Climate discussions of late have focused a lot of so-called short-lived forcers. These are substances such as methane and black carbon that don’t stay in the atmosphere for all that long but trap a lot of heat while they’re there. Analysts use global warming potentials (GWPs) as shorthand to compare these gases with carbon dioxide. For example, over a 20-year period, methane traps 72 times as much heat as carbon dioxide, giving methane a 20-year GWP of 72.
The problem, as many readers of this blog know, is that it’s never clear what time period one should focus on. Methane may have a 20-year GWP of 72, but it also has a 100-year GWP of 25. Which one should analysts use when thinking about the dangers posed by short-lived forcers?
An EPA working paper that I wish I’d read when it was published in January 2011 (it was also published as a peer-reviewed paper in Energy Policy in 2012) suggests a useful way of thinking about this. Instead of focusing on how much heat is trapped, we should be looking at how much damage is done.
There’s a huge literature aimed at estimating the damage done by a ton of carbon dioxide, a quantity known as the social cost of carbon (SCC). The literature is highly controversial, in part because damage estimates are uncertain, but also because the results depend sensitively on how deeply you discount the future economic impact of emissions. When the U.S. government estimated the SCC in 2010, for example, it found a value of $21 per ton. But had it assumed a discount rate of 5 percent rather than 3 percent, the answer would have come out at $5, and had it assumed a discount rate of 2.5 percent, the SCC would have been $35. Moreover, had it focused on the 95th percentile of potential damages, it would have found an SCC of $65.
Here’s the neat thing that makes looking at damages a great way to think about short-lived forcers: the estimated damages caused by short-lived forcers are a lot less sensitive to the discount rate. That’s because they’re concentrated in the near future, which makes them less sensitive to the choice of discount rate. That makes it easier to approach agreement on what the social costs of short-lived forcers are.
So what’s the upshot? According to the EPA authors’ paper, if you assume a 5 percent discount rate, methane is 39 times as damaging as carbon dioxide when integrated over time; if you assume a 3 percent discount rate, methane is only 25 times as damaging, similar to the ratio suggested by the 100-year GWP; and if you assume a 2.5 percent discount rate, that factor drops to 21. (The ratio would be even smaller for the ultra-low discount rates that some have encouraged.) A similar pattern would prevail if one modeled other short-lived forcers. Moreover, if you focus on the 95th percentile damages, you find methane is 27 times worse that carbon dioxide.
Those who claim that climate impacts from carbon dioxide emissions are well above the $21 figure used by the U.S. government typically argue for low discount rates and high-end damages to justify their stance. In order to be self-consistent, then, they should be treating methane and other short-lived-forcers based on something closer to their 100-year (or longer) GWPs than the 20-year ones that have become popular in some quarters in recent years.