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Register a new userA recent tipping point in the Arctic sea-ice cover: abrupt and persistent increase in the seasonal cycle since 2007
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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 studies argue that the loss of summer sea ice does not involve an irreversible bifurcation, because it is highly reversible in models. However, a broader definition of a `tipping point also includes other abrupt, non-linear changes that are neither bifurcations nor necessarily irreversible. Examination of satellite data for Arctic sea-ice area reveals an abrupt increase in the amplitude of seasonal variability in 2007 that has persisted since then. We identified this abrupt transition using recently developed methods that can detect multi-modality in time-series data and sometimes forewarn of bifurcations. When removing the mean seasonal cycle (up to 2008) from the satellite data, the residual sea-ice fluctuations switch from uni-modal to multi-modal behaviour around 2007. We originally interpreted this as a bifurcation in which a new lower ice cover attractor appears in deseasonalised fluctuations and is sampled in every summer-autumn from 2007 onwards. However, this interpretation is clearly sensitive to how the seasonal cycle is removed from the raw data, and to the presence of continental land masses restricting winter-spring ice fluctuations. Furthermore, there was no robust early warning signal of critical slowing down prior to the hypothesized bifurcation. Early warning indicators do however show destabilization of the summer-autumn sea-ice cover since 2007. Thus, the bifurcation hypothesis lacks consistent support, but there was an abrupt and persistent increase in the amplitude of the seasonal cycle of Arctic sea-ice cover in 2007, which we describe as a (non-bifurcation) `tipping point. Our statistical methods detect this `tipping point and its time of onset.
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The Arctic sea ice represents an important energy reservoir for the climate of the northern hemisphere. The shrinking of the polar ice in the past decades decreases the stored energy and raises serious concerns about future climate changes.[1-4] Model calculations of the present authors [5,6] suggest that half of the global warming during the past fifty years is directly related to the retreat of the sea ice, while the cause is not well understood, e.g. the role of surface pollution [7-10]. We have analysed the reported annual melting and freezing data of the northern sea ice in the years 1979 to 2018 [11] to gain some insight. Two features can be deduced from our simple model: (i) recent results [12,13] are confirmed that approximately 60 % of the loss of sea ice stems from energy transport to the arctic region. (ii) We find evidence that the remaining part of the ice retreat originates from an increasing surface absorption of solar radiation, obviously due to the rising surface pollution of the sea ice. While the phenomenon was previously considered by several authors in a qualitative way, our analysis contributes semi-quantitative information on the situation. We estimate that the relevant fall-out of light absorbing aerosols onto the sea ice increased by 17 +/- 5 % during the past fifty years. A deposition of additional 3 +/- 1 % of solar radiation in the melting region results that accounts for the ice retreat. Recalling the important role of the ice loss for the terrestrial climate,[3,5,9] the precipitation of air pollution in the Arctic seems to be an important factor for the global warming.
The importance of the sea ice retreat in the polar regions for the global warming and the role of ice-albedo feedback was recognized by various authors [1,2]. Similar to a recent study of the phenomenon in the Arctic [3] we present a semi-quantitative estimate of the mechanism for the Southern Hemisphere (SH). Using a simple model, we estimate the contribution of ice-albedo feedback to the mean temperature increase in the SH to be 0.5 +/- 0.1 K in the years 1955 to 2015, while from the simultaneous growth of the greenhouse gases (GHG) we derive a direct warming of only 0.2 +/- 0.05 K in the same period. These numbers are in nice accordance with the reported mean temperature rise of 0.75 +/- 0.1 K of the SH in 2015 since 1955 (and relative to 1880). Our data also confirm previously noticed correlations between the annual fluctuations of solar intensity and El Nino observations on the one hand and the annual variability of the SH surface temperature on the other hand. Our calculations indicate a slowing down of the temperature increase during the past few years that is likely to persist. Assuming a continuation of the present trends for the southern sea ice and GHG concentration we predict the further temperature rise to decrease by 33 % in 2015 to 2025 as compared to the previous decade.
The BLLac object S4 0954+65 is one of the main targets of the Urumqi monitoring program targeting IntraDay Variable (IDV) sources. Between August 2005 and December 2009, the source was included in 41 observing sessions, carried out at a frequency of 4.8 GHz. The time analysis of the collected light curves, performed by applying both a structure function analysis and a specifically developed wavelet-based algorithm, discovered an annual cycle in the variability timescales, suggesting that there is a fundamental contribution by interstellar scintillation to the IDV pattern of the source. The combined use of the two analysis methods also revealed that there was a dramatic change in the variability characteristics of the source between February and March 2008, at the starting time of a strong outburst phase. The analysis results suggest that the flaring state of the source coincides with the appearance of multiple timescales in its light curves, indicating that changes in the structure of the relativistically moving emitting region may strongly influence the variability observed on IDV timescales.
The growing concentrations of the greenhouse gases CO2, CH4 and N2O (GHG) in the atmosphere are often considered as the dominant cause for the global warming during the past decades. The reported temperature data however do not display a simple correlation with the concentration changes since 1880 so that other reasons are to be considered to contribute notably. An important feature in this context is the shrinking of the polar ice caps observed in recent years. We have studied the direct effect of the loss of global sea ice since 1955 on the mean global temperature estimating the corresponding decrease of the terrestrial albedo. Using a simple 1-dimensional model the global warming of the surface is computed that is generated by the increase of GHG and the albedo change. A modest effect by the GHG of 0.08 K is calculated for the period 1880 to 1955 with a further increase by 0.18K for 1955 to 2015. A larger contribution of 0.55 +/-0.05 K is estimated for the melting of polar sea ice (MSI) in the latter period, i.e. it notably exceeds that of the GHG and may be compared with the observed global temperature rise of 1.0 +/- 0.1 K during the past 60 years. Our data also suggest a delayed response of the mean global temperature to the loss of sea ice with a time constant of approximately 20 years. The validity of the theoretical model and the interrelation between GHG-warming and MSI-effect are discussed.
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