What is a "Global Warming Potential"? Part 2 of 2.
In my last blog post, I introduced the concept of Global Warming Potentials (GWPs) and how they are used to equate releases of different greenhouse gases – carbon dioxide, methane, nitrous oxide, etc – by converting them into a common unit, known as carbon dioxide equivalents (CO2e). This allows facility owners, policy makers and concerned citizens to make more informed decisions about what actions will have the biggest impact on their carbon footprint.
However, to complicate matters, a GWP isn’t a single, fixed value. Asides from the changing understanding of science, which leads to revisions to the GWP of gases from time to time, each gas has a different GWP depending on what time horizon it is being considered over. This is because some gases are persistent in the atmosphere: highly resistant to destruction and/or mixing, they can remain in our atmosphere for hundreds, thousands or even tens of thousands of years. Other gases, such as methane (CH4) have a short atmospheric lifetime (CH4 lasts for around 10 years, on average).
International agreements have resulted in GWP100 being the standard GWP that is used to quantify greenhouse gas emissions. In other words, the impact of each gas over a 100-year time horizon is used to compare the overall contribution of each GHG.
This does matter. Why? Well, let’s consider methane. Methane is a by-product of incomplete combustion of fossil fuels and is also released directly into the atmosphere from processes such as venting from oil and gas facilities, and “enteric fermentation” (that’s belching cows). It’s a major greenhouse gas; in Canada’s latest National Inventory Report, it’s second only to CO2 (79%) and is responsible for 14% of all emissions in Canada (NIR, Figure S-3).
Whereas the very latest scientific assessment has placed methane’s GWP100 at 28, the same assessment has placed its GWP20 at 86. Meaning that, over a 20 year time horizon, methane is almost 3.5 times more potent than when assessed over a 100 year time horizon. (This is due to its short atmospheric lifetime, meaning that most of the impact of methane is realized quickly).
And here’s the issue: using a GWP100 isn’t wrong, but, as the IPCC notes, it is a value judgement. In the context of methane releases, using a GWP100, instead of a GWP20, substantially underplays the short-term contribution of methane to global warming. An approximate recalculation of Canada’s national footprint using GWP20 for CH4 and N2O puts methane at 31% of all emissions in Canada – more than twice the 14% reported using GWP100.
Our resources are not unlimited, and scientists are concerned that we are rapidly approaching one or more climate tipping points; we don’t have 100 years to fix this problem. So, the question that needs to be asked is whether or not using GWP100 appropriate, given the impact it may be having on how climate mitigation policies and actions are being designed and assessed?