Early Warning Signals (EWS)
Early Warning Signals (EWS) are statistical and mathematical methods used to forecast critical transitions in complex systems, with critical (or catastrophic) transition (or tipping points) denoting the sudden shift of a system from one state (or equilibrium) to another due to complex interactions (Scheffer, 2009). Such shifts have been described for a wide array of biomedical (Litt et al., 2001; McSharry et al., 2003; Eiger/Lehnertz, 1998), economic (Bates, 1991, 1996), ecological and climate systems such as tropical forests (Hirota et al., 2011), savannah (Kéfi et al., 2007), fisheries (Berkes et al., 2006, Litzow et al., 2008), coral reefs (Mumby et al., 2007), and lake ecosystems (Carpenter, 2011).
In recent years, various statistical patterns have been identified that seem to be associated with these sudden critical transitions. It is assumed that these patterns can be used as EWS of a system approaching a tipping point (Scheffer et al., 2009; Lenton, 2011). At the time being these statistical patterns comprise dynamics like “critical slowing down”, “flickering”, “increasing variance”, “increasing autocorrelation”, “self-organized spatial patterns” (for an overview see Scheffer et al., 2009; for discussions see Carpenter/Brock, 2006; van Nes/Scheffer, 2007). Some of these indicators have been tested in both experimental manipulation (Drake/Griffen, 2010; Carpenter, 2011; Veraart et al., 2011; Dai et al., 2012) and historical observations (Livina and Lenton, 2007; Dakos et al., 2008; Thompson/Sieber, 2010), with the latter focusing particularly on global climate dynamics. As one example for this, Litzow et al. (2008) were able to identify increased spatial variance one year ahead of a climate associated regime shift in the Gulf of Alaska, and three years ahead of the overfishing related regime shift on the Scotian shelf.
Although lately there have been reservations (about EWS partially being due to misinterpretation of data, see Ditlevsen/Johnsen, 2010; Boettiger/Hastings, 2012), research on EWS seems to offer interesting possibilities for the promotion and advancement of Climate Service research. In any case it conveys interesting insights into the dynamics and stabilities of complex systems.
Manfred Füllsack is Professor for Systems Sciences at the University of Graz. His research includes: systems, complexity, networks, games and computational theory, work – its history, its sociology, its economy, and computer-based simulations.
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