AP Syllabus focus:
‘Urbanization increases atmospheric carbon dioxide through fossil-fuel burning and landfills, affecting the carbon cycle.’
Urban areas concentrate people, energy use, and waste production. This concentration changes how carbon moves between reservoirs, especially by accelerating transfers of long-stored carbon into the atmosphere as carbon dioxide.
The carbon cycle connection
Urbanization influences the carbon cycle by changing the size and direction of key carbon fluxes, particularly those involving the atmosphere.
Carbon cycle: The movement of carbon among major reservoirs (atmosphere, living biomass, soils, oceans, and geologic stores) through processes such as photosynthesis, respiration, decomposition, and combustion.
In most cities, the dominant urban carbon flux is from combustion of fossil fuels to the atmosphere. A second important pathway is carbon released from landfills as waste decomposes.
Fossil-fuel burning: the primary urban CO2 source
Urbanization typically increases fossil-fuel combustion, moving carbon from geologic reservoirs (coal, oil, natural gas) to atmospheric CO2 on very short timescales.
This plot shows how atmospheric has varied over hundreds of thousands of years (from Antarctic ice cores) and how it rises sharply in the modern instrumental record (Mauna Loa). The steep modern increase highlights how rapidly carbon can be transferred into the atmosphere when geologic carbon is oxidized through fossil-fuel combustion. Source
Where the CO2 comes from in cities
Transportation
Gasoline and diesel burned by cars, trucks, buses, ships, and aircraft servicing urban economies.
Electricity generation for urban demand
Power plants may be inside or outside city boundaries, but demand is driven by urban activity (lighting, appliances, data use).
Buildings (direct fuel use)
Natural gas, fuel oil, or propane for space heating, water heating, and cooking.
Industry linked to urban growth
Manufacturing and material processing that support construction and consumption, often involving energy-intensive combustion.
Why urbanization raises emissions
Higher energy throughput: Dense economic activity increases total energy demand even when energy use per person varies.
Infrastructure lock-in: Roads, building stock, and heating systems can commit a city to decades of fuel use.
Urban heat island effects: Warmer urban microclimates can increase air-conditioning demand, indirectly raising CO2 if electricity is fossil-fuel based.
Landfills: waste as a carbon source
Urbanization also increases municipal solid waste, and much of that waste ends up in landfills, where carbon is converted into atmospheric gases.
Carbon processes in landfills
Organic waste decomposition (food scraps, paper, yard waste) transfers carbon from biomass to gases.
Low-oxygen (anaerobic) conditions common in landfills slow complete breakdown and favor production of methane (CH4) and carbon dioxide (CO2).
This diagram summarizes the four-phase progression of landfill decomposition and shows how gas composition changes over time after waste is buried. It connects anaerobic microbial activity to the production of and , clarifying why landfills become long-term sources of atmospheric carbon gases without effective gas capture. Source
Over time, carbon leaves the landfill system primarily as:
Landfill gas emitted to the atmosphere (a mix including CO2 and CH4)
Captured gas that is flared or used for energy (often converting CH4 to CO2 during combustion)
Even when methane is a major product, the landfill still affects the carbon cycle by increasing atmospheric carbon in gas form, including CO2 directly and CO2 formed after methane oxidation in the atmosphere.
What controls landfill carbon emissions
Waste composition: More biodegradable carbon generally increases gas generation potential.
Moisture and temperature: Influence microbial activity and the rate of decomposition.
Landfill design and management
Daily cover, compaction, and liners affect oxygen availability and gas movement.
Gas collection systems can reduce emissions by capturing landfill gas.
How these pathways “affect the carbon cycle”
Urbanization alters carbon cycling by:
Accelerating transfer of carbon from long-term stores (fossil fuels) to the short-term atmospheric pool
Increasing the atmospheric pool of carbon gases via waste decomposition in landfills
Shifting the balance between carbon inputs to the atmosphere (combustion and decomposition) and carbon removals (e.g., uptake by plants and oceans), contributing to higher atmospheric CO2 concentration
FAQ
Many inventories separate emissions into scopes (direct fuel use vs purchased electricity vs supply-chain).
Different accounting choices change reported totals, especially when electricity is generated outside city boundaries.
$CH_4$ contains carbon and is part of the same carbon flow from waste to the atmosphere.
Over time, atmospheric reactions oxidise much $CH_4$ to $CO_2$, shifting carbon into the $CO_2$ pool.
Gas capture wells and piping networks
Flaring or energy recovery
Diverting organics (e.g., separate collection)
Effectiveness depends on installation timing, maintenance, and waste characteristics.
They can store carbon in biomass and sometimes soils, but storage is limited by space, growth rates, and maintenance emissions.
Net benefit depends on species, survival, and long-term canopy retention.
Producing cement, steel, and glass often emits $CO_2$ through high-temperature fuel use and industrial chemistry.
Rapid urban development can therefore raise carbon fluxes even before buildings are occupied.
Practice Questions
State two ways urbanization can increase atmospheric and briefly identify the source in each case. (2 marks)
mark: Fossil-fuel burning increases atmospheric (e.g., vehicles, electricity generation, heating).
1 mark: Landfills increase atmospheric via decomposition of organic waste (direct and/or after landfill gas management/oxidation).
Explain how urbanization affects the carbon cycle through fossil-fuel burning and landfills. In your answer, describe the carbon reservoirs involved and the direction of carbon movement. (5 marks)
1 mark: Identifies fossil fuels as a long-term/geologic carbon reservoir.
1 mark: Explains combustion transfers carbon from fossil fuels to atmospheric .
1 mark: Identifies waste/biomass in landfills as an organic carbon source/reservoir.
1 mark: Explains decomposition in landfills releases carbon gases, including (and may mention as part of landfill gas).
1 mark: Clearly states that these processes increase atmospheric carbon (net movement toward the atmospheric reservoir), altering the carbon cycle balance.
