www hoddereducation co ukgeographyreview The carbon cycle A

Key terms System A set of interrelated objects. A system can be either closed — with no import or export of materials across its boundary (e. g. the global carbon cycle) — or open, where imports occur (e. g. the carbon cycle of a local woodland). Equilibrium A state of balance between inputs and outputs in a system Do you know what these terms mean? such as the carbon cycle. A steady-state equilibrium means there is balance in the long term but the system fluctuates in the short term. Click to reveal the definitions Carbon cycle Hodder & Stoughton © 2018 The biogeochemical cycle by which carbon moves from one part of the global system to another. At the global scale, it is a closed system made up of linked inputs, outputs, flows and stores. At the local scale, it is an open system.

Curriculum relevance All A-level geography specifications require students to understand: • The distribution and size of global carbon stores, and the factors driving change in the size of these stores over time and in space. • The pathways and processes which control the cycling of carbon within and between land, oceans and atmosphere at a range of time and space scales. • The links between the carbon and water cycles using climate as a key context for exploring these linkages. Carbon cycle studies at A-level require you to: • show understanding of scientific concepts and processes • apply your knowledge in unexpected geographical contexts, including synoptic exam questions Hodder & Stoughton © 2018

Carbon cycle components • Stores are the amounts of carbon held the global system, e. g. the atmosphere, hydrosphere (oceans), lithosphere and biosphere. • Flows are movements or transfers of carbon between stores, e. g. volcanic activity adds 0. 1 gigatonnes of carbon to the atmosphere annually. • Flows into stores are called inputs, e. g. carbon dioxide added to the atmosphere by human activities. • Flows leaving stores are outputs, e. g. dissolved carbon carried from the land in solution by rivers. Hodder & Stoughton © 2018

The global carbon cycle • • The global carbon cycle is shown here as a ‘black box’ model One gigatonne equals one billion tonnes (or one trillion kg).

Global carbon stores • The sizes shown here are natural – the data have not been modified to take into account human activity. • How do you think human activity has changed the actual sizes of these global stores? Which have grown larger and which have shrunk? Hodder & Stoughton © 2018

Ecosystem carbon storage Green plants Nearly 20% of carbon in Earth’s biosphere is stored in plants, including root systems. Animals These play a small role in carbon storage because the biomass of animals is much less than plant biomass. Litter This is fresh and un-decomposed plant debris. It includes leaves, pine needles and twigs. Soil Humus is a black substance that remains in the soil after most of the organic litter has decomposed. It gets dispersed throughout the soil by earthworms. Hodder & Stoughton © 2018

Global biomes Using these data, can you (1) describe how and (2) explain why the amount of carbon stored as biomass varies with latitude?

Carbon cycle fluxes & processes Over what timescale do the processes shown here operate? Which are faster and which are slower?

The concept of net balance • • • Hodder & Stoughton © 2018 For each country, calculate the net carbon flow shown. This is calculated by subtracting outputs from inputs. A positive final calculation indicates a net gain in storage, and vice-versa.

Carbon pathways: ecosystems • • • Hodder & Stoughton © 2018 This diagram shows the processes that transfer carbon between different stores in an ecosystem. Can you define all of these processes? Why might the importance of these processes vary seasonally and between different places?

Carbon pathways Process How it works Photosynthesis Respiration Hodder & Stoughton © 2018 Carbohydrate molecules are produced from carbon dioxide and water using energy from light. Plants ‘fix’ gaseous carbon dioxide into solid form in their living tissues. The net primary productivity (NPP) of an ecosystem is the rate at which new organic matter is produced by photosynthesis per unit area per unit time. Carbon dioxide is released into the atmosphere by organisms through the process of respiration. Plants create energy for respiration by breaking down stored glucose (sugars). Carbon dioxide is given off as a by-product.

Carbon pathways Process Decomposition How it works Carbon dioxide is returned to the atmosphere when living organisms die: their cells break down as a result of physical (wind and water), chemical (leaching and oxidation) and biological (feeding and digestion) mechanisms (carried out by bacteria and fungi). Fossil fuel combustion Hodder & Stoughton © 2018 Hydrocarbon (fossil fuel) combustion takes place rapidly in the presence of oxygen and releases carbon dioxide Around 85% of global energy consumption is derived from coal, oil and gas fuels. Traditional societies burn biomass on demand.

Carbon pathways: oceans Oceans take up carbon dioxide by two carbon cycle pumps: (1) the biological pump (2) the physical pump Can you describe and explain the biological and physical carbon transfers shown here?

Carbon pathways: oceans • The physical (inorganic) pump involves the movement of carbon dioxide from the atmosphere to the ocean by a process called diffusion. • Carbon dioxide dissolved in the surface of the ocean can be transferred to the deep ocean in areas where cold, dense surface waters sink. • This downwelling carries carbon molecules to great depths where they may remain for centuries. • Carbon dioxide diffusion determines the acidity of the oceans. Hodder & Stoughton © 2018

Carbon pathways: oceans • The biological (organic) pump is driven by ocean phytoplankton absorbing carbon dioxide via photosynthesis. • These organisms form the bottom of the marine food web, and live in the ocean’s surface layer. • Phytoplankton are consumed by other organisms and carbon is transferred along food chains. • Organic carbon may eventually be transferred to the deep ocean when dead organisms sink towards the ocean floor. Hodder & Stoughton © 2018

Slow carbon pathways Some carbon is eventually returned to the atmosphere by volcanism, as carbon dioxide is released from melted rocks when subduction occurs at plate boundaries.

Slow carbon pathways • Cycling of carbon between terrestrial bedrock and oceans occurs over millions of years. • Rainwater containing weak carbonic acid reacts with limestone to produce soluble calcium bicarbonate. • Calcium bicarbonate moves through the water cycle in solution into rivers and the ocean. • Next, calcium bicarbonate is used by marine organisms to create shells; these are deposited later as carbonate-rich sediment on the ocean floor where they are lithified. Hodder & Stoughton © 2018

Summary 1. The global carbon cycle consists of stores, flows, inputs and outputs e. g. carbon stored in sedimentary rocks and deep ocean sediments. 2. Photosynthesis, respiration, decomposition and fossil fuel combustion are important processes controlling carbon flows between the atmosphere and the land. 3. Carbon flows from the atmosphere to the ocean take place via the physical pump and the biological pump. 4. A slow carbon cycle operates between the land the ocean over millions of years, involving carbonation, river transport and plate tectonic movement. Hodder & Stoughton © 2018

This resource is part of GEOGRAPHY REVIEW, a magazine written for A-level students by subject experts. To subscribe to the full magazine go to: http: //www. hoddereducation. co. uk/geographyreview Hodder & Stoughton © 2018