by Emil Morhardt
Everyone agrees that it would be helpful in making climate policy if we had some advance warning of climate tipping points. Williamson et al. (2016) set out to look for one associated with the melting of the Arctic sea ice. They asked if there is any evidence that the sea ice is about to undergo a local tipping point (which they also refer to as a bifurcation) that would lead to much faster melting. In the case of Arctic sea ice, the annual melting is caused by the summer sun, and a long-term parameter change that is enhancing it is the gradual buildup of CO2 in the atmosphere. Melting is limited partially by the fact that much of the incoming solar radiation is reflected back to space by the ice. A tipping point might be reached when enough of the ice is melted that the amount of sunlight absorbed by the ice-free open ocean begins to warm it faster each cycle. Although this study did not find any evidence for an imminent tipping point the authors noted that the sort of signal they were looking for might not occur until very close to a tipping point. That would, of course, make it useless for long-term policymaking. Their approach to looking for a signal is interesting though, and could be applied to many other potential climate tipping points.
A general feature of tipping points is that a small smooth change in parameters controlling a system triggers an out-of-proportion response. The parameter undergoing the small smooth change in this case is atmospheric CO2 concentration; at some point the warming from it might exceed that needed to melt enough ice that runaway melting is triggered.
The authors begin by describing the series of changes that they thought might be observed in the annual cycles of sea ice area if it were approaching a tipping point. At the moment, for example, there is a three month lag between the annual maximum solar radiation and the minimum ice (and conversely, between the annual minimum solar radiation and the maximum extent of sea ice.) Any change in this phase lag might have indicated a coming tipping point, but there was none. There also might have been an increase in the annual amount of melting out of proportion with the CO2 increase, but apparently this did not occur either. Subjecting the annual pattern to a Fourier transform—a mathematical analysis of the frequency of change—would have revealed any trend in second and third harmonics; changes in the magnitude of these might portend a tipping point, but there were no consistent ones. The authors also tried calculating the lag 1 autocorrelation of each cycle to look for trends, but did not find any and concluded that this test was not likely to reveal anything in this system anyway.
Nevertheless, the approach used in this analysis is potentially very effective; the academic researchers accessed satellite data in the public domain and analyzed it in ways not previously tried. While they turned nothing up in this case, they might well have, and probably will eventually as they keep trying.
Williamson, M., Bathiany, S., Lenton, T., 2016. Early warning signals of tipping points in periodically forced systems. Earth System Dynamics Discussions 6, doi: 10.5915/esd-5917-5313-2016.