Okay, not really, but I might need to change them. Something that I’ve been stressing here is that internal (natural) variability can’t produce long-term warming. The reason being that it just moves energy around the climate system. If it’s not associated with some kind of change in radiative forcing, then if it heats the surface, the extra energy will be lost quickly, and if it cools the surface, the energy will be recovered quickly. For a system already in equilibrium, internal variability will simply cause the temperatures to vary about the externally determined equilibrium temperature.
I have, however, been made aware of a recent paper (thanks Victor; some people just don’t know how to stick to the script) that shows that it may be more complex than this. The paper (Top-of-atmosphere radiative contribution to unforced decadal global temperature variability in climate models by Brown, Li, Li & Ming) uses 36 pre-industrial control runs from the CMIP5 ensemble, to investigate how decadal variability in the top-of-the-atmosphere (TOA) flux influences surface temperatures. As I understand it, all these runs are unforced, so all the variability is internal (natural) and not forced.
The main figure is probably the one below. The two largest-magnitude warming decades and the two largest-magnitude cooling decades were selected from each of the 36 controls runs. In each case, they also determined the TOA flux for each of the decades that were selected. What they found was that warming decades were associated with a mean net TOA flux of -0.06 Wm-2 (upward orientated, so negative means gaining energy), and the cooling decades were associated with a mean net TOA flux of +0.06 Wm-2. So, even though this variation is internal, the warming is not simply associated with a movement of energy around the climate system, but is also associated with a net TOA flux that increases the total energy in the system. Similarly – but in reverse – for cooling decades. The paper also illustrates that about half the warming (cooling) in each of these decades was associated with the net TOA flux.
So, this seems to be an interesting result : internal variability isn’t necessarily simply associated with energy moving around the climate system, it could also associated with a net TOA flux. The paper indicates that this is mainly a consequence of small changes (0.1%) in the albedo that temporarily counteracts the change in the outgoing long-wavelength radiation.
The one question, though, is how significant this is with respect to recent warming. The paper addresses this, saying,
From the years 1955 to 2010, 0–2000 m ocean heat content accumulation suggested that the mean QTOA over this period was ~0.27 W/m2 (due mostly to external radiative forcings, F) [Levitus et al., 2012]. This study has shown that in extreme episodes of decadal scale unforced T change, mean TOA imbalances were on the order of ± ~0.06 W/m2 averaged across all GCMs (the most extreme imbalances observed were ± ~0.2 W/m2, Figure S4). This would imply that in certain circumstances, unforced variability in QTOA may have been able to modulate the long-term forced imbalance by ~22% (~74% in the most extreme circumstances) over the course of a given decade. Currently, however, measurements indicate that the energy imbalance at the TOA is between 0.5 and 1.0 W/m2 [Abraham et al., 2013; Trenberth et al., 2014]. At this magnitude, unforced QTOA variability would only be able to modulate the background forced imbalance by ~6–12% (~20–40% in the most extreme circumstances) over the entirety of a given decade.
So, over the course of a given decade it could modulate the forced imbalance by more than 50% when the imbalance is less than 0.5 Wm-2. However, given that the imbalance today is probably around 0.5 Wm-2 or greater, even in the most extreme scenario, internal variability will probably only be able to modulate the imbalance, today, by about 40%, at most. Of course, one should remember that this could both increase and decrease the imbalance, and – if I understand the paper correctly – is unlikely to extend beyond about a decade.
Even though this paper indicates that internal variability may not be only associated with moving energy around the climate system, I don’t think this means that internal variability can – by itself – be associated with long-term (multi-decade) warming, or cooling. I thought I might lay out some reasons why. Of course, these are just how I understand it, so if others disagree or want to elaborate or clarify, please do so.
- Unforced, climate model control runs do not show evidence for multi-decade trends associated with internal variability.
- Even though internal variability may not be only associated with moving energy around the climate system, this is still the process that triggers the warming or cooling. Since the available energy is finite, the process is self-limiting.
- There is little evidence to suggest that internal variability played a significant role in past climate variability. Maybe, more correctly, in most cases we can associate past changes to our climate with external triggers or changes in external forcings.
- As I understand it, the main reason for the change in the TOA flux is because of changes in albedo, presumably driven by changes in cloud cover. One might think that this could feedback on itself and consequently produce longer-term warming or cooling. Water vapour is, however, quite sensitive to temperature changes and so when the cycle reverses, the changes in water vapour and clouds should reverse and the system should evolve back towards to its pre-warming/cooling state (I may not have explained this all that well, so if someone can do a better job, go ahead).
- The annual variation in global temperature is actually greater than the kind of variations being considered here. If this kind of variability could produce long-term warming, then that would suggest that the same should apply to the annual variations.
- If internal variability can drive long-term warming, it would suggest that our climate is quite sensitive to small changes. This should then apply to changes in external forcings too (increased CO2 concentrations, for example), but might also imply that if it were that sensitive to small changes, we wouldn’t be here to have this discussion.
Given the recent interest in the role of internal variability, I thought this might be something interesting to post about. I don’t, however, think it really changes anything with respect to the significance of internal variability (and it is only one paper) and have laid out some ideas why, above. Of course, if others disagree or have other thoughts, feel free to make them through the comments.