Global Warming Potential (GWP) is a handy shorthand that allows us to compare and contrast the climate impact of different greenhouse gases. On its surface, it’s a straightforward measurement that looks at, per molecule, how fast a greenhouse gas warms the earth.
CO2 Standard
GWP uses the global warming potential of carbon dioxide as its standard, 1. The climate impact of all other greenhouse gases is compared to that of carbon dioxide. So, something with a global warming potential of 14 is, per molecule, 14 times more harmful than carbon dioxide.
Since there are ten primary greenhouse gases, it’s important to have a baseline comparison.
Half-life Complications
However, GWP isn’t as straightforward as one comparison. Since each gas has a different half-life (the amount of time that it takes to break down in the atmosphere), we need to introduce another unit of measurement.
Over twenty years, the impact of a molecule of methane compared to a molecule of carbon dioxide is 84-87.
But methane is a relatively short-lived climate pollutant. It lasts about a decade in the atmosphere. If we extend our timeframe to 100 years, the impact of a molecule of methane compared to a molecule of carbon dioxide is 28-36.
Methane absorbs much more energy than carbon dioxide, but it lasts much shorter in the atmosphere. The net climate effect is reflected in the GWP.
Why is this important? Let’s look at it this way: “Strong” can mean something different to everyone. We could ask “how much do you benchpress?” but that still presents us with limited information. A better, more thorough understanding would be to ask “how much of your body weight can you bench press?” That metric helps us understand the full picture and make a better determination of strength.
IPCC Reports
One last hurdle in determining GWP comes down to keeping up with the science. Our tools, methods, and approaches are constantly refining and improving. As such, the data changes. The International Panel on Climate Change reassesses the data every few years, and it can take some time for organizations to adopt the changes.
To keep it simple, we’ve opted to follow the California Air Resources Board’s lead and use IPCC’s Fourth Assessment Report where possible. When needed (for gases that weren’t addressed in the AR4), we’ll use the Fifth Report.
GWP of Refrigerants
The global warming potential of refrigerants varies widely. For example, natural refrigerants have a GWP of about 2. Transition refrigerants have a GWP of 238-1,387. HFCs have a significantly larger impact, in the 10s of thousands.
This chart shows the natural refrigerant GWP compared to transition refrigerant GWP. As you can see, the natural refrigerant GWP is much smaller than the transition refrigerant GWP. This comparison is helpful as we decide to transition to natural refrigerant systems. We can calculate the climate impact based on the GWP of the replacement refrigerant.