Michael Mann, Stefan Rahmstorf and colleagues have a new paper on the likelihood of the recent warmth. What they’re investigating is the run of warm years we’ve seen recently – 13 of the warmest 15 years have happened since 2000, and 9 of the 10 warmest years have happened since 2000. They want to determine how likely this is from internal variability alone, and how likely it is if they then include anthropogenic and natural forcings.
Essentially, they generate a large number of time series and then test the likelihood of observing these runs of warmest years. For time series that are intended to represent internal variability only (estimated using the residuals after the CMIP5-estimated forced response is subtracted from the observed temperatures) it is 1-in-10000 for the 13 in 15 warmest years, and 1-in-770 for the 9 in 10 warmest years. When anthropogenic and natural forcings are included, it becomes 72% and 83%. They also considered a scenario in which internal variability was assumed to have much more persistence than is considered likely, which then increases the likelihood due to internal variability only, to 1-in-100 and 1-in-80. However, as the paper says
even using a too-conservative null hypothesis of persistent red noise, the recent observed record warmth is still unlikely to have occurred from natural variability alone.
and they conclude that
the recent record temperature years are are roughly 600 to 130,000 times more likely to have occurred under conditions of anthropogenic than in its absence.
I should probably add that they also considered individual years and found that the likelihood of these warm individual years occuring due to internal variability alone is much smaller than the likelihood of the runs of warmest years. This is because it has to actually cross some warming threshold, rather than simply have a run of warmest years in the record.
As far as I can tell, this all seems pretty obvious. Judith Curry, on the other hand, seems less than impressed, and appears to be suggesting that we should simply assume that we don’t know anything. Nic Lewis, surprise surprise, seems to think that
[i]t is a paper that would be of very little scientific value even if it were 100% correct.
You might imagine that this is because Nic also thinks that it’s a pretty obvious result. You might also be wrong. He then goes on to list various criticisms of the paper, including that the record is too short to determine internal variability, that a detailed attribution study should have been performed, that they should have considered models with lower sensitivity (as if only a few hundred to a few hundred thousand times more likely would change the overall conclusion significantly), and that there are problems with their assumptions about long memory noise.
The latter issue – which I’ll comment on briefly – is essentially whether or not internal variability could drive long-term warming or cooling. The answer to this is almost certainly “no”. You could try reading this Realclimate post. Richard Telford has written about this in the context of Doug Keenan’s claims. I’ve written about it too.
The basic issue is very simple. If you want internal variability to drive, for example, long-term warming, then the energy has to come from somewhere. It could come from the oceans, but you can’t extract energy from the oceans indefinitely and, if the temperature exceeds the equilibrium temperature, it would radiate away quite rapidly (the heat capacity of the land and atmosphere is low relative to the oceans). Alternatively, maybe some internal warming could drive a radiative response that sustains a planetary energy imbalance. The problem here is that the physical processes involved would essentially be the same as those that act as feedbacks to forced warming. So, if you want to argue for high sensitivity to internally-forced warming, you’re essentially arguing for high climate sensitivity overall, and most of our observed warming would be anthropogenic anyway – which is essentially what this paper is illustrating.
Anyway, it’s been a long day and that’s about all I can think of saying. If anyone has anything to add, feel free to do so through the comments.