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The Earth's climate has experienced numerous critical transitions during its history, which have often been accompanied by massive and rapid changes in the biosphere. Such transitions are evidenced in various proxy records covering different timescales. The goal is then to identify, date, and rank past critical transitions in terms of importance, thus possibly yielding a more thorough perspective on climatic history. To illustrate such an angle, which inspired the punctuated equilibrium angle on the theory of evolution, we have analyzed 2 key high-resolution datasets: the CENOGRID marine compilation (past 66 Myr), and North Atlantic U1308 record (past 3.3 Myr). By combining recurrence analysis of the individual time series with a multivariate representation of the system based on the theory of the quasi-potential, we identify the key abrupt transitions associated with major regime changes that differentiate various clusters of climate variability. This allows interpreting the time-evolution of the system as a trajectory taking place in a dynamical landscape, whose multiscale features are associated with a hierarchy of tipping points., 27 pages, 5 figures plus supplementary material

Paleoclimate proxy records contain abrupt transitions that may represent former instances of the climate system crossing a tipping point (TP). Properly identifying these TPs in the Earth’s past helps determine critical thresholds in present-day climate and better understand the climate system’s underlying bifurcation mechanisms.Information contained in paleoclimate proxy records is often ambiguous because of the complexity of the system, which includes both deterministic and stochastic processes. Furthermore, paleoclimate time series differ in their time spans and periodicities, and often have high levels of noise and a nonuniform resolution. These combined sources of uncertainty highlight the need for using advanced statistical methods for robustly identifying and comparing TPs.A recently developed method that uses an augmented Kolmogorov-Smirnov test has been shown to be highly effective for transition detection in different types of records. Here, we apply this method to a set of high-quality paleoproxy records exhibiting centennial-to millennial-scale variability that have been compiled in the PaleoJump database. We thereby detect previously unrecognized transitions in these records and identify potential TPs. Furthermore, we investigate regime changes with recurrence analysis and spectral analysis. This study is supported by the H2020-funded Tipping Points in the Earth System (TiPES) project.

The Earth’s climate has experienced numerous abrupt and critical transitions during its long history. Such transitions are evidenced in precise, high-resolution records at different timescales. This type of evidence suggests the possibility of identifying a hierarchy of past critical events, which would yield a more complex perspective on climatic history of the than the classical saddle-node two-dimension representation of tipping points. Such a context allows defining a tipping, or dynamical, landscape (Lucarini and Bódai, 2020), similar to the epigenetic landscape of Waddington (1957).To illustrate a richer structure of critical transitions, we have analyzed 3 key high-resolution datasets covering the past 66 Ma and provided evidences of abrupt transitions detected with the augmented Kolmogorov-Smirnov test and a recurrence analysis (Bagniewski et al., 2021). These time series are the CENOGRID benthic d18O and d13C (Westerhold et al., 2020), the U1308 benthic d18O, d13C and the d18bulk carbonate (Hodell and Channell, 2016), and the NGRIP d18O (Rasmussen et al., 2014) records. The aim was to examine objectively the observed visual evidence of abrupt transitions and to identify among them the key thresholds indicating regime changes that differentiate among major clusters of variability. This identification is followed by establishing a hierarchy in the observed thresholds organized through a domino-like cascade of abrupt transitions that shaped the Earth’s climate system over the past 66 Ma.This study is supported by the H2020-funded Tipping Points in the Earth System (TiPES) project.ReferencesBagniewski, W., Ghil, M., and Rousseau, D. D.: Automatic detection of abrupt transitions in paleoclimate records, Chaos, 31, https://doi.org/10.1063/5.0062543, 2021.Hodell, D. A. and Channell, J. E. T.: Mode transitions in Northern Hemisphere glaciation: co-evolution of millennial and orbital variability in Quaternary climate, Clim. Past, 12, 1805–1828, https://doi.org/10.5194/cp-12-1805-2016, 2016.Lucarini, V. and Bódai, T.: Global stability properties of the climate: Melancholia states, invariant measures, and phase transitions, Nonlinearity, 33, R59–R92, https://doi.org/10.1088/1361-6544/ab86cc, 2020.Rasmussen, S. O., Bigler, M., Blockley, S. P., et al.: A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy, Quat. Sci. Rev., 106, 14–28, https://doi.org/10.1016/j.quascirev.2014.09.007, 2014.Waddington, C. H.: The strategy of the genes., Allen & Unwin., London, 1957.Westerhold, T., Marwan, N., Drury, A. J., et al.: An astronomically dated record of Earth’s climate and its predictability over the last 66 million years, Science, 369, 1383-+, https://doi.org/10.1126/science.aba6853, 2020.

