Displays

In this talk, we will attempt to cover, as time permits, issues pertaining to a self-consistent, unified treatment of the climate system’s natural and forced variability, i.e. climate change, sensitivity and intrinsic variability. To set the stage, key features of short-, intermediate, and long-term prediction will be sketched, followed by the effects of the system’s multiple scales of motion. After summarizing the main results and uncertainties of successive assessment reports of the International Panel on Climate Change (IPCC), time-dependent forcing will be introduced, in both its natural and anthropogenic forms.

   We will outline the generalization of strange attractors to this non-autonomous setting, namely pullback and random attractors (PBAs & RAs), as well as the generalization of the bifurcations known from classical, autonomous dynamical systems to the tipping points (TPs) of non-autonomous ones. The case of the Lorenz convection model with stochastic forcing and of its RA will be used as an illustrative example. The talk will conclude with a list of questions and a selected bibliography.

How to cite: Ghil, M.: Bifurcations, Global Change, Tipping Points and All That, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18559, https://doi.org/10.5194/egusphere-egu2020-18559, 2020.

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/s41598-019-53284-1

This work is performed under the TiPES project funded by the European Union’s Horizon 2020 research and innovation program under grant agreement # 820970 <https://tipes.sites.ku.dk/>

How to cite: Rousseau, D.-D., Barbosa, S., Bagniewski, W., Boers, N., Cook, E., Fohlmeister, J., Goswami, B., Marwan, N., Rasmussen, S. O., Sime, L., and Svensson, A.: Data quality in different paleo archives and covering different time scales: a key issue in studying tipping elements., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14267, https://doi.org/10.5194/egusphere-egu2020-14267, 2020.

The Dansgaard-Oeschger (D-O) oscillation recorded in isotopic analyses of Greenland ice cores is a climate oscillation with millennial scale variability alternating very rapidly between cold climate and warm climate states. In contrast to theories invoking Heinrich event forced oscillations or stochastic noise induced transitions between on or off states of the Atlantic Meridional Overturning Circulation, theories are emerging that propose that the D-O oscillation is an intrinsic stable glacial limit cycle relaxation oscillation that can be perturbed by internal and external forcing.  Here we use the Community Earth System Model (CESM), run with glacial boundary conditions, which accurately simulates internal unforced D-O oscillations that can be modulated by radiative forcing, freshwater forcing, and changes in ocean mixing. Based on our set of CESM climate simulations, we propose a clear process-based framework that explains the natural intrinsic timescale for the millennial scale climate transitions. We build a reduced dimensional planar dynamical system model in which the parameters of the simple model are informed by the fully coupled glacial climate model. This simple system can produce self-sustained millennial scale abrupt climate transitions, which can be modulated by forcing and display a behaviour like that observed in the complex model. We conclude that the physics underlying the glacial climate system is characterized by an excitable system susceptible to coherence resonance with similar analogues in biological systems that operate on vastly different spatial and time scales.

How to cite: Vettoretti, G., Ditlevsen, P., Jochum, M., Rasmussen, S., and Nisancioglu, K.: A Noisy Excitable Model of Millennial Scale Glacial Climate Variability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9618, https://doi.org/10.5194/egusphere-egu2020-9618, 2020.

The Dansgaard-Oeschger (DO) events of the last glacial period provide a unique example of large-scale climate change on centennial time scales. Despite significant progress in modeling DO-like transitions with realistic climate models, it is still unknown what ultimately drives these changes. It is an outstanding problem whether they are driven by a self-sustained oscillation of the earth system, or by stochastic perturbations in terms of freshwater discharges into the North Atlantic or extremes in atmospheric dynamics.

This work addresses the question of whether DO events fall into the realm of tipping points in the mathematical sense, either driven by an underlying bifurcation, noise or a rate-dependent instability, or whether they are a true and possibly chaotic oscillation. To do this, different ice core proxy data and empirical predictability can be used as a discriminator.

The complex temporal pattern of DO events has been investigated previously to suggest that the transitions in between cold (stadial) and warm (interstadial) phases are purely noise-induced and thus unpredictable. In contrast, evidence is presented that trends in proxy records of Greenland ice cores within the stadial and interstadial phases pre-determine the impending abrupt transitions and allow their prediction. As a result, they cannot be purely noise-induced.

