Topic 6: The Carbon Cycle and Carbon Insecurity Flashcards
(188 cards)
What is the natural carbon cycle
- the movement and storage of carbon between the land, ocean and the atmosphere.
What are the three forms of carbon in the Carbon Cycle?
- inorganic: found in rocks as bicarbonates and carbonates
- organic: found in plant material and living organisms
- gaseous: found as CO2 and CH4 (methane)
Why is carbon important?
It plays a major role in regulating global climate, particularly temperature and the acidity of rain, rivers and oceans.
How is carbon stored?
- Atmosphere - as carbon dioxide and compounds such as methane
- Hydrosphere - as dissolved carbon dioxide
- Lithosphere - as carbonates in limestone and fossil fuels (e.g. coal, oil and gas)
- Biosphere - in living and dead organisms
Three components of carbon cycle:
- Stores - where carbon is held, e.g. atmosphere or lithosphere
- Fluxes (transfers) - the flows which move carbon between stores (from one sphere to another) measured in petagrams or gigatonnes of carbon per year. e.g. photosynthesis and respiration.
- Processes - the physical mechanisms which drive the fluxes between stores e.g. photosynthesis and diffusion
What is the geological carbon store?
- natural cycle that moves carbon between land, oceans and atmosphere.
- involves a number of chemical reactions that create new stores which trap carbon for significant periods of time.
- tends to be a natural balance between carbon production and absoption. However there can be occasional disruptions and short periods before balance is restored, e.g. major volcanic eruptions emitting large quantities of carbon.
Process of geological carbon cycle:
- terrestrial carbon, held within mantle, is released into the atmosphere as CO2 when volcanoes erupt. This is known as out-gassing.
- CO2 within the atmosphere combines with rainfall to produce carbonic acid that dissolves carbon-rich rocks, releasing bicarbonates. This is chemical weathering.
- rivers transport weathered carbon and calcium sediments to the oceans, where they are deposited.
- carbon in organic matter from plants and animal shells and skeletons sinks to the ocean bed when they die, building up strata of coal, chalk and limestone.
- carbon-rich rocks are subducted along plate boundaries and eventually emerge again when volcanoes erupt.
- the presence of intense heating along subduction plate boundaries metamorphoses sedimentary rock by baking, creating metamorphic rocks. CO2 is released by the metamorphism of rocks rich in carbonates during this process.
geological stores of carbon - out-gassing:
- The Earth’s crust contains pockets of carbon dioxide which can be disturbed by volcanic eruptions or seismic activity
- This release of gas that has been dissolved, trapped, frozen or absorbed in rock is called outgassing
- Outgassing happens at:
+ Volcanic zones associated with plate boundaries (including subduction zones and spreading ridges)
+ Areas with no current volcanic activity, e.g., the geysers in Yellowstone National Park, USA
+Direct emissions from fractures in the Earth’s crust - The gas released by volcanic eruptions is relatively insignificant in comparison to human activity
+ Volcanoes currently emit 0-15 - 0.26 Gt carbon dioxide annually
+ Fossil Fuel use emits about 35 Gt
geological stores of carbon - chemical weathering:
- chemical weathering water reacts with carbon dioxide in the atmosphere forming carbonic acid, which reaches the surface as rain dissolving surface minerals.
- rivers transport calcium ions from the land to oceans which combine with bicarbonate ions to form calcium carbonate
- deposition and burial turns the calcite sediment into limestone.
- subduction of the sea floor (tectonic uplift can expose buried limestone)
- some carbon rises back to the surface within magma which is returned to the atmosphere as carbon dioxide.
What are the four key processes involved in the bio-geochemical carbon cycle?
- photosynthesis: removing CO2 from the atmosphere to promote plant growth.
- respiration: releasing CO2 into the atmosphere as animals consume plant growth and breathe.
- decomposition: breaking down organic matter and releasing CO2 into the atmosphere.
- combustion of biomass and fossil fuels - releasing CO2 and other greenhouse gases into the atmosphere.
Variations in fluxes - fast and slow:
- ## if it’s too dark, hot or cold, and low levels of CO2 the speed of the cycle reduces.
Variations in fluxes - geographical patterns:
- levels are always higher in the Northern hemisphere as it contains greater landmasses and greater temperature variations than in the Southern Hemisphere.
What is carbon sequestration?
Sequestering is the movement of carbon into carbon stores which can lower the amount of carbon dioxide in the atmosphere
Photosynthesis (by land based plants and phytoplankton) is the main process responsible for sequestering carbon from the atmosphere
What is ocean sequestration?
