I’ve been pondering today if being anonymous means that others think it’s okay to be rude and unpleasant, or if they are simply rude and unpleasant. I’m tending towards the latter and have seen nothing to convince me otherwise. A reason I was pondering this, was that I had another lengthy Twitter discussion yesterday that ended in the standard question as to whether feedbacks were operating or not? I responded that they were and provided a calculation that I initially got wrong. As an aside, it’s interesting that those who will likely end up on the wrong side of this “debate” are very quick to highlight silly mistakes made by others. You might think that those who are likely to end up wrong, would be more circumspect, but maybe if you’re going to gamble you may as well go the whole way (also, you could refer back to the beginning of this post).
Anyway, there’s a really easy way to show why feedbacks probably are operating, and I’ll try to explain it here. It’s probably reasonable to assume that the planetary energy imbalance in the mid-1800s was small (quite close to zero). Since then surface temperatures have risen by about 1 degree (this is all going to be ballpark numbers so let’s not quibble over a few tenths here and there). Everyone – I think – agrees that a doubling of CO2 produces an increase in radiative forcing of around 3.5 Wm-2, and would ultimately result in an increase in surface temperatures of about 1 degree. Therefore, if nothing else were to change, a 1 degree increase in surface temperature, should increase the outgoing flux by about 3.5 Wm-2.
What we observe, however, is that we have an energy excess of about 0.5 Wm-2 (i.e., we’re gaining 0.5 Joules per square metre per second, rather than losing 3.5 Joules per square metre per second). This means that there must also have been an increase in radiative forcing – since the mid-1800s – of about 4 Wm-2 (i.e, 3.5 Wm-2 + 0.5 Wm-2). Where can this come from? If you consider the IPCC radiative forcing diagram below, solar forcing could make a small contribution, and anthropogenic forcing probably contributes about 2 Wm-2. So external forcings could be contributing just over 2 Wm-2.
So, if external forcings (solar + anthropogenic) can contribute just over 2 Wm-2 of the 4 Wm-2, where does the rest come from? Well, that would probably be the feedback response; mainly water vapour. So, there you go. That’s a simple explanation for why feedbacks are probably operating and are probably comparable in magnitude (between 1 and 2 Wm-2) to the anthropogenic forcings. Of course, if anyone thinks I’ve made a mistake or would like to correct anything, feel free to do so (bearing in mind that these were just ballpark numbers).
Having said the above, if people do want to argue that feedbacks are not operating or are small, they would seem seem to have 3 options.
- They could argue that the measurements are wrong. This is possible, but I don’t really see how we can have sensible scientific discussions if some can simply assert that the measurements aren’t right.
- They could argue that the aerosol forcing is really small. If this were the case, then the change in anthropogenic forcings would be much larger than we currently think, and could produce most of the change in radiative forcing. This, however, seems unlikely as the best estimate for the aerosol forcing is around -1 Wm-2.
- They could argue that, coincidentally, some kind of internal variability has produced a change in radiative forcing that is unassociated with the change in anthropogenic forcing and that will simply go away at some point in the future. Well, there’s no real evidence for this and it would also make it difficult to explain the greenhouse effect and past climate variability.
So, that’s it. I’ve explained why feedbacks probably are operating and even give some arguments that could be made if one wants to assert that they’re not or are small. Of course, it’s possible I’ve made some kind of silly mistake or misunderstood something, so please point that out if so – ideally taking the comment and moderation policies into account.