babakar carbon

Why is carbon important and what forms does it take?

Carbon is an essential building block for all life on Earth. Plays major role in regulating global climate, particularly temperature and acidity of rain, rivers and oceans.

Major carbon compounds:

· Carbon dioxide (CO2): Gas found in oceans, soils and atmosphere; waste product in respiration

· Methane (CH4): Greenhouse gas found in rocks, oceans, permafrost, soils

· Hydrocarbons (fossil fuels): Found in sedimentary rocks in gas, liquid or solid form

· Calcium carbonate (CaCO3): Found in limestone rock, shells, eggs

· Carbon biomolecules: Organic molecules including carbohydrates, fats, proteins; form 50% of total dry mass of living things

Describe the carbon cycle as a system.

Carbon cycles operate at global 'sphere' level - lithosphere, hydrosphere, biosphere etc. Have inputs, stores, fluxes/flows and outputs that transfer carbon from one place to another and either deplete or build carbon stores.

Two systems:

· Long-term/slow carbon cycle: Movement of carbon between atmospheric, oceanic and lithospheric stores (100-200 million years)

· Short-term/fast carbon cycle: Movement of carbon from living things to atmosphere and oceans (up to a thousand times more carbon in shorter space of time)

Global carbon system can be subdivided into systems operating on land, oceans and atmosphere - inter-related through fluxes/flows but also distinct subsystems

What are the main carbon stores and their residence times?

Biosphere:

· % of total: 0.001%

· Amount: 3,170 GtC

· Forms: Living plants and animals, including marine and aquatic life

· Residence time: 18 years

Lithosphere:

· % of total: 99.983%

· Amount: 110 million GtC

· Forms: Largest store - sedimentary rocks contain carbon (limestone/calcium carbonate), hydrocarbons (fossil fuels), marine sediments from shells and skeletons

· Residence time: 240-300 million years

GtC = Gigatonnes of carbon dioxide equivalent (one billion tonnes)

What are the remaining main carbon stores?

Pedosphere (soil):

· % total: 0.003%

· Amount: 12,300 GtC

· Forms: Organic matter, soil organisms, remains of dead plants & animals

· Residence time: Days to 1000s of years (peat soils contain highest carbon)

Cryosphere (frozen ground):

· % total: 0.001%

· Amount: 1,700 GtC

· Forms: Permafrost of tundra and arctic regions contains plant material

· Residence time: 1000s of years (ice cores show millions of years)

Atmosphere:

· % total: 0.001%

· Amount: 750 GtC

· Forms: Mainly CO2 and methane (CH4)

· Residence time: 6 years

Hydrosphere (oceans):

· % total: 0.007%

· Amount: 38,000 GtC

· Forms: 90% dissolved as bicarbonate, carbonate ions and dissolved CO2

· Residence time: Surface 25 years, Deep 1250 years

How do carbon stores vary over time and location?

Global distribution of vegetation changes stored carbon:

· Arctic and Sahara Desert = virtually no plant storage

· Amazon rainforest = all-year-round storage

· Carbon uptake higher in middle/high latitudes of northern hemisphere

· Less uptake in southern hemisphere (less land mass)

Seasonal changes:

· Plants grow and decay differently in summer vs winter

· CO2 emissions change with seasons - as plants grow they intake more CO2

· During dormant stage, less CO2 needed

Different terrestrial ecosystems store different amounts - large tropical trees store more carbon than small brambles.

How does carbon transfer occur at a plant scale?

A tree's wood acts as a carbon store (wood is approximately 50% carbon).

Transfers between atmosphere, biosphere and pedosphere through:

· Photosynthesis: Removes CO2 directly from atmosphere

· Respiration: Tree and microbes in soil return carbon to atmosphere as CO2

· Decomposition: Leaf litter or death of tree returns carbon to atmosphere or soil

· Combustion: Wildfires release large amounts of stored carbon in tree

What is a sere and how does carbon transfer occur at sere scale?

A 'sere' is a stage in vegetation succession:

· Lithosere: Succession on bare rock

· Hydroserce: Succession in freshwater (e.g. pond)

· Halosere: Succession in salt-rich conditions (e.g. salt marshes)

· Psammosere: Succession in sandy areas (e.g. sand dunes)

When environmental equilibrium is reached, succession stops - final stage is 'climatic climax community'. In UK, usual climatic climax community for lithosere is deciduous wood.

Carbon cycle at sere scale is much more complex - numerous different stores and transfers vary over space and time.

How does carbon transfer occur at continental scale?

Involves all fast and slow carbon cycles. Connections between stores very complex. Rate of transfers varies over time due to changing conditions on planet. Human activity has added another dimension, particularly with increased additions of carbon dioxide to atmosphere.

