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There is ongoing debate over whether Arctic sea-ice has already passed a`tipping point', or whether it will do so in the future. Several recent studiesargue that the loss of summer sea ice does not involve an irreversiblebifurcation, because it is highly reversible in models. However, a broaderdefinition of a `tipping point' also includes other abrupt, non-linear changesthat are neither bifurcations nor necessarily irreversible. Examination ofsatellite data for Arctic sea-ice area reveals an abrupt increase in theamplitude of seasonal variability in 2007 that has persisted since then. Weidentified this abrupt transition using recently developed methods that candetect multi-modality in time-series data and sometimes forewarn ofbifurcations. When removing the mean seasonal cycle (up to 2008) from thesatellite data, the residual sea-ice fluctuations switch from uni-modal tomulti-modal behaviour around 2007. We originally interpreted this as abifurcation in which a new lower ice cover attractor appears in deseasonalisedfluctuations and is sampled in every summer-autumn from 2007 onwards. However,this interpretation is clearly sensitive to how the seasonal cycle is removedfrom the raw data, and to the presence of continental land masses restrictingwinter-spring ice fluctuations. Furthermore, there was no robust early warningsignal of critical slowing down prior to the hypothesized bifurcation. Earlywarning indicators do however show destabilization of the summer-autumn sea-icecover since 2007. Thus, the bifurcation hypothesis lacks consistent support,but there was an abrupt and persistent increase in the amplitude of theseasonal cycle of Arctic sea-ice cover in 2007, which we describe as a(non-bifurcation) `tipping point'. Our statistical methods detect this `tippingpoint' and its time of onset.

A recent tipping point in the Arctic sea-ice cover: abrupt and persistent increase in the seasonal cycle since 2007