Climate change driven by greenhouse gas emissions is one of the most pressing issues facing the world today. Carbon dioxide (CO2) is the primary greenhouse gas contributing to global warming. As nations, corporations, and individuals work to reduce their carbon footprints, accurately measuring carbon emissions has become increasingly important. This allows us to set emission reduction goals, track progress, and identify the most impactful actions we can take. But how are carbon emissions measured?
The Carbon Cycle
To understand carbon measurement, we must first understand the carbon cycle. Carbon is constantly being exchanged between the atmosphere, oceans, soils, plants, animals and fossil fuel deposits through processes like photosynthesis, respiration, decay and combustion.
During photosynthesis, plants absorb CO2 from the atmosphere and use solar energy to convert it into carbohydrates they use for food. The CO2 becomes part of the plant tissues. Animals then eat the plants, incorporating the carbon compounds into their bodies. When plants and animals die and decay or are eaten, some percentage of the carbon returns to the atmosphere or soil through respiration and decomposition. Over millions of years, a portion of decaying organic matter was buried and converted through heat and pressure into fossil fuel deposits of coal, oil and natural gas.
When we extract and burn fossil fuels to power our modern society, carbon stored underground for millennia is released back into the atmosphere. Deforestation and land use changes also emit stored carbon. Prior to industrialization, the carbon cycle was roughly in balance. Today, by burning over 11 billion metric tons of fossil fuels each year, humans are altering the carbon cycle and increasing atmospheric CO2 faster than natural processes can absorb it.
Measuring Carbon Dioxide Emissions
There are two main approaches to estimating CO2 emissions:
1. Bottom-Up Calculations: This involves directly measuring or calculating emissions from specific sources and adding them together. Examples include measuring the CO2 emitted from vehicle tailpipes or power plant smokestacks or calculating emissions based on fuel consumption statistics.
2. Top-Down Calculations: This estimates total CO2 emissions over a large area by taking atmospheric concentration measurements and using models to work backwards. The emissions are then attributed to different sources.
Bottom-up calculations offer detailed, source-specific information useful for compiling inventories and allocating emissions. However, identifying and measuring every source is challenging. Top-down methods provide a big-picture view of total emissions that can validate bottom-up inventories. But they have less specificity. Most carbon accounting combines the two approaches.
Here are some key methodologies used:
Governments, companies and other entities measure and compile data on emissions from sources like power plants, transportation, industry and agriculture over a given time period. This generates an emissions inventory. For example, the EPA annually tracks U.S. greenhouse gas emissions at the national and state level. Companies might track their emissions over a year. Inventories allow tracking emission trends, setting reduction goals and allocating emissions to different source categories.
It is often not possible to directly measure emissions from every car, truck, factory, etc. Instead, emissions factors are used to calculate emissions based on activity data. For example, an emissions factor may specify the amount of CO2 emitted per kilowatt-hour of electricity generated from a specific power plant. Multiplying the emissions factor by the facility’s total electricity generation provides its estimated emissions. Government agencies and industry groups compile emissions factors for different fuels, processes and equipment that entities reference to estimate their emissions.
The emissions intensity expresses the amount of CO2 emitted per unit of a product or activity. Common metrics include emissions per kilowatt-hour of electricity, per barrel of oil produced, per mile driven, or per dollar of GDP. Intensities allow comparing and benchmarking the emissions efficiency of products, companies and countries. Tracking intensity over time shows decarbonization progress as economies improve efficiency and transition toward lower-carbon activities.
Life cycle assessments:
LCAs estimate the emissions of a product over its entire life cycle from material extraction, production, use and disposal. This helps identify high-impact stages and improvement opportunities. For example, an LCA on vehicle emissions would assess CO2 not just from tailpipe exhaust but from producing the metals, plastics, electronics and fuels needed to manufacture and run the vehicle. Companies and governments use LCAs to direct R&D, set product regulations and guide purchasing.
The carbon footprint sums up greenhouse gas emissions from a person, product, company, event or group over a given period. It might include direct emissions like fuel use plus indirect emissions from electricity and materials. Carbon footprints help motivate and guide emission reduction efforts. For example, people can calculate their personal footprints and identify lifestyle changes to reduce emissions. Products like food or clothing can display carbon labels. Companies track footprints and set reduction targets.
Networks of sensors across the globe precisely measure atmospheric concentrations of CO2 and other greenhouse gases. Combining this data with atmospheric circulation models allows scientists to estimate total emissions for regions like North America, pinpoint emission hotspots and analyze trends. This provides independent validation of bottom-up inventories.
Some of the fundamental calculations used in emissions accounting include:
- Fuel-based: Emissions are calculated based on the amount of fuel burned and its emission factor. Examples include gallons of gasoline used by vehicles, tons of coal burned by a power plant or cubic feet of natural gas consumed to heat a building. The emission factor for the fuel gives the tons of CO2 emitted per unit. Fuel quantities are typically obtained from purchase receipts and meters.
