Understanding greenhouse gas emissions | Part 1: Generalities

Greenhouse Gas (GHG) emissions have been part of public and private debates for two decades. Yet, the notion of emissions and what they relate to can be difficult to comprehend in the first place. This series article intends to give readers keys to understand them better.

What is a CO2 Emission?

Carbon Dioxide (C02) is a gas that has the particularity of trapping heat in the atmosphere. It is part of a large group of gases sharing that characteristic: greenhouse gases. Other gases part of GHG are methane (CH4), nitrous oxide (N20), and Fluorinated Gases. We refer to emissions when these gases are released into the atmosphere during any natural or human process. GHGs present different characteristics and various origins. Among these characteristics, two present high importance for the global warming debate:

• The lifetime in the atmosphere: duration while the molecules stay in the atmosphere before they decay. The longer they stay, the longer they trap heat.
• Global Warming Potential (GWP): all gases do not trap heat at the same intensity. To normalize this intensity, we use the GWP. The base unit is the amount of heat trapped by CO2, usually over a 100 year period. For example, methane has a GWP of 25, meaning 1 ton of methane will trap as much heat in the atmosphere as 25 tons of CO2.

Two other essential notions to understand about GHG emissions are:

• MtCO2e: the unit of magnitude used when dealing with large-scale emissions (state, industry). Stands for “1 Metric Ton equivalent CO2”, which represents the amount of heat trapped by this mass of CO2. To find the MtCO2e of other gases, we multiply the amount of gas studied by the associated GWP factor. What is 1 MtCO2e worth: according to the EPA, that’s what is emitted by driving an average car from San Francisco to Atlanta, or charging 127,000 smartphones.
• ppm: stands for “part per million”. Measures the concentration of a gas in the atmosphere. 1ppm means that we find 1 molecule of a given gas for every million molecules in the atmosphere. As of January 2021, CO2 level in the atmosphere is 415ppm (pre-industrial era level in the late 18th century was around 278ppm). Past 530ppm, the +2°C limit goal would become extremely unlikely.

The pie chart below presents the global GHC emissions recorded in 2017, in equivalent C02 percentage.

Total emissions in 2017, all GHC included : 49,947.4 MtCO2e

It's important to note that although today’s technologies allow us to gather a lot of pertinent data, they are not infallible. Also, IPCC’s 5th Assessment Report set confidence levels as follow: CO2 (90%), CH4 (80%), N20 (40%), F-gases (80%). In addition, CO2 emissions associated with FOLU (forestry and other land use) have a 50% confidence interval. It’s important data to know about, as this is an argument used by both climate-skeptical and radical ecologist movements. My personal take on that is to prepare for the worst, hope for the best.

We’ll dig more into the macro and micro levels of the topic in other articles of that Kweek series. The table below presents GHG characteristics:

Recommended reading: IPCC AR6

Sources: IPCC, CAIT, EPA, Mauna Loa Observatory, GIEC