Carbon dioxide emissions come from both natural processes and human activities, but the distinction between gross natural carbon exchange and the additional human caused imbalance is essential. Natural systems continually move enormous quantities of carbon dioxide between the atmosphere, oceans, soils, vegetation and geological reservoirs through photosynthesis, respiration, decomposition, ocean gas exchange, weathering and volcanic activity.
These natural flows largely operate as a dynamic cycle in which increases in one reservoir are often balanced by corresponding removals elsewhere over relatively short periods. Human activities add substantial quantities of carbon dioxide to the atmosphere by transferring carbon that has been stored underground or in long lived geological deposits into the active carbon cycle.
The principal anthropogenic source is the combustion of fossil fuels such as coal, oil and natural gas, followed by land use change and industrial processes, particularly cement manufacture. The important scientific issue is therefore not that humans have invented carbon dioxide emissions, because carbon dioxide has always existed and been naturally exchanged, but that human activities have substantially increased the amount of carbon entering the atmosphere faster than natural sinks can remove it, producing a persistent increase in atmospheric carbon dioxide concentration.
Fossil fuel combustion is the largest human source of carbon dioxide emissions globally. Coal, petroleum and natural gas contain carbon that was removed from the atmosphere by ancient organisms and subsequently buried, transformed and stored over geological timescales. When these fuels are extracted and burned for electricity generation, heating, industrial production, transport and other purposes, their carbon combines with oxygen to form carbon dioxide.
Coal combustion is particularly carbon intensive per unit of energy because coal generally contains substantial carbon and produces relatively high emissions for the useful energy obtained. Oil is heavily associated with road transport, aviation, shipping and petrochemical production, while natural gas is widely used for electricity, heating and industry. The amount of carbon dioxide produced depends on the carbon content of the fuel, the quantity consumed and the efficiency of the conversion process. Burning the same mass of different fuels therefore does not produce identical emissions. Although technologies such as carbon capture can reduce the fraction released to the atmosphere, conventional unabated fossil fuel combustion remains a major source of anthropogenic carbon dioxide.
Electricity and heat production have historically represented a major component of fossil fuel related carbon dioxide emissions because large quantities of coal and natural gas are burned in power stations and heating systems. Electricity itself is not an emission source in the physical sense; emissions arise from the fuels and industrial processes used to generate it. The carbon intensity of electricity therefore varies substantially among countries and over time according to their energy mix.
Coal dominated grids generally have higher operational carbon dioxide emissions than systems relying heavily on nuclear power, hydroelectricity, wind or solar generation, while natural gas generally produces less carbon dioxide per unit of electricity than coal but still produces substantial fossil carbon emissions. Improvements in power station efficiency can reduce emissions per unit of electricity, but efficiency improvements alone do not necessarily reduce total emissions if electricity demand increases sufficiently. Replacing high carbon generation with low carbon generation, reducing energy demand where practical, improving efficiency and changing industrial processes are therefore distinct strategies that affect total emissions through different mechanisms.
Transport is another major source of carbon dioxide because petrol, diesel, jet fuel and marine fuels are predominantly derived from petroleum. Cars and vans burn liquid fuels in internal combustion engines, while aircraft rely heavily on aviation turbine fuel and ships commonly use marine fuels. The carbon dioxide released is broadly proportional to the amount of carbon in the fuel that is oxidised.
Electric vehicles do not produce direct exhaust carbon dioxide during operation, but their overall climate impact depends partly on how the electricity used for charging is generated and on emissions associated with vehicle and battery manufacturing. Rail systems can have very different emissions profiles depending on whether trains use electricity, diesel or other energy sources. Transport emissions are also affected by vehicle efficiency, occupancy, travel demand, freight patterns, infrastructure and urban design. Consequently, reducing transport carbon dioxide can involve technological changes as well as changes in energy supply and patterns of mobility.
Industrial processes produce carbon dioxide through mechanisms that are not necessarily equivalent to simply burning fuel. Cement manufacture is a particularly important example because limestone, principally calcium carbonate, is heated during clinker production. The process causes calcium carbonate to decompose into calcium oxide and carbon dioxide, a chemical reaction known as calcination. Some carbon dioxide is also produced from the fuels supplying heat to the kiln.
Other industrial sources include iron and steel production, chemicals, mineral processing and certain manufacturing operations. Industrial emissions can therefore remain significant even when the energy used is decarbonised because some processes inherently release carbon dioxide from raw materials. Technologies under development or deployment include alternative binders, material efficiency, electrification, hydrogen based industrial processes and carbon capture and storage. The distinction between energy related emissions and process emissions is important when assessing industrial decarbonisation because eliminating fossil fuel combustion does not automatically eliminate every source of industrial carbon dioxide.
Land use and land use change can either emit carbon dioxide or remove it, depending on what happens to vegetation and soils. Deforestation releases carbon when trees and other vegetation are burned or decomposed, while soil disturbance can accelerate the loss of previously stored organic carbon. Converting forests to agricultural land can therefore transfer carbon from biomass and soils into the atmosphere.
Conversely, forest growth, ecosystem restoration and some agricultural practices can remove atmospheric carbon dioxide and store carbon in vegetation and soils. The balance is complicated because ecosystems simultaneously absorb carbon through photosynthesis and return carbon through respiration and decomposition. Fire, drought, pests, land management and climate change can alter these processes. Land based carbon accounting therefore considers both emissions and removals, and the net result can vary strongly among regions and years. Land use change is particularly important because tropical forests and other carbon rich ecosystems contain substantial stores of carbon whose disturbance can produce significant emissions.
Natural carbon dioxide sources include respiration by organisms, decomposition of dead organic matter, ocean release, volcanic activity and chemical weathering processes. These sources are part of the natural carbon cycle and do not automatically represent a net long term increase in atmospheric carbon dioxide because natural sinks remove large quantities as well. Volcanic carbon dioxide, for example, is a genuine natural source, but its present day global contribution is much smaller than carbon dioxide emissions from human fossil fuel use.
Oceans exchange carbon dioxide continuously with the atmosphere, with some regions acting as net absorbers and others as net sources depending on temperature, circulation, biology and dissolved carbon chemistry. Natural variability can therefore cause atmospheric carbon dioxide concentrations to rise or fall over shorter timescales, but the sustained modern increase is primarily associated with human emissions. Isotopic measurements, atmospheric observations, emissions inventories and carbon cycle models provide independent evidence for the dominant role of fossil carbon and land use in the modern atmospheric increase.
Understanding sources is important because emissions are not interchangeable in terms of timescale, controllability or accounting method. Fossil carbon extracted from geological reservoirs represents a transfer into the active carbon cycle that can persist in the atmosphere, ocean and biosphere for centuries to millennia. A tonne of carbon dioxide released from fossil fuel combustion and a tonne released from biomass burning are chemically identical in the atmosphere, but the wider carbon cycle consequences depend on whether the biomass subsequently regrows and reabsorbs carbon.
Carbon dioxide inventories therefore distinguish fossil fuel and industrial emissions, land use emissions and removals, while climate analysis also considers methane, nitrous oxide and other greenhouse gases separately because they have different atmospheric lifetimes and radiative properties. The major human sources can consequently be understood as fossil fuel and industrial activity, land use change and several specific industrial processes, with the largest individual categories varying by country and sector. Accurate inventories are essential for tracking trends, evaluating mitigation measures and understanding why atmospheric carbon dioxide concentrations continue to change.