How does the physical pump/thermohaline circulation transfer carbon?

Considered most important transfer:

· CO2 absorbed by ocean surface through diffusion

· Dissolved CO2 taken from surface down to intermediate and deep ocean stores through downwelling currents (96 GtC per year)

· Thermohaline circulation distributes carbon around planet

· Cold water absorbs more CO2 - as equatorial waters move toward poles, more CO2 absorbed

· Salinity increases, water denser, sinks (downwelling), taking CO2 to deep ocean stores

· Allows more diffusion at surface, regulating atmospheric carbon

· Upwelling of carbon from intermediate/deep oceans to surface (105.6 GtC yr-1)

· Through upwelling currents and turbulence from surface winds, stored carbon returns to surface and back to atmosphere

Oceans absorb more carbon than they emit (net gain of 0.6 GtC yr-1).

How does the biological cycle sequester carbon in oceans?

Biological cycle sequesters carbon through photosynthesis by phytoplankton and other marine animals (10 GtC per year):

· Converts CO2 into organic matter

· Acts as biological pump transporting carbon from ocean surface to intermediate and deep ocean stores (10 GtC per year)

· As organisms die, dead cells, shells and other parts sink into mid and deep water

· Decay of organisms releases CO2 into intermediate and deep water stores

Oceans regulate atmospheric composition by moving carbon from surface (where it may vent back) and storing it in mid/deep ocean stores, along with dissolved carbon store.

How does the solubility cycle/carbonate pump transfer carbon?

Occurs when CO2 absorbed by oceans from atmosphere:

· Forms carbonic acid which reacts with hydrogen ions to form bicarbonate

· Further reactions form carbonates stored in upper ocean

· Some organisms use carbonates to make shells or skeletons

· When organisms die, some material sinks to ocean floor and forms sea bed sediment store (1750 GtC)

· Over time, through chemical and physical processes, carbon transformed into rocks (e.g. limestone)

· Locks up carbon in long-term cycle, prevents easy return to ocean surface and possible venting into atmosphere (unlike physical pump)

Explain weathering and burial/compaction as carbon transfers.

Weathering (carbonation - chemical weathering):

· Atmosphere has CO2 that combines with water vapour to produce carbonic acid (weak acid)

· Makes precipitation slightly acidic

· Calcium carbonate (calcite) in rocks reacts with acidic water, forms calcium bicarbonate

· Soluble and removed in solution by percolating water via streams, rivers and oceans, eventually back to atmosphere

Burial and compaction:

· Oceans absorb carbon from atmosphere, goes into shells/skeletons of marine creatures as calcium carbonate

· When creatures die, sink to bottom and build up layers

· Over time compact to create sedimentary rocks (e.g. limestone)

· Under heat and pressure, carbon from organic matter trapped in sediment, converted into hydrocarbons

Explain sequestration and combustion as carbon transfers.

Sequestration:

· Natural: Process where carbon removed from atmosphere and stored in liquid or solid form (e.g. rocks of lithosphere, plants of biosphere)

· Formation of hydrocarbons is good example - carbon removed from atmosphere and stored for very long time before release

· Carbon capture and storage (CCS): Technological process of capturing CO2 from industrial sources, separated, treated and transported to long-term storage location

Combustion:

· Tectonic activity over thousands of years moves sea floor toward destructive plate boundaries, subducted into mantle

· Extreme heat and pressure release carbon in rock back to surface, returns to atmosphere through volcanic eruptions (200 million tonnes per year)

· Burning organic material releases energy, water and CO2

· Industrial processes return carbon to atmosphere that would otherwise remain stored in rocks for millions of years

· Wildfires release stored carbon in vegetation back to atmosphere

Explain decomposition and respiration as carbon transfers.

Decomposition:

· When plants/animals die, decompose through animals (worms), bacteria and fungi (collectively decomposers)

· Breaking down carbohydrates releases CO2 and methane back to atmosphere

· Some carbon transferred to soil as humus

· Soil contains millions of micro-organisms forming part of carbon cycle

· Temperature dependent - warmer temperatures show greater microbial activity and faster decomposition

· Water regulates rate - heavily waterlogged areas slow down decomposition (e.g. peat)

Respiration:

· Plants and animals that feed on them break down carbohydrates to release energy to grow and survive

· Release CO2 as by-product through respiration and waste gases as they digest food

· Life on Earth fuelled by breakdown of carbohydrates which releases CO2 back to atmosphere

Explain photosynthesis as a carbon transfer.