The observed proxy trends manifest consistent reorganizations of the climate system at specific time scales, and can give some hints on the physical processes being involved. Nevertheless, the complex temporal pattern, i.e., what sets the highly variable and largely uncorrelated time scales of individual DO excursions remains to be explained.

How to cite: Lohmann, J. and Ditlevsen, P.: Predicting past tipping points: The Dansgaard-Oeschger events of the last glacial period, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16665, https://doi.org/10.5194/egusphere-egu2020-16665, 2020.

The Last Interglacial (LIG) is a period of great importance as an analog for future climate change. Global sea level was 6-9 m higher than present. Stronger LIG summertime insolation at high northern latitudes drove Arctic land summer temperatures around 4-5 K higher than during the preindustrial era. Climate-model simulations have previously failed to capture these elevated temperatures. This may be because these models failed to correctly capture LIG sea ice changes.

Here, we show that the latest version of the UK Hadley Center coupled ocean-atmosphere climate model (HadGEM3) simulates a much improved Arctic LIG climate, including the observed high temperatures. Improved model physics in HadGEM3, including a sophisticated sea ice melt-pond scheme, results in the first-ever simulation of the complete loss of Arctic sea ice in summer during the LIG.

Our ice-free Arctic yields a compelling solution to the long-standing puzzle of what drove LIG Arctic warmth. The LIG simulation result is a new independent constraint on the strength of Arctic sea ice decline in climate-model projections, and provides support for a fast retreat of Arctic summer sea ice in the future.

How to cite: Guarino, M. V., Sime, L., Schroeder, D., Malmierca-Vallet, I., Rosenblum, E., Ringer, M., Ridley, J., Feltham, D., Bitz, C., Steig, E., Wolff, E., Stroeve, J., and Sellar, A.: A sea ice-free Arctic during the Last Interglacial supports fast future loss, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1399, https://doi.org/10.5194/egusphere-egu2020-1399, 2020.

The Holocene ’greening’ and subsequent, possibly abrupt desertification of the Sahara is a fascinating example of natural environmental change. It was driven by a gradual decline in summer insolation but with land-atmosphere coupling by vegetation likely providing additional reinforcing feedbacks. However, the majority of general circulation models (GCMs) cannot produce enough precipitation to sustain a ’Green’ Sahara, and the transient evolution through the Holocene has therefore only been studied with a few models. We present a suite of transient simulations with the coupled atmosphere-ocean GCM HadCM3, the CMIP3 version of the Met Office’s Hadley Centre model. These simulations cover the Holocene from 10,000 years before present, and optionally include recently developed optimisations of the atmospheric convection and dynamic vegetation parameterisations. In the model run with both optimisations, HadCM3 shows a convincing ‘greening’ for the first time. This is followed by a series of abrupt oscillations in vegetation cover and hydrology, that culminates in an abrupt collapse at around 6000 years before present. We compare the behaviour in four model versions and make a detailed evaluation with available geological evidence. Our results show that the stability of climate models is determined by chosen parameter values and formulations. We conclude that novel methods of inferring suitable model state-space regions using both present day observations and palaeoclimate reconstructions are needed.

How to cite: Hopcroft, P., Valdes, P., Ingram, W., and Ivanovic, R.: Simulating an abrupt termination of the Holocene African Humid period using an optimised configuration of HadCM3, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19531, https://doi.org/10.5194/egusphere-egu2020-19531, 2020.

A superrotating atmosphere, one in which the angular momentum of the atmosphere exceeds the solid body rotation of the planet, occurs on Venus and Titan. However, it may have occurred on the Earth in the hot house climates of the Early Cenozoic and some climate models have transitioned abruptly to a superrotating state under the more extreme global warming scenarios. Applied to the Earth, the transition to superrotation causes the prevailing easterlies at the equator to become westerlies and accompanying large changes in global circulation patterns. Although current thinking is that this scenario is unlikely, it shares features of other global tipping points in that it is a low probability, high risk event.

More than anything though, this tipping point serves as an ideal example to test some spatial early warning methods. I’ll show some preliminary results how the critical spatial modes and time scales change through the transition to superrotation using an idealized general circulation model (GCM), Isca.

How to cite: williamson, M.: Spatial early warnings of the transition to superrotation: Studying a bifurcation in the general circulation using an idealized GCM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9484, https://doi.org/10.5194/egusphere-egu2020-9484, 2020.

