During my last few months at South Pole two words kicked around my head more than any others; ‘voluntary redundancy’, ‘emission factors’ and I need to get them out. I expect I’ll make reference to them again in the future as I think they’re an interesting topic, so this ones for the nerds who don’t know and the writers who want to reference EFs without explaining them.  

The first step organisations should take towards reducing their emissions is to understand them; to work out where their emissions come from, where they’re high, and where they’re low, so they can take targeted action to reduce them. This kind of understanding is achieved by doing greenhouse gas (GHG) accounting, a complicated series of calculations that works out emissions based on ‘activity data’ and ‘emission factors’.  

What is an emission factor?

An emission factor (EF) is a coefficient that describes the amount of GHG released from a given activity per unit of that activity. They are typically expressed in terms of weight of CO2e per reference unit. For example kilograms of CO2e per megajoule of energy (kgCO2e/MJ). 

There are EFs for just about everything and they differ based on where you are and the exact kind of activity you are doing. There are global databases of EFs that cover everything from jet joule to ice cream that are publicly available.

What is CO2e

Notice the little ‘e’ after CO2. This means ‘carbon dioxide equivalent’. When you look at the GHGs given off from something like burning fuel, CO2 is not the only gas produced. In a lot of activities there are too other very common GHGs given off: methane (CH4) and nitrous oxide (N2O). Measuring their individual impact on the climate is complicated because different gases vary in how much heat they can trap in the atmosphere, and how long they remain up there.

For example, nitrous oxide has a ‘100-year’ warming effect, 265-298 times more than CO2. This relative index is called its Global Warming Potential (GWP), which expresses how much warming a gas contributes over time. We use 100-year GWP specifically because it is the time-frame that the ‘climate emergency’ is typically described over.

So, given an amount of any GHG, CO2e expresses the amount of CO2 that would warm the atmosphere as much as the same amount of the gas in question over a 100-years from the moment of release.

As CO₂ is the gas that acts as a base for the relative unit, the GWP for CO2 is 1. The GWPs for other gases are provided by the Intergovernmental Panel on Climate Change (IPCC) in their periodic reports. 

• Carbon dioxide (CO2) (GWP = 1)
• Methane (CH4) (GWP = 28)
• Nitrous oxide (N2O) (GWP = 265)

Based on the IPCC's 5th (2013) Assessment Reports

How are Emission Factors Derived?

There are many many, many emission factors and probably hundreds of data bases full of them. There are the public sources I referenced earlier, private sources that you need to purchase to have access to, and custom emission factors derived by various organisations for their own specific purposes. For example you wouldn’t expect the IPCC to bother deriving an emission factor for Nestle’s milkshakes would you? No, let Nestle do that themselves. 

Emission factors are derived through a process that involves extensive data collection, analysis, and modelling. The following steps outline the typical procedure for deriving emission factors:

1. Data Collection: Scientists gather comprehensive data on the source or activity being studied. This data may include but is not limited to measurements of emissions and operating parameters.
2. Laboratory Testing: In some cases, controlled experiments in a labs are conducted to measure emissions under controlled conditions. This helps establish baselines for EFs.
3. Field Measurements: For real-world sources like vehicles or industrial processes, field measurements are taken using specialised equipment to assess emissions in various operating scenarios.
4. Data Analysis: Collected data is analysed to determine the relationships between emissions and relevant factors such as fuel type, equipment efficiency, and operating conditions.
5. Modelling: Statistical and mathematical models are used to extrapolate emissions data to a broader range of scenarios, ensuring that EFs are representative of real-world conditions.
6. Quality Assurance: The derived emission factors undergo thorough quality assurance and peer review to ensure their accuracy and reliability.

Why are Emission Factors Important?

The two most obvious uses for emission factors are probably ‘footprint assessment’, and ‘reporting’ respectively. EFs are used to calculate emissions of products, services, whole organisations, and entire countries so these entities can create reports on their emissions and 1. Understand their emissions and plan to reduce them, and 2. track their progress toward their emission reduction goals. 

But a quick search tells me three more important uses for EFs too:

1. Policy Development: Policymakers rely on EFs to formulate effective regulations to reduce issues like pollution, they provide a basis for setting emission standards.
2. Air Quality Management: EFs can help to assess air quality and identify pollution sources. This data can then be used to develop strategies for improving air quality in urban areas.
3. Climate Change Mitigation: EFs enable researchers to model the impact of emissions on global climate and assess the effectiveness of mitigation actions taken.

Conclusion

Emission factors are simple coefficients derived from a lot of research that allow people to calculate emissions with the intention of reducing them in the long term. They’re good stuff.