This document covers the entirety of Unit 3, Topic 6, 'The Carbon Cycle and Energy Security' in depth. It is formatted in accordance with the three 'Enquiry Questions' from the Pearson Edexcel A-Level specification.
HOW DOES THE CARBON CYCLE OPERATE TO MAINTAIN PLANETARY
HEALTH?
3
, 1. THE CARBON CYCLE
a | A SYSTEMS APPROACH
The carbon cycle is the movement of carbon from one ‘sphere’ to another by a set
of processes and is understood through a systems approach. Open systems are
bounded and have inputs, outputs and throughputs.
The throughputs are controlled by processes within the system which are often grouped together
as a sub-system (a forest).
There are three concepts key to understanding biogeochemical cycling within a systems
framework:
The total amount of the material of interest held within a part of the system.
This is effectively how much of the material there is and where it is. For
STORES example, soils are a major store of carbon within the terrestrial carbon system.
(or stocks) Stocks are usually expressed in units of mass, e.g. total global soil carbon
storage is estimated at 1500-2400 PgC (PgC is petagrams of carbon, a unit
equivalent to grams or one gigatonne.)
Measurements of the rate of flow of material between the stores. Because
FLUXES fluxes are a rate the units are mass per unit time, commonly for global cycles
these are expressed as Pg per year.
The physical mechanisms which drive the flux of material between stores. For
PROCESSES example, one of the key processes which drive the flux of carbon from the
atmosphere to the vegetation store is photosynthesis.
KEY CONCEPT | SYSTEM FEEDBACK (write in the definition from page 78)
Earth systems normally operate by negative (stabilising) feedback, maintaining a stable state by
preventing the system from moving beyond certain thresholds. Any change is cancelled out,
maintaining equilibrium.
Positive (amplifying) feedback loops occur when a small change in one component causes changes
in other components. This shifts the system away from its previous state and toward a new one.
b | UNDERSTANDING CARBON
Carbon is integral to life on earth and provides the major building blocks for all life on earth. It
regulates our climate, making it warm enough to survive, and is stored within rocks, plants and the
oceans. It exists in gas, liquid and solid forms, and in biotic and abiotic forms. Human activity is
part of the carbon cycle, and planetary health is placed at risk as more carbon enters the
atmosphere.
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