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  • Emily Six

20 vs 100 Years: Putting GWP into Practice

Humankind must reduce the number of greenhouse gases in the atmosphere as quickly as practical to minimize man-caused global warming. The Paris Agreement’s goal is to limit global warming to well below 2 degrees Celsius, preferably to 1.5 degrees Celsius, compared to pre-industrial levels. To stay below 1.5 °C of global warming, greenhouse gas emissions need to be cut by roughly 50% by 2030only 7.5 years away.

Are all greenhouse gases equal?

The mandate "cut greenhouse gas emissions by 50%" is somewhat of an oversimplification. In reality, different greenhouse gases have different effects on our atmosphere. Our understanding of the impact of each gas continues to evolve as we study the sources and sinks of emissions. Reviewing each greenhouse gas's lifespan in our atmosphere is essential to create the most effective strategy required to limit catastrophic outcomes.

The easiest way to compare greenhouse gases is by their Global Warming Potential GWP. (Read a comprehensive introduction to GWP here.) Global Warming Potential is a measurement that compares each greenhouse gas's impact over time to that of carbon dioxide. The most widely used GWP timeframe is 100 years. GWP-100 presents a valuable comparison of climate pollutants and is often used to prioritize climate solutions.

Short-lived climate pollutants

However, the matter isn't quite so straightforward. Using 100-year GWP alone doesn’t take the 20-year effect of short-lived climate pollutants into account. This might adversely skew the prioritization of high-GWP, short-lived gases.

Short-lived climate pollutants have a half-life of fewer than 20 years, meaning they enact the bulk of their extraordinarily large damage in their first 20 years in the atmosphere. Evaluating that negative impact over 100 years creates a false understanding of their devastating legacy.

Let's think about this from a creative angle: beach safety and shark attacks. I invite you to a surf lesson and tell you that over the last 100 years, my neighborhood beach has seen only 30 unprovoked shark attacks. You might assume that's 1 attack every 3 years. However, upon clarification, you learn that 25 of those attacks occurred in the last 20 yearsand 19 of those were in the last 5 years. That certainly changes your view of taking a carefree dip, doesn't it?

Despite the fact that I live in a landlocked state and fabricated those numbers, a silly analogy like the one above helps illustrate why it is important to accurately understand and reflect the nuances of each greenhouse gas. We won't dive into the nitty gritty of each climate pollutant here, but we will look at a few types of gases in greater detail.

Choosing the best perspective

New York state’s Climate Act uses the 20-year GWP methodology for hydrofluorocarbons (HFCs) precisely because they are short-lived climate pollutants. For example, HFC-32 has a half-life of 5.4 years. Its 20-year GWP is 2,690. Its 100-year GWP decreases to 771 and decays to 220 after 500 years. HFC-134a has a half-life of 13.8 years and a 20-year GWP of 4,140. When evaluated over 100 years, its GWP dwindles to 1,530 and decays to 436 after 500 years.

The climate impact of longer-lived pollutants is best measured over 100 years – for example Nitrous Oxide. N2O has a lifetime of 114 years. Therefore, its GWP doesn’t change over 100 years; it stays steady at 298, decaying slowly to 130 after 500 years.

Why it matters

In general, we support using a 100-year GWP as a standardized unit of measurement for carbon projects. It’s important to establish credible comparisons not only between gases but companies and solutions. Assessing overall climate trends is made easier and more accurate when we can compare apples and apples.

We value accuracy as much as the next scientist, but the crux of this issue isn't just accuracyit's urgency. We simply don't have 100 years to address the climate crisis. As mentioned in the opening paragraph, we have 7.5 years to accomplish our goals. Together, we must prioritize the elimination of short-lived climate pollutants in order to properly address our mid-century goals. Discussing and evaluating short-lived climate pollutants in terms of GWP-20 helps convey the urgency around these gases.

Addressing high-GWP refrigerants isn't a bandaid to the climate crisis; it's applying pressure to an arterial bleed. We must stop the bleeding before we can even hope to address the myriad other issues and establish a climate equilibrium.

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