Papering over “overshoot”

What’s going on with these “overshoot” scenarios coming out of the IPCC?

Photo by Mick Haupt on Unsplash

In his latest ode to collapse, Umair, our resident Casandra here on Medium, mentions how many of our more recent IPCC climate scenarios involve the idea of “overshoot”, which he describes as a process in which:

“the temperature rises, and then it comes down, doing a whole lot of damage along the way”

One commentator replied that maybe Umair didn’t understand the term “overshoot”, which has a well-known and long-established different meaning. Another commentator replied that Umair was using the term correctly, but this was a newer meaning of “overshoot”, not the original one.

I added a reply to the thread, raising a related question: why obscure a well-established term by giving it a new meaning unrelated to its original meaning? Let me just say it got my Spidey Sense tingling. Here’s what I wrote (I’ve added some references for context).

You are right that Umair is not using the term “overshoot” here as it has traditionally been used since Catton’s 1980 book. This is a new use of the term introduced recently to describe climate scenarios that go over a threshold temp in an initial period, but then are brought back down to below the threshold in later years.

Most future climate scenarios considered in the IPCC’s Sixth Assessment Report (AR6) now factor in an overshoot in their projections, although there are significant variations in the severity and longevity of the predicted overshoot, and hence the consequences that it will entail. (source)

This sort of scenario has become popular in recent years because IPCC modelers have been unable to find a formula for reducing emissions to “net zero” that is compatible with ongoing economic growth (source, see also this excellent piece by Paul Abela here on Medium). So these models now posit that we grow and blow more CO2 into the atmosphere, then rely on magical thinking and unproven, unlikely to scale, but incredibly expensive technologies (carbon capture and storage, CCS, carbon capture and removal, CCR) to pull that CO2 back out of the air, bring that “overshoot” back down to “net zero”, and bring warming back down to 1.5°C (source). (On the questionable…