Michael Mann and Lawrence Torcello have an article in the Conversation about the philosophy and science of limiting global warming to 2^{o}C. This caused some controversy on Twitter because it wasn’t clear if the article referred to the Northern Hemisphere only, or to the whole globe. It also cause some controversy because 2036 seems very soon for us to reach the 2 degree limit (self-imposed limit, at least).

However, if – as the article assumes – the ECS is 3 degrees and we follow a high-emission pathway (RCP8.5) then I think 2 degrees by 2036 is plausible. An ECS of 3 degrees suggests that feedbacks amplify the warming by a factor of about 2.7 (3/1.1). This tells us that the feedbacks must be about 2.2Wm^{-2}K^{-1}. If we follow a high emission pathway then we will increase anthropogenic forcings by about 2Wm^{-2} by 2036. Given that we have a planetary energy imbalance of around 0.6Wm^{-2} today, if we did not warm at all, we’d have a planetary energy imbalance of 2.6Wm^{-2} in 2036.

To reach 2 degrees by 2036, we’d need to warm by 1.1 degrees from today (1.1 + 0.9) which would produce a negative feedback of 3.7Wm^{-2}, and positive feedbacks of 2.4Wm^{-2} (2.2 x 1.1). This would then leave a planetary energy imbalance of 1.3 Wm^{-2} (2.6 + 2.4 – 3.7) which may actually be a little high, but not implausible. So, 1.1 degrees from today seems entirely possible, especially as we might expect it unlikely that the planetary energy imbalance could grow much larger than around 1Wm^{-2}.

This also seems broadly consistent with a post by Ed Hawkins which seems to suggest that 2 degrees by around 2040 is plausible if we follow an RCP8.5 emission pathway. So, I’m not sure I quite get what the fuss is all about. If the ECS is indeed 3 degrees (which is close to the expected value) then reaching 2 degrees by 2036 is indeed quite possible, if we choose to follow a high emission pathway. Of course, if we do reach 2 degrees by 2036, this will not be where warming stops, since we’d still warm by about another degree or so, so as to reach equilibrium. Of course, I’m not suggesting that the exact date would be 2036, simply that reaching 2 degrees by that time is quite possible if we continue along a high emission pathway. I guess we’ll know soon enough if this is indeed possible or not; at which point we can all collectively say “shit, Michael Mann was right!”.