why?
-a rate of 3.1 mm/year since the early 1990s
-due to the melting of terrestrial ice and thermal expansion as when water heats up, it expands
what is an example of a positive and negative feedback loop in the carbon cycle?
-positive = greenhouse effect increases, so temperatures rise and thus the rate of respiration in plants increases, increasing the amount of CO2 ( a GHG ) in the atmosphere, which enhances the greenhouse effect
-negative = atmospheric CO2 increases, which causes more plant growth and so plants remove and store more CO2 from the atmosphere, which in turn reduces atmospheric CO2
what is an example of a tropical rainforest and why is it important?
-the Amazon Basin is the world’s largest rainforest and one of the most biodiverse, with 300 billion trees and 15,000 species which store 1/5 of all the carbon in the planet’s biomass
-it is home to over 500 species of mammals and 2000 species of fish, many of which are endangered ( such as the Amazonian manatee, black caiman and pirarucu )
-it covers 40% of the South American landmass ( i.e. 9 countries ) and is home to 34 million people who depend on the resources it provides
how has the amount of carbon absorbed by and stored in the Amazon changed over time and what is the impact of this?
-decreased from 2 billion tonnes per year in the 1990s to 600 million tonnes in 2019, storing a total of 76 billion tonnes and forming a carbon sink of 1-3 GtC/year
-the increasing concentration of CO2 in the atmosphere has led to increased productivity in the Amazon rainforest because the vegetation is able to access more CO2 for photosynthesis, so the amount of biomass has been increasing
-however, although trees are growing more quickly, they are also dying younger, so we may not be able to rely on the Amazon rainforest to continue to be such an effective carbon sink in the future, e.g. the south-eastern part of the forest ( about 20% of the total area ) has become a carbon source and lost its ability to absorb large quantities of CO2
what are the human drivers of change in the Amazon?
-deforestation
-climate change
how does deforestation drive change in the Amazon?
-between 2005 and 2012, deforestation in the Brazilian Amazon rapidly decreased from over 18,000 km2 to just under 5,000 km2 ( by 13,000km2 in 7 years ), which was followed by a more gradual increase by 7000 km2 to about 12,000 km2 in 2019, with pasture ( beef ) consistently being the greatest driver of forest loss, primarily using slash and burn techniques
what is the impact of deforestation on the water cycle?
-there is no tree canopy to intercept rainfall, so more water reaches the ground surface but there is too much to infiltrate into the soil, so instead the water moves to rivers as surface runoff, which increases the risk of flooding
-the rate of EVT is reduced, meaning that less water vapour reaches the atmosphere, fewer clouds form and rainfall is reduced, which increases the risk of drought
what is the impact of deforestation on the carbon cycle?
-without roots to hold the soil together, heavy rain washes away the nutrient-rich top layer of soil, transferring carbon stored in the soil to the hydrosphere
-there is less leaf litter, so humus isn’t formed and the soil cannot support much new growth, which limits the amount of carbon that is absorbed
-trees remove CO2 from the atmosphere and store it, so fewer trees means more atmospheric CO2, which enhances the greenhouse effect and global warming
how does climate change drive change in the Amazon?
-in some areas, temperature is increasing and rainfall is decreasing, which leads to drought, e.g. the Amazon had severe droughts in 2005, 2010 and 2015-16
-plants and animals living in tropical rainforests are adapted to moist conditions, so many species die in dry weather, and frequent or long periods of drought could lead to extinction of some species, in addition to forest fires, which can destroy large areas of forest, releasing lots of CO2 into the atmosphere
-scientists predict that a 4°C temperature rise could kill 85% of the Amazon rainforest, which would result in lots of carbon being released into the atmosphere as the dead material decomposed, and less CO2 being taken in from the air by trees for photosynthesis