- Miles-based: For mobile sources like vehicles, planes or ships, CO2 emissions can be calculated from the number of miles traveled and a fuel efficiency factor. Common metrics include grams CO2 per mile, mile per gallon, or gallons per 100 miles. The fuel efficiency reflects the vehicle model, engine, load and operating conditions. Emissions increase with more miles driven or lower fuel efficiency.
- Process-based: Emissions from industrial facilities and processes like cement and steel production are calculated using emissions factors for material inputs and different steps in the production sequence. This factors in efficiencies that determine how much raw CO2 is generated per ton of product.
- Deforestation: Clearing forests for timber harvesting or land development releases stored carbon. Estimates are based on the total acres deforested and inventories assessing metric tons of carbon stored per acre in vegetation biomass and soils.
- Carbon content: Fuels contain different amounts of carbon per unit. Burning one gallon of gasoline emits about 8.9 kg CO2 while one gallon of diesel emits about 10.2 kg. The carbon content helps determine fuel-specific emissions factors.
Why Accurate Carbon Accounting Matters
Precise, comprehensive carbon accounting is crucial for:
Tracking emissions trends: Consistent accounting allows us to accurately track changes in emissions over time and assess the effectiveness of mitigation policies and actions.
Setting reduction goals: Robust data enables nations and subnational entities to establish measurable, science-based emissions reduction targets.
Creating mitigation strategies: Detailed emissions data helps model scenarios and impacts of different mitigation pathways like cleaner energy, electrification, efficiency gains or carbon capture.
Prioritizing high-impact actions: Identifying major emission sources helps focus reduction efforts where they can have the most impact.
Evaluating progress: Standardized, transparent accounting enables benchmarking of emitters and progress toward reduction goals.
Allocating responsibility: Attributional analyses define contributions to emissions from different sectors, technologies, regions, companies, and products to guide responsibility.
Facilitating carbon markets and pricing: Markets and taxes require accurate accounting to set permit allocations and prices. Uncertaintyraises costs and inhibits participation.
Informing policy: Emissions data guides effective policy development by governments at the international, national and local levels.
Motivating action: Quantified footprints spur mitigation by increasing awareness of emissions impacts and opportunities.
Limitations and Uncertainty
While emissions accounting methods continue improving, they still have limitations:
Incomplete data: Unknowns in quantities like fuel consumption along with missing or outdated emissions factors introduce uncertainty in bottom-up inventories.
Estimations: It is often impossible to directly measure every emission source continuously. Some estimates can have uncertainties of 10% or more.
Inconsistent methodologies: Different entities or groups may use varying accounting approaches, assumptions and system boundaries, limiting result comparability.
Attribution challenges: Attributing emissions to specific regions, economic sectors or fuel types involves modelling with inherent approximations.
Natural carbon fluxes: Natural processes like wildfires, soil disturbances and wetlands can emit substantial CO2 in ways that are difficult to quantify.
Data verification: Top-down validation of bottom-up inventories remains limited, though improving through satellite monitoring.
While uncertainties remain, carbon accounting practices continue advancing, recently aided by technologies like remote sensing, cheaper sensors and automated data collection. Integrating multiple methodologies provides cross-verification. Despite limitations, current emissions data still gives directionally accurate trend information to guide mitigation. Perfect accounting is not needed to act on climate change, and individuals can make a significant impact by adopting eco-friendly practices at home. But improving accounting to narrow uncertainties will allow more targeted, cost-effective solutions.
Frequently Asked Questions
1. What are the main greenhouse gases contributing to climate change?
The primary greenhouse gases are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and fluorinated gases such as hydrofluorocarbons (HFCs). CO2 accounts for about 80% of total greenhouse gas emissions globally due to our heavy use of fossil fuels. Methane, nitrous oxide and fluorinated gases have a much higher global warming potential per molecule compared to CO2 but are emitted in smaller quantities.
2. What are common units used to report carbon emissions?
Emissions are typically reported in terms of metric tons or tonnes of carbon dioxide equivalent (tCO2e). This standardizes emissions from all greenhouse gases based on their relative global warming potential compared to CO2. Emissions can also be reported as million metric tons of carbon dioxide equivalent (MMtCO2e) or billion metric tons (GtCO2e) for large totals.
3. How do different countries compare in total greenhouse gas emissions?
China emits the most greenhouse gases, over 14 GtCO2e annually, followed by the United States at over 6.5 GtCO2e. However, many developed countries like the US have far higher per capita emissions. Some major emitters have recently pledged to reach net zero emissions by 2050-2060, including the US, UK, EU and Japan.
4. What are the main limitations or uncertainties in current carbon accounting?
Key limitations include data gaps, inconsistent methodologies, difficulties attributing emissions sources, quantifying natural fluxes and limited verification. Uncertainties in annual national inventories are typically estimated to range from around 5% to 10% but can be higher for some source categories. There are ongoing efforts to improve accounting and reduce uncertainty.