Abrupt climate changes constitute a relatively new field of research, which addresses variations occurring in a relatively short time interval of tens to a hundred years. Such time scales do not correspond to the tens or hundreds of thousands of years that the astronomical theory of climate addresses. The latter theory involves parameters that are external to the climate system and whose multi-periodic variations are reliably known and almost constant for a large extent of Earth history. Abrupt changes, conversely, appear to involve fast processes that are internal to the climate system; these processes varied considerably during the past 2.6 Myr, and yielded more irregular fluctuations. In this paper, we re-examine the main climate variations determined from the U1308 North Atlantic marine record, which yields a detailed calving history of the Northern Hemisphere ice sheets over the past 3.2 Myr. The magnitude and periodicity of the ice-rafted debris (IRD) events observed in the U1308 record allow one to determine the timing of several abrupt climate changes, the larger ones corresponding to the massive iceberg discharges labeled Heinrich events (HEs). In parallel, abrupt warmings, called Dansgaard-Oeschger (DO) events, have been identified in the Greenland records of the last glaciation cycle. Combining the HE and DO observations, we study a complex mechanism that may lead to the observed millennial-scale variability corresponding to the abrupt climate changes of last 0.9 Myr. This mechanism relies on amended Bond cycles, which group DO events and the associated Greenland stadials into a trend of increased cooling, with IRD events embedded into every stadial, the latest of these being an HE. These Bond cycles may have occurred during the last 0.9 Ma when Northern Hemisphere ice sheets reached their maximum extent and volume, thus becoming a major player in this time interval’s climate dynamics. Since the waxing and waning of ice sheets during the Quaternary period are orbitally paced, we conclude that the abrupt climate changes observed during the Mid and Upper Pleistocene are therewith indirectly linked to the astronomical theory of climate.

Tipping points (TPs) in the Earth system have been studied with growing interest and concern in recent years due to the potential risk of anthropogenic forcing causing abrupt, and possibly irreversible, climate transitions. Paleoclimate records are essential for identifying TPs in the Earth’s past and to properly understand the climate system’s underlying bifurcation mechanisms. Due to their varying quality, resolution, and dating methods, it is often necessary to select the records that give the best representation of past climate. Furthermore, as paleoclimate records vary in their origin, time spans, and periodicities, an objective, automated methodology is crucial for identifying and comparing TPs. To reach this goal, here we present the PaleoJump database of carefully selected, high-resolution records originating in ice, marine sediments, speleothems, loess, and lake sediments. These records, which include tipping elements, cover long time intervals and represent a global distribution from all continents and ocean basins. For every record, a transition detection methodology based on an augmented Kolmogorov-Smirnov test is applied to identify abrupt transitions. The PaleoJump database highlights these automatically detected transitions for every record together with other essential information, including location, temporal scale and resolution, as well as temporal plots; it therefore represents a valuable resource for researchers investigating TPs in past climates. This study is supported by the H2020-funded TiPES project.

Early evidence of abrupt transitions in Camp Century and Dye 3 Greenland ice cores (Dansgaard et al. 1982) has recently been reinforced by the identification of additional abrupt transitions in the NGRIP ice core (Rasmussen et al. 2014). These additional events correspond to changes of either short duration or amplitude of d18O that visual or statistical inspections do not necessarily validate. Abrupt transitions have been described for marine (Bond et al. 1992) and continental (Wang et al. 2001) records as well, and they provide a broader spatial perspective. Finally, abrupt transitions have also been documented over much deeper timescales (Zachos et al., 2001, Hodell & Channel, 2016, Westerhold et al. 2020). In spite of the variable time resolution of all these records, the abrupt transitions seem to reflect the individual impact of external forcing, of internal climate variability, or a combination of the two on Earth’s climate system. To illustrate this, we have analyzed 4 reference datasets with timescales ranging from one glacial cycle — i.e., the last 130,000 years — to the last 70 Ma. We show patterns that repeat within a single glacial cycle and seem to be related to internal variability, along with patterns associated with longer time periods and possibly related to external forcing; such forcing may arise from changes in either Earth’s orbit or its dynamics. This study is supported by the H2020-funded Tipping Points in the Earth System (TiPES) project.

