Outgoing longwave radiation

Something that often strikes me is that when I think I understand something quite well, there often turns out to be an aspect that I haven’t understood particularly well. I sometimes think that this is can be an important thing to realise; just because you think you have a good understanding of something doesn’t always make it so.

The context here is that a typical way to explain global warming is to point out that adding a greenhouse gas to the atmosphere reduces the outgoing longwave (OLR) flux, and that warming of the surface and troposphere then returns the system to energy balance. This tends to suggest that the OLR will go down when the greenhouse gases are added, and then slowly recover as the system returns to equilibrium.

A particularly persistent climate “skeptic” has, on a number of occasions, promoted the figure on the right, which is from this paper. Because it shows a larger increase in OLR than would be expected if global warming simply involved the OLR recovering, it has been suggested that this disproves anthropogenic global warming (AGW).

As indicated at the beginning of this post, I was also somewhat confused by this. However, thanks to a comment from Chris Colose on this Realcimate post, I’ve cleared up my confusion. Chris highlights this paper which points out that the response to an initial external perturbation (such as increasing atmospheric CO₂) involves both longwave and shortwave feedbacks.

In fact, if we increase atmospheric CO₂, the expectation is that the shortwave feedback will lead to an increase in absorbed shortwave radiation (ASR). As illustrated by the figure on the left, this means that the OLR will end up increasing to above the level it had prior to the increase in atmospheric CO₂. In fact, it potentially recovers in less than 20 years, which means that the subsequent warming is due to the increased ASR. To be clear, this does not mean that global warming isn’t due to increased atmospheric CO₂, since the increased ASR is a response (feedback) to this increased atmospheric CO₂.

I always find it quite fun to solve what was a bit of a puzzle (to me, at least). I would also argue that if someone thinks that they’ve found some supposedly obvious reason why a large number of other experts are wrong, it’s often best to take a step back and consider that there might be something that they’ve missed.

Links:
Realclimate comment – Chris Colose.
Shortwave and longwave radiative contributions to global warming under increasing CO2 – Donohoe et al. (2014).
Global warming due to increasing absorbed solar radiation – Trenberth & Fasullo (2009).