Judith Curry has a new post about Ocean heat content uncertainties. The post itself seems to just be the standard uncertainty monster rearing it’s ugly head. Maybe it is too soon to be confident about anything. Maybe there are still uncertainties in the data. Maybe we don’t understand radiative physics as well as we think we do. Maybe feedbacks won’t operate as we think they should. Maybe everything will be fine. Okay, Judith didn’t actually say all of these things, but this seems to be the basic theme of her post. Would I bet my reputation on the possibility that we’ve made some fundamental mistake with regards to the ocean heat content. That maybe the planet isn’t warming as we think it is. I wouldn’t, but I guess if you want to win big you should take the long odds.
So, maybe Judith will turn out the be correct and will have saved us from damaging our economies by trying to mitigate against a non-existent problem. On the other hand, maybe (probably) not. What I was really wanting to comment on, though, was how Judith finishes her post. She says,
All in all, I don’t see a very convincing case for deep ocean sequestration of heat. And even if the heat from surface heating of the ocean did make it into the deep ocean, presumably the only way for this to happen involves mixing (rather than adiabatic processes), so it is very difficult to imagine how this heat could reappear at the surface in light of the 2nd law of thermodynamics.
The first part is really just an illustration of the uncertainty monster. The latter part, however, is – I think – a misunderstanding that I would have expected Judith to appreciate. I’m not actually sure if anyone has actually said this heat could reappear at the surface, but if they have, I don’t think they mean it the way Judith has interpreted it.
I don’t think anyone is really suggesting that the energy going into the deep ocean will suddenly roar to the surface and accelerate surface warming. What they’re suggesting is that the planet has an energy imbalance (we’re receiving more energy than we lose) and – at the moment – more of this energy is going into the oceans than was the case in past decades. Some of this energy appears to be going into the deep oceans. This cannot carry on indefinitely. The oceans are not an infinite heat sink, and the surface has to eventually warm so as to reduce the energy imbalance (we can’t maintain an energy imbalance forever). Hence, we would expect – in the future – the fraction of the energy excess going into the oceans to reduce and the fraction heating the surface (land and atmosphere) to increase, and for surface warming to accelerate. So, it’s not the actual energy quanta that went into the deep ocean that will suddenly reappear, it’s the portion of the energy imbalance currently heating the deep ocean that will suddenly act to heat the surface.
Precisely when this happen is hard/impossible to know. As I discuss in this post the trade winds are nudging up sea levels in the western Pacific. Eventually that water (with it’s energy) will have to run back towards the East. Also, the longer it takes, the larger the energy imbalance will be and the faster the surface warming will be. So, yes, the energy going into the deep ocean will likely stay there (for the foreseeable future, at least) but basic physics tells us that we can’t continue to sequester energy in the deep ocean and eventually the portion of the energy excess that is currently going into the deep ocean will have to start heating the surface, and we will have to see accelerated surface warming (unless, as I said at the beginning, our understanding of radiative physics turns out to be wrong).