Over the past 15 years climate tipping points have emerged as both an important research topic and source of public concern. Some articles have suggested that some tipping points could begin within the 1.5-2^oC Paris climate target range, with many more potentially starting by the ~3-4^oC of warming that current policy is projected to be committed to. Recent work has also proposed that these tipping points could interact and potentially ‘cascade’ – with the impacts of passing one tipping point being sufficient to trigger the next and so on – resulting in an emergent global tipping point for a long-term commitment to a ‘Hothouse Earth’ trajectory of 4+^oC (Steffen et al., 2018). However, much of the recent discussion relies largely on a decade-old characterisation of climate tipping points, based on a literature review and expert elicitation exercise. An updated characterisation would fully utilise more recent results from coupled and offline models, model inter-comparisons, and palaeoclimate studies. The ‘tipping cascade’ hypothesis has also not yet been tested, with the suggestion of 2^oC as the global tipping point remaining speculative. Furthermore, the definition of what counts as a climate tipping point is often inconsistent, with some purported tipping points represented more accurately as threshold-free positive feedbacks. Here we perform an updated systematic review of climate tipping points, cataloguing the current evidence for each suggested element with reference to rigorously-applied tipping point definitions. Based on this we test the potential for a global tipping cascade using a stylised model, from which we will present preliminary results.

References

Steffen, W., et al.: Trajectories of the Earth System in the Anthropocene, Proc. Natl. Acad. Sci., 115(33), 8252–8259, doi:10.1073/pnas.1810141115, 2018.

How to cite: Armstrong McKay, D., Staal, A., Cornell, S., Lenton, T., and Fetzer, I.: Climate Tipping Points: Can they trigger a Global Cascade?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17889, https://doi.org/10.5194/egusphere-egu2020-17889, 2020.

Tipping elements in the Earth's climate system are continental-scale subsystems that are characterized by a threshold behavior with potentially large short- to long-term impacts on human societies. It has been suggested that these include biosphere components (e.g. the Amazon rainforest and coral reefs), cryosphere components (e.g. the Greenland and Antarctic ice sheets) and large-scale atmospheric and oceanic circulations (e.g. the AMOC, ENSO and Indian summer monsoon). Interactions and feedbacks of climate tipping elements via various processes could increase the likelihood of crossing tipping points under a given level of global warming and interaction strength. However, studying these potential domino effects and tipping cascades with process-detailed state-of-the-art Earth system models is difficult so far, because relevant tipping elements are often not represented and uncertainties in their properties and interactions are large.

To bridge this current gap in the model hierarchy, we present a risk analysis approach based on a paradigmatic model of interacting tipping elements that propagates uncertainties in interaction structure, sign and strength as well as critical thresholds and other parameters via large Monte Carlo ensembles. Our approach allows to study the likelihood of domino effects and tipping cascades to emerge due to pairwise interactions and feedbacks to global mean temperature. We apply our approach to a subset of five potential tipping elements (Greenland and West Antarctic ice sheets, AMOC, Amazon rainforest and ENSO) with known parameter uncertainty estimates and find that their interactions overall tend to be destabilizing. The presented framework is flexible and can be adapted to study the interaction effects of other or additional tipping elements and more detailed submodels for describing their individual dynamics.

How to cite: Donges, J., Wunderling, N., Kurths, J., and Winkelmann, R.: Risk analysis approach for tipping cascades and domino effects in the Earth system under global warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21507, https://doi.org/10.5194/egusphere-egu2020-21507, 2020.

Climate models show a weakening Atlantic meridional overturning circulation (AMOC) under global warming. Limited by short direct measurements, this AMOC slowdown has been inferred, with some uncertainties, indirectly from some AMOC fingerprints locally over the subpolar North Atlantic region. Here we present observational and modeling evidences of the first remote fingerprint of AMOC slowdown outside the North Atlantic. Under global warming, the weakening AMOC reduces the salinity divergence and then leads to a remote fingerprint of “salinity pileup” in the South Atlantic. Our study supports the AMOC slowdown under anthropogenic warming and, furthermore, shows that this weakening has occurred all the way into the South Atlantic.

How to cite: Zhu, C. and Liu, Z.: Atlantic Salinity Pileup as a Remote Fingerprint of Weakening Atlantic Overturning Circulation under Anthropogenic Warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3790, https://doi.org/10.5194/egusphere-egu2020-3790, 2020.

