Beginners Guide to Greenhouse Gases

What is CO₂-e?

Greenhouse gas (GHG) emissions are a combination of gases that trap heat in the Earth's atmosphere.

• Carbon Dioxide (CO₂) makes up roughly 75% of greenhouse gases. CO2 is released through activities like burning fossil fuels, deforestation, and various industrial processes.
• Methane (CH₄) makes up roughly 17% of greenhouse gases. It comes from natural sources like wetlands and human activities such as agriculture and waste management.
• Nitrous Oxide (N₂O) makes up roughly 6% of greenhouse gases. It is linked to agricultural and industrial activities.
• Fluorinated Gases make up roughly 2% of greenhouse gases. They are used in refrigeration and manufacturing.

Each of these gases have a different capacity to warm the atmosphere.

In particular, Methane has a higher warming potential than Carbon Dioxide. It also has a shorter atmospheric lifespan.

Scientists use the Global Warming Potential (GWP) metric to quantify how much a given amount of a particular gas contributes to global warming over a specified time (typically 100 years) compared to the same amount of Carbon Dioxide.

This metric is Carbon Dioxide equivalent (CO₂-e).

What is Mt?

Greenhouse gases are measured in tonnes (t), which is often in the millions of tonnes (Mt).

GHGs are measured by mass (i.e. weight) instead of volume (i.e. size), because the volume of gases changes depending on temperature and pressure. For example, a cubic metre of carbon dioxide at sea level contains less CO₂ than a cubic metre at high pressure.

How are Greenhouse Gases measured?

It is more correct to refer to GHG estimates, rather than measurements.

Countries and companies use the 'greenhouse gas inventory' approach to report emissions.

• Inventories start by listing all potential emission sources (e.g. power stations, factories, equipment, transport, animals).
• Protocols and methods are then used to estimate the emissions from each source. This involves applying averages or multipliers to activities. (E.g. Burning X amount of coal = Y amount of CO₂). The protocols and emission factors are based on past studies.
• Direct measurements are occassionally used. This means obtaining actual measurements, such as at power plants or refineries, where sensors track emissions in real-time. For most sectors, this is impractical, however new technologies are making it more common.
• Adjustments are common as methods and standards evolve.
• Inventories are often self-reported and may be audited.