Plants are primary producing organisms (make their own food). Use CO2 from atmosphere and water in soil using energy from sunlight to produce carbohydrates. Plants 'fix' gaseous CO2 into solid form in living tissues as glucose. Oxygen released as by-product. In oceans, microscopic organisms (phytoplankton) also photosynthesise.

Outline the process of decomposition in the carbon cycle. (4 marks)

Decomposition is the decay of organic matter by decomposers such as bacteria, worms and fungi. This releases carbon dioxide from the biosphere stored into the atmospheric store. The rate of decomposition is reliant on temperature and availability of water. A higher temperature normally leads to greater microbial activity and therefore higher levels of decomposition. A water-logged area reduces the rate of decomposition.

How does natural climate change affect the carbon cycle?

Over past 2.6 million years, climates fluctuated between interglacial (warm) and glacial (cold) periods. Antarctic ice core data shows temperatures and CO2 levels roughly match. Higher temperatures associated with high CO2 concentrations. Quaternary period had 60 cold periods and warmer interglacial periods. Last ice age ended approximately 25,000 years ago.

· Increase in CO2 leads to enhanced global warming and temperature increase

· Lower CO2 reduces effectiveness of natural greenhouse effect

· Change in temperature can affect CO2 levels due to melting of permafrost releasing methane (positive feedback loop)

How do cold and warm conditions affect the carbon cycle?

Cold conditions:

· Cold water can hold more CO2 but freeze-thaw cycle exposes more rock to chemical weathering

· Extent and location of forests affected - treeline of tundra

· Respiration and photosynthesis processes reduced

· Carbon transfers to soil reduced (decomposers less effective)

· Less sediment transported by rivers and deposited in seas/oceans (more water stored as snow/ice)

· Frozen soil stops transfer of carbon

Warm conditions:

· Increases rate of decomposition, increasing carbon transfer to soil

· Melts permafrost, releasing carbon and methane - enhances greenhouse effect, increases atmospheric warming

· This is positive feedback - further destabilises system

How do wildfires impact the carbon cycle?

Burning transfers carbon from biosphere to atmosphere as CO2. Causes carbon emissions to spike in already rising trend. Extreme wildfires can turn vast areas from carbon sinks to carbon sources.

However, burning can encourage plant growth in long term - some plants need wildfires to grow as it reduces competition for space.

As temperatures increase, risk of wildfires increases. 2019 Arctic wildfires (Siberia, Alaska, Canada, Greenland) contributed significant greenhouse gases. In June alone, these wildfires (some size of 100,000 football pitches) emitted 50 megatonnes of CO2 - equivalent to Sweden's annual total CO2 emissions, more than Arctic fires in same month between 2010-2018 combined.

How does volcanic activity impact the carbon cycle?

Carbon stored in rocks for millions of years released, mainly as CO2 gas, through volcanic eruptions. During Palaeozoic era (542-251 million years ago), volcanoes more active than now, releasing vast quantities of CO2. Today, 130-380 million tonnes of CO2 released annually through volcanic activity. Compared to 30 billion tonnes released by human activities (fossil fuels), volcanic contribution is relatively low.

However, volcanic activity can impact carbon cycle through reducing photosynthesis rates, also impacting water cycle. Example: 1815 Indonesian Mt Tambora eruption emitted sulphur dioxide gas, reduced insolation and lowered global temperatures by 0.4-0.7°C in 1816.

Outline the process of combustion in the carbon cycle. (3 marks)

Combustion (burning) is one method where carbon is transferred between the stores of the biosphere and pedosphere/lithosphere as a solid to the atmospheric store as a gas. As a result of combustion, the magnitude of these stores is changed. Wild fires and volcanic activity are examples of natural drivers of combustion. Burning fossil fuels is an example of a human cause of combustion.

How does extracting hydrocarbons impact the carbon cycle?

Hydrocarbons lock up carbon within lithosphere over long periods. Carbon diffuses from atmosphere allowing phytoplankton and animal life to use carbonate ions to form shells/skeletons. Upon death, sink to ocean floor where (over long time) compressed into oil, coal and gas.

Rate of extraction increased over time, moving lithospheric carbon stores to atmosphere. Extractive industries (e.g. Canada tar sands) responsible for half of Earth's carbon emissions. Since 1970, hydrocarbon extraction risen from 6bn to 15bn tonnes - three times faster despite population only doubling. Extraction processes destroy environment, biodiversity lost, reducing vegetation available to photosynthesise CO2 from atmosphere.

How does burning hydrocarbons impact the carbon cycle?

Burning fossil fuels affects finely balanced climate. Since 1960s, global CO2 concentrations risen from 320 ppm to over 418 ppm (2022) - highest recorded level ever. Levels remained fairly cyclical prior to industrial revolution, then continued increasing with no sign of dropping.