Extensive degradation of near-surface permafrost is projected during the 21st century, which will have detrimental effects on northern communities, ecosystems and engineering systems. This degradation will expectedly have consequences for many processes, which most previous modelling studies suggested would occur gradually. Here, we project that soil moisture will decrease abruptly (within a few months) in response to permafrost degradation over large areas of the present-day permafrost region, based on analysis of transient climate change simulations performed using a state-of-the-art regional climate model. This regime shift is reflected in abrupt increases in summer near-surface temperature and convective precipitation, and decreases in relative humidity and surface runoff. Of particular relevance to northern systems are changes to the bearing capacity of the soil due to increased drainage, increases in the potential for intense rainfall events and increases in lightning frequency, which combined with increases in forest fuel combustibility are projected to abruptly and substantially increase the severity of wildfires, which constitute one of the greatest risks to northern ecosystems, communities and infrastructure. The fact that these changes are projected to occur abruptly further increases the challenges associated with climate change adaptation and potential retrofitting measures.

How to cite: Teufel, B. and Sushama, L.: Abrupt changes across the Arctic permafrost region endanger northern development, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5847, https://doi.org/10.5194/egusphere-egu2020-5847, 2020.

Nonlinear feedbacks, such as the melt-elevation feedback, may produce a critical temperature threshold beyond which the current state of the Greenland Ice Sheet loses stability. Hence, the ice sheet may exhibit an abrupt transition under ongoing global warming, with substantial impacts on global sea level and the Atlantic Meridional Overturning Circulation. Melting rates across Greenland and solid ice discharge at the ice sheet's margins have recently accelerated. In this work, we analyze ice sheet runoff reconstructions and process-based simulations using new methods. We compare the acceleration in the runoff with the statistical properties of fluctuations around the system's equilibrium. The analysis uncovers significant early-warning signals for an ongoing destabilization and substantial further mass loss in the near future. 

How to cite: Rypdal, M. and Boers, N.: Observed early-warning signals for a Greenland-ice-sheet tipping point , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7369, https://doi.org/10.5194/egusphere-egu2020-7369, 2020.

Antarctica is loosing mass in an accelerating way and these losses are considered as the major source of sea-level rise in the coming centuries. Ice-sheet mass loss is mainly triggered by the decreased buttressing from ice shelves mainly due to ice-ocean interaction. This loss could be self-sustained in potentially unstable regions where the grounded ice lies on a bedrock below sea level sloping down towards the interior of the ice sheet, leading to the so-called marine ice sheet instability (MISI).
Recent observations on accelerated grounding-line retreat and insights in modelling the West Antarctica ice sheet give evidence that MISI is already on its way. Moreover, similar topographic configurations are also observed in East Antarctica, particularly in Wilkes Land. We present an ensemble of simulations of the Antarctic ice sheet using the f.ETISh ice-sheet model to evaluate tipping points that trigger MISI by forcing the model with sub-shelf melt pulses of varying amplitude and duration. As uncertainties in ice-sheet models limit the ability to provide precise sea-level rise projections, we implement probabilistic methods to investigate the influence of several sources of uncertainty, such as basal conditions. From the uncertainty analysis, we identify confidence regions for grounded ice interpreted as regions of the Antarctic ice sheet that remain ice-covered for a given level of probability. Finally, we discuss for each Antarctic basin the total melt energy needed to reach tipping points leading to sustained MISI. 

How to cite: Sun, S., Pattyn, F., Durand, G., Zipf, L., Bulthuis, K., Goelzer, H., and Haubner, K.: Potential Tipping Points of Antarctic Ice Sheet Basins, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7454, https://doi.org/10.5194/egusphere-egu2020-7454, 2020.

Global change is not only about climate change. Several changes in the Earth System occur concurrently and sequentially, and still, novel factors are being identified as emerging problems such as microplastic pollutants. Global change is diverse; nonetheless, little is known about the role of multiple global change co-occurrences. Can we safely anticipate that the effects of multiple global change factors are independent of each other? Or, should we be concerned about the potential of their synergistic interaction, where the joint effect of multiple factors can be larger than the addition of their single effects?