- 93% of carbon dioxide is stored in undersea algae, plants, coral and dissolved form, making oceans the largest carbon store on Earth
- The movement of carbon within oceans is controlled:
+Vertically by carbon cycle pumps
+Horizontally by thermohaline circulation - There are three carbon cycle pumps which move carbon dioxide to the sea floor and to the ocean surface to be released into the atmosphere
Ocean sequestration - biological pump:
- The biological cycle sequesters carbon in the ocean through photosynthesis by phytoplankton and other marine animals which converts CO2 into organic matter (10GtC per year)
- This acts as a biological pump transporting carbon from the oceans’ surface to the intermediate and deep ocean stores (10 GtC per year)
- As the biological organisms die, their dead cells, shells and other parts sink into the mid and deep water
- Also, the decay of these organisms releases carbon dioxide into the intermediate and deep water stores
- Oceans regulate the composition of the atmosphere by moving carbon from the ocean’s surface (where it may vent back into the atmosphere) and storing it in the mid and deep ocean store, along with the dissolved carbon store, which regulates the carbon cycle
Ocean sequestration - carbonate pump:
- Relies on inorganic carbon sedimentation
- When organisms die and starts to sink, many shells dissolve before they reach the ocean floor entering the deep ocean currents
- The solubility cycle occurs when CO2, absorbed by the oceans from the atmosphere, forms carbonic acid which in turn reacts with hydrogen ions to form bicarbonates and then further reactions form carbonates which are stored in the upper ocean
- Some organisms use these carbonates to make their shells or skeletons
- When these organisms die some material sinks to the ocean floor and forms the sea bed sediment store (1750 GtC)
- Over time, through chemical and physical processes, the carbon is transformed into rocks such as limestone
- This process locks up carbon in the long-term carbon cycle and does not allow an easy return to the ocean surface and so prevents possible venting into the atmosphere as the physical pump does.
Ocean sequestration - physical pump:
- Considered the most important transfer
- Carbon dioxide (CO2) is absorbed by the ocean’s surface through diffusion
- Dissolved CO2 is then taken from the surface down to the intermediate and deep ocean stores through downwelling currents (96 GtC per year)
- The thermohaline circulation then distributes the carbon around the planet
- Cold water absorbs more CO2, therefore, as the equatorial waters move toward the poles, more CO2 is absorbed
- Salinity increases at the same time, making the water denser, therefore, the water sinks (downwelling) taking CO2 from the ocean’s surface to the deep ocean stores
- Allowing more diffusion to occur at the surface and helping to regulate the carbon stored in the atmosphere
- However, there is also the upwelling of carbon from intermediate and deep oceans to the surface oceans (105.6 GtC yr-1)
- Through upwelling currents and turbulence created by surface winds, previously stored carbon in the intermediate and deep ocean stores, return to the ocean’s surface and then back into the atmosphere
The role of trees - carbon sequestration:
- 95% of a tree’s biomass is made up from the CO2 that it sequesters and converts into cellulose.
- carbon fixation turns gaseous carbon - CO2 - into living organic compounds that grow.
- the amount of carbon stored within a tree, woodland or forest depends on the balance between photosynthesis and respiration.
Mangroves and the role of soil:
- mangroves are vital processors - sequestering almost 1.5 metric tonnes of carbon per hectare per year.
- their soils are anaerobic and due to this the decomposition of plant matter is slow. As a result, little of the carbon can be respired back to the atmosphere and the store remains intact.
- however if mangroves are drained or cleared, carbon is released into the atmosphere. Mangroves are being closed for tourism. Even 2% is cleared results in carbon released being 50 times the natural sequestration rate.
Tundra soils - carbon sequestration:
- much of soil in the tundra is permanently frozen and contains ancient carbon.
- this locks any carbon into an icy store due to the decayed organic matter being frozen.
- tundra soils contain carbon that has been trapped for hundreds of thousands of years.
Tropical rainforests as carbon stores:
- tropical rainforests are huge carbon sinks, but are fragile and can quickly disappear.
- carbon within rainforests is mainly stored in trees, plant litter and dead wood.
- as litter and dead wood decay they are recycled so quickly a soil store does not develop.
- TR absorb more atmospheric CO2 than any other terrestrial biome, accounting for 30% of global net primary production, although they cover 17% of the earth’s surface.
- if they died off the world would lose a massive carbon sink.
What is the natural greenhouse effect:
- solar energy is received from the sun, fark surfaces on Earth absorb this sunlight and some is reflected back into space.
- the greenhouse gases in the atmosphere act like a ‘blanket’ to trap some of the heat and keep the earth warm (without them we would be 16 degrees Celsius). This means life on earth is sustained.
- CO2 is the most common greenhouse has and it has the highest radioactive forcing effect (RFE) - holds onto heat for longer.
Enhanced greenhouse effect:
-concentration of greenhouse gases such as CO2 and CH4 in the atmosphere have increased 25% since 1750. 75% of CO2 emissions have come from burning fossil fuels.
- many believe that this is the cause of increased global temperatures and leading to enhanced greenhouse effect.
- human activities, e.g. burning fossil fuels and deforestation release natural carbon stores and nitrogen, which then combine with oxygen to form greenhouse gases. e.g. carbon combines with oxygen to form CO2.
- level of water vapour increases as well as global temperatures. Higher temps results in greater evaporation of water leading to greater condensation. This causes increased cloud cover, trapping heat in the atmosphere.
enhanced greenhouse effect - temperature:
- due to the angle of the sun’s rays solar insolation is intense at the Equator, but dispersed over a wider area at the Poles.
- different characteristics of the Earth’s surface (e.g. whether it is light or dark) also affect how much heat is absorbed or reflected (albedo) snow reflects heat and dark forests absorb it.
- heat is redistributed around the globe by air movement, caused by both pressure differences and ocean currents.