Introduction to Carbon Footprinting: Principles, Methodology, and Applications

The Kyoto Basket: There are seven primary greenhouse gases identified in the Kyoto Basket that are central to climate management:
- Carbon Dioxide ( $CO_2$ )
- Methane ( $CH_4$ )
- Nitrous Oxide ( $N_2O$ )
- Sulfur Hexafluoride ( $SF_6$ )
- Hydrofluorocarbons ( $HFCs$ )
- Perfluorocarbons ( $PFCs$ )
- Nitrogen Trifluoride ( $NF_3$ )
Carbon Dioxide Trends: There has been a significant increase in atmospheric $CO_2$ since the mid-19th Century. Data from the Mauna Loa Observatory (Keeling Curve) indicates:
- Concentrations have risen from approximately $310\,\text{ppm}$ in $1960$ to over $420\,\text{ppm}$ by early $2025$ .
- The current level is $420\,\text{ppm}$ , which is $17\% \text{ above the } 1870 \text{ average}$ .
- Timeline of concentration increases:
  - $270 \rightarrow 280\,\text{ppm}$ : ~ $5000\,\text{years}$
  - $330 \rightarrow 340\,\text{ppm}$ : $8\,\text{years}$
  - $400 \rightarrow 410\,\text{ppm}$ : $4\,\text{years}$
  - Most recent data (February 2025): $424.2\,\text{ppm}$ .
Methane ( $CH_4$ ) Case Study: Kazakhstan Mega-Leak:
- A blowout at a remote well in the Mangistau region began on June 9, 2023, and was only controlled by December 25, 2023.
- An estimated $127,000\,\text{tonnes}$ of methane escaped.
- Impact: Comparable to driving more than $717,000\,\text{petrol cars}$ for a year.
- Atmospheric methane concentration reached $1,930\,\text{ppb}$ in January 2023, which is a factor of $2.68$ above pre-industrial levels ( $722\,\text{ppb}$ ).

Radiative Forcing Definition: This is the difference between insolation (sunlight) absorbed by the Earth and the energy radiated back to space.
Climate Forcings: These cause temperatures to rise or fall over decadal periods.
- Positive Radiative Forcing: Earth receives more incoming energy than it radiates; net gain causes warming.
- Negative Radiative Forcing: Earth loses more energy to space than it receives; produces cooling.
Global Temperature: Recent years have shown global annual mean temperatures nearly $1.2\,^{\circ}\text{C}$ above the pre-industrial reference period ( $1850-1900$ ).

Origin: The concept emerged in the mid-1990s from "Ecological Footprint," which estimates the Earth's surface area needed to provide resources and process waste for a specific population or activity.
Wiedman and Minx (2008): Defined carbon footprint as the total amount of $CO_2$ emissions directly and indirectly caused by an activity or accumulated over the life stages of a product.
Moss, Lambert and Rennie (2008): Argued it should include the total mass of all greenhouse gases (direct and indirect).
Wright et al (2011) Definition: "A measure of the total amount of $CO_2$ and $CH_4$ emissions of a defined population, system or activity, considering all relevant sources, sinks and storage… calculated as $CO_2$ equivalents using the relevant 100-year global warming potential."
Climate Footprint (Wright et al 2012): A more comprehensive metric including all Kyoto Basket GHGs (specifically adding $NF_3$ in 2013).

Emissions Source Identification: Categorizing direct and indirect emissions.
Boundary Setting: Defining system, geographic, and temporal (e.g., financial year) boundaries.
Selection of Emission Calculation Method: Choosing between process analysis, EEIOA, or hybrid models.
Data Collection: Gathering primary or secondary data.
Calculating Scope 1 and 2 Emissions: Focusing on direct impacts and energy use.
Capturing Scope 3 Emissions: Addressing indirect life cycle and supply chain impacts.

Scope 1 (Direct): Emissions occurring within the organizational or geographical boundaries (e.g., fuel combusted in company vehicles, boilers, hobs, and fugitive emissions).
Scope 2 (Indirect - Energy): Emissions from energy generation (electricity, steam, heating/cooling) purchased for own consumption.
Scope 3 (Indirect - Other): Emissions that occur as a consequence of activities but outside the boundary (e.g., purchased goods/services, waste disposal, employee commuting, business travel, leased assets, and end-of-life treatment of sold products).

Primary Formula for Carbon Calculation: $E = AD \times EF$ Where:
- $E = \text{emission (kg or tonnes } CO_2)$
- $AD = \text{activity data}$
- $EF = \text{emission factor}$
Carbon Equivalent Calculation: $E_{equiv} = \sum [gas] (E_{[gas]} \times GWP_{[gas]})$
GWP Values (100-year horizon):
- $CO_2$ : $1$
- $CH_4$ : $27.9$ (or a range of $28-36$ depending on source)
- $N_2O$ : $310$
- $HFCs$ : $140 - 11,700$
- $PFCs$ : $6,500 - 9,200$
- $SF_6$ : $22,800$
- $NF_3$ : $17,200$

Tier One: Use of non-specific data (e.g., national average fuel use per capita, IPCC default factors).
Tier Two: Use of country-specific data (e.g., engineering estimates, fuel use calculated from expenditure).
Tier Three: Use of technology-specific data (e.g., direct monitoring with specialized equipment, metered energy use).

Process Analysis (PA): A "bottom-up" approach. High accuracy and transparency, better suited for products, but requires significant time and resources.
Environmentally Extended Input-Output Analysis (EEIOA): A "top-down" approach. Uses national economic/environmental data. Quick and inexpensive but less accurate.
Hybrid (Hybrid-EIO-LCA): Combines process analysis for the defined system boundary with EEIOA for analyzing in-flows/supply chains.

Cities and GHG Management: More than $50\% \text{ of the world's population}$ is urban. Cities are hubs for significant emissions and are targeted for "low-carbon" agendas.
PAS 2070:2013: A specification for the assessment of GHG emissions of a city, utilizing direct, supply chain, and consumption-based methodologies.
Waste Management Sector:
- The sector accounts for approximately $3\% \text{ of UK emissions}$ , with $89\% \text{ stemming from landfill } CH_4$ .
- Key strategies include maximizing resource efficiency and material recycling.
- Case Study (Cardiff): Landfilling was the dominant GHG source; benefits were primarily from reuse, reprocessing, and Anaerobic Digestion (AD).
- Wales is noted as unlikely to hit its $70\% \text{ recycling target by 2025}$ without aggressive waste prevention policies.

Biogenic Carbon: $CO_2$ released from biomass decomposition or burning is often considered "carbon neutral" due to previous photosynthesis. However, decomposition of biomass can create potent $CH_4$ .
Carbon Sinks/Stores: Reservoirs that store GHGs indefinitely. Categories include:
- Soil organic carbon
- Biomass
- Man-made products
Estimation Levels:
- Tier 1: Land use types and forestry.
- Tier 2: Computer modeling.
- Tier 3: Direct measurements of soil and biomass (e.g., trees).