Although the Earth system is described to react relatively abruptly to present anthropogenic forcings, the notion of abruptness remains questionable as it refers to a time scale that is difficult to constrain properly. Recognizing this issue, the tipping elements as listed in Lenton et al. (2008) rely on long-term observations under controlled conditions, which enabled the associated tipping points to be identified. For example, there is evidence nowadays that if the rate of deforestation from forest fires and the climate change does not decrease, the Amazonian forest will reach a tipping point towards savanna (Nobre, 2019), which would impact the regional and global climate systems as well as various other ecosystems, directly or indirectly (Magalhães et al., 2020). However, if the present tipping elements, which are now evidenced, are mostly related to the present climate change and thus directly or indirectly related to anthropogenic forcing, their interpretation must still rely on former cases detected in the past, and especially from studies of abrupt climatic transitions evidenced in paleoclimate proxy records. Moreover, recent studies of past changes have shown that addressing abrupt transitions in the past raises the issue of data quality of individual records, including the precision of the time scale and the quantification of associated uncertainties. Investigating past abrupt transitions and the mechanisms involved requires the best data quality possible. This can be a serious limitation when considering the sparse spatial coverage of high resolution paleo-records where dating is critical and corresponding errors often challenging to control. In theory, this would therefore almost limit our investigations to ice-core records of the last climate cycle, because they offer the best possible time resolution. However, evidence shows that abrupt transitions can also be identified in deeper time with lower resolution records, but still revealing changes or transitions that have impacted the dynamics of the Earth system globally. TiPES Work Package 1 will address these issues and collect paleorecords permitting to evidence the temporal behavior of tipping elements in past climates, including several examples.Lenton T. et al. (2008). PNAS 105, 1786-1793.Nobre C. (2019). Nature 574, 455.Magalhães N.d. et al. (2020). Sci. Rep. 16914 (2019) doi:10.1038/s41598-019-53284-1This work is performed under the TiPES project funded by the European Union’s Horizon 2020 research and innovation program under grant agreement # 820970

We present the first transient simulations of Marine Isotope Stage 3 (MIS 3) performed with an oxygen isotope-enabled climate model. Our simulations span several Dansgaard-Oeschger cycles and three Heinrich stadials and are directly compared with oxygen isotope records from 13 sediment and 2 ice cores. Our results are consistent with a 30–50% weakening of the Atlantic Meridional Overturning Circulation during Dansgaard-Oeschger stadials and a complete shutdown during Heinrich stadials. We find that the simulated δ 18 O anomalies differ significantly between Heinrich stadials and non-Heinrich stadials. This difference is mainly due to different responses in ocean circulation, and therefore climate, impacting oceanic δ 18 O, while the volume of 18 O-depleted meltwater plays a secondary role.

Bifurcations and tipping points (TPs) are an important part of the Earth system’s behavior. These critical points represent thresholds at which small changes in the system’s parameters or in the fo...

We perform idealized experiments of a Heinrich Stadial using an oxygen-isotope-enabled Earth System Climate Model. Our results compare relatively well with the planktic and benthic δ18O records of Heinrich Stadials 1 and 4. We find that changes in surface δ18Oc can be equally attributed to the “temperature effect” due to fractionation during calcification; the “circulation and climate effect” due to changes in circulation, precipitation, and evaporation; and the “meltwater effect” due to the addition and subsequent advection of depleted fresh water from continental ice sheets. In contrast, the meltwater effect and circulation and climate effect have only a small impact on benthic δ18Oc during the cessation of the Atlantic Meridional Overturning Circulation (AMOC). Significant temperature changes simulated in both the deep Atlantic and Pacific Oceans substantially influence benthic δ18Oc during that time. During the AMOC recovery, however, anomalously low δ18Ow surface waters are advected to the deep Atlantic, thus generating a large (∼0.5‰) δ18Oc decrease in the deep North Atlantic. This is in contrast to a hypothesis that links such depletion seen in Atlantic sediment cores during the last glacial stadials to deepwater formation caused by brine rejection and sea ice formation.

International audience; Bifurcations and tipping points (TPs) are an important part of the Earth system’s behavior. These critical points represent thresholds at which small changes in the system’s parameters or in the forcing abruptly switch it from one state or type of behavior to another. Current concern with TPs is largely due to the potential of slow anthropogenic forcing to bring about abrupt, and possibly irreversible, change to the physical climate system and impacted ecosystems. Paleoclimate proxy records have been shown to contain abrupt transitions, or “jumps,” which may represent former instances of such dramatic climate change events. These transitions can provide valuable information for identifying critical TPs in current and future climate evolution. Here, we present a robust methodology for detecting abrupt transitions in proxy records that is applied to ice core and speleothem records of the last climate cycle. This methodology is based on the nonparametric Kolmogorov–Smirnov (KS) test for the equality, or not, of the probability distributions associated with two samples drawn from a time series, before and after any potential jump. To improve the detection of abrupt transitions in proxy records, the KS test is augmented by several other criteria and it is compared with recurrence analysis. The augmented KS test results show substantial skill when compared with more subjective criteria for jump detection. This test can also usefully complement recurrence analysis and improve upon certain aspects of its results