Thursday Reading

Very interesting article on tipping points that seems to answer a question I put to Vasilis back in 2009 at IIASA: whether critical transitions due to bifurcations could be distinguished from stochastic transitions due to rare events Ditlevsen & Johnsen, 2010.  Argues that some of the well-known climate transitions may have been stochastic transitions instead.  Will need a closer read to better assess how they are distinguishing between such transitions in the data.

Provocative Nature paper by Frank et al.  claims that fishery collapse represents only a long transient on the road to recovery, rather than an alternate stable state (Frank et. al. 2011).  An intriguing debate – setting aside the trouble of actually defining what it means rigorously, since either description is a low-dimensional approximation of the true dynamics anyhow.  Of course the next worry is that we probably don’t have the data to tell definitively, even if we can define the question; but we can certainly do our best.  This kind of data on fisheries seems to go through all kinds of opaque filtering before it becomes the numbers we analyze (getting from catch effort to information about abundance and size that we don’t observe by using phenomenological models seems to leave room for doubt).  So if we set those concerns aside too and do the best we can to answer the question – we can at least confront the paper.  The hypothesis seems plausible, the argument that the system is in a transient because Figure 3 looks like the pattern of a damped harmonic oscillator is hardly convincing.  The match of theory to data should be compared between plausible hypotheses.  Nor does it seem that just because the forage fish dynamics have a stable spiral that we’ve eliminated the alternate stable states hypothesis.  Still, I’d need to actually look at the data to really be able to make much judgment, and it looks like reconstructing the time series from the public data would take some doing.

Hypothesis they wish to displace runs something like this: Large benthic species like cod normally predate the smaller forage fish.  Destruction of cod releases the forage fish who then prevent cod recovery due to their increased biomass (through competition with smaller cod, or predation, etc) (Jackson, 2001) (Persson et. al. 2007) (Fauchald, 2010).

Interesting review of “Early warning of climate tipping points” in Nature Climate Change: (Lenton, 2011); a remarkable number of the examples are ecological rather than more meteorological.

References