As CO2, methane and water vapour released, act as greenhouse gases and trap heat within Earth's atmosphere. Anthropogenic activities responsible for almost all increase in greenhouse gases over last 150 years. 90% of global CO2 emissions from burning fossil fuels and industry. Emissions from coal (42%), oil (33%), gas (19%), cement (6%) and gas flaring (1%). Natural gas (promoted as cleaner) accounts for fifth of world's total carbon emissions. China and USA dominate CO2 emissions

How do farming practices impact the carbon cycle?

Ten per cent of all released carbon due to land use changes. Agricultural practices release carbon indirectly (burning fossil fuels to run machinery, fertilisers based on fossil fuels) and directly through:

· Ploughing and harvesting - disturbing soil, burning straw stubble

· Slash-and-burn techniques for livestock rearing

· Livestock digestive processes produce methane (5.5 tonnes/20% of USA's total methane emission from cattle)

· Bacteria in waterlogged rice fields produce methane as by-product

· Rice field soil microbes produce nitrous oxide (very potent greenhouse gas)

Impact of these gases roughly equivalent to 1,200 coal power plants. 50% of world's population relies on rice as primary food source.

How does urbanisation impact the carbon cycle?

Significant impact on local carbon cycles - trees removed, parks replaced with housing, surfaces replaced with concrete pavements. Just 2% of Earth's total land area is urban but responsible for 97% of anthropogenic CO2 emissions. Emissions originate from transport, industry and land development.

Cement and concrete key building materials - chemical conversion of limestone to lime creates CO2. Globally, cement industry contributes around 7.5% of total anthropogenic CO2 emissions. Amount of CO2 released depends on materials used, firing of limestone, and types of fossil fuels burned through various stages. If cement industry were a country, it would be third largest emitter in world (behind China and US) - more CO2 than aviation fuel (2.5%) and not far behind global agriculture (12%).

How does deforestation impact the carbon cycle?

Deforestation generates 20% of global CO2 emissions. Undisturbed forestry is carbon neutral - decomposition and slow CO2 release compensated through growth of new vegetation.

Burning wood for heating or clearing woodland (slash and burn), along with wildfires, immediately releases CO2 into atmosphere. Conversion of woodland (grass for cattle rearing) reduces system's ability to absorb CO2 in future - making it carbon source instead of sink.

Deforestation concentrated around tropical areas for building, mining, ranching and commercial crops (soya, palm oil). Timber valuable source of income through wood products.

What is the carbon budget?

The carbon budget is the amount of carbon stored and transferred within the carbon cycle on global or local scale. Includes carbon emissions by various processes (i.e. burning fossil fuels) against natural or human sequestration.

Calculated using carbon footprint calculator and defined as: the total amount of greenhouse gases produced (directly and indirectly) to support human activities, expressed in gigatonnes of carbon dioxide equivalent per year (GtC/yr).

One GtC = one billion (1,000,000,000) tonnes = also referred to as a petagram (Pg).

How does the carbon cycle impact the atmosphere and land?

Atmosphere:

· CO2 in atmosphere warms Earth through natural greenhouse effect

· Increases in carbon emissions leads to enhanced greenhouse effect, threatens delicate atmospheric balance

· Carbon stored by vegetation acts as carbon sinks, holding carbon on short-term basis

· Wildfires and deliberate burning release carbon quickly into atmosphere

Land:

· Soil made from organic matter and cycled through carbon system, brings important trace nutrients to assist further vegetation growth

· Carbon stored in grass provides food for animals (hay, silage)

· Carbon provides energy as fossil fuels (hydrocarbons) and wood

· Deforestation disturbs balance between length and capability of trees to store carbon - removal not only releases carbon but reduces availability of oxygen and water

· Carbon also provides valuable resource in charcoal, diamonds and graphite

How does the carbon cycle impact the ocean?

Calcium carbonate derived from carbon is utilised by marine animals to build shells and skeletons

· Plant and animal remains dropped to ocean floor compress into sedimentary rock and hydrocarbons - effectively locking carbon in long-term cycle

· Phytoplankton utilise CO2 during photosynthesis, transferred along marine food chain

· Increased levels of CO2 and warming of oceans has led to algal blooms and blocking of sunlight for photosynthesis by phytoplankton

What is the examiner's tip regarding CO2?

Remember that CO2 is necessary in the atmosphere as it is just one of the greenhouse gases that maintains the Earth's temperature at an average of 14°C. The issue is the amount of extra CO2 that is being released into the atmosphere, which is driving the normal temperature up.