Our talk focuses on ‘the diversity of global change factors’—How the diversity of global change factors can increase, and how the diversity of global change can affect environmental systems in the context of tipping points. We also show empirical evidence that an increasing number of global change factors can cause abrupt shifts in a soil system (cf. Rillig et al. 2019 in Science). We emphasize the urgent need to investigate the expected roles of an increasing diversity of global change factors as an emerging threat to nature and society.

How to cite: Ryo, M. and Rillig, M.: Diversity of Global Change Factors and Tipping Points, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3797, https://doi.org/10.5194/egusphere-egu2020-3797, 2020.

For an autonomous dynamical system, an invariant measure is called physical or natural if it describes the statistics of a typically chosen trajectory that started an arbitrarily long time ago in the past, i.e. without transients. In order to apply such a concept to systems where there is time-varying forcing, we need to develop an analogous notion for such nonautonomous dynamical systems, where the measure is not fixed but evolves in time under the action of the nonautonomous system. The importance of such measures, and the pullback attractors on which they are supported, for interpreting climate statistics have been highlighted by Chekroun, Simmonet and Ghil (2011) Physica D 240:1685. We seek to gain a deeper understanding of these measures and implications for tipping points. We present some results for two classes of nonautonomous systems: autonomous random dynamical systems driven by stationary memoryless noise, and deterministic nonautonomous systems that are asymptotically autonomous in the negative-time limit. In both cases we show existence of a physical measure under suitable assumptions. We  highlight further questions about defining rates of mixing in such a setting, as well as implications for prediction of tipping points.

This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 820970 (TiPES).

How to cite: Ashwin, P. and Newman, J.: Physical measures and tipping points in a changing climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5979, https://doi.org/10.5194/egusphere-egu2020-5979, 2020.

A classical scenario for tipping is that a dynamical system experiences a slow parameter drift across a fold tipping point, caused by a run-away positive
feedback loop. We study what happens if one turns around after one has crossed the threshold. We derive a simple criterion that relates how far the parameter exceeds the tipping threshold maximally and how long the parameter stays above the threshold to avoid tipping in an inverse-square law to observable properties of the dynamical system near the fold. We demonstrate the inverse-square law relationship using simple models of recognised potential future tipping points in the climate system. 

How to cite: Ritchie, P., Cox, P., and Sieber, J.: How fast to turn around: preventing tipping after a system has crossed a climate tipping threshold, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5555, https://doi.org/10.5194/egusphere-egu2020-5555, 2020.

There is an urgent need to better understand the how climatic change may cause abrupt transitions and tipping points in the underlying dynamic process (κ →κ′) that can result in more severe extremes. The variability in precipitation based flooding and arid events in the Sahel and SE Asia may be related to alterations to Atlantic (AMOC) and Pacific (ENSO) modes, and how they teleconnect. Extreme value process distributions are widely used for assessing the  environment. In this work we apply a spatial Dominant Frequency State Analysis (DFSA) to GPCC reanalysis data to evaluate the extreme properties of precipitation extremes and dry arid events in these regions. The spatial variation we find implies that how the wave interaction properties vary and that wave guide teleconnection is important. The physical wave interaction reasons for why extremes occur and how they vary has not been fully explained to date: that is a statistical mechanics problem. For earth system climate analysis General Circulation Model simulation sizes are too small, 10 to 30 ensemble members (due to computational complexity), to carry out such a large ensemble analysis. However large ensembles are intrinsic to the study of Anderson localization and Random Matrix Theory (RMT) transport study. So we use a theoretical based approach to provide a wave interaction explanation of how differing forms of extreme can occur. This theory work is a generic advance in the study of wave propagation phenomena and extremes in the presence of disorder.  To do this we merge the universal wave transport approach used in solids with the geometrical extreme type max stable universal law to evaluate the ensemble based on wave interaction principles. This provides a generic ensemble random Hamiltonian and characteristic polynomial to give a physical proof for encountering extreme value processes. This shows that the Generalized Extreme Value (GEV) shape parameter ξ is a diagnostic tool that accurately distinguishes localized from unlocalized systems and this property should hold for all wave based transport phenomena. This work establishes that ξ(κ) can change when the dynamical system fundamentally changes its physical structure κ →κ′ and that this is a universal result. For our earth system a disorder induced state transition to a heavy tailed process could indicate a wave localization state has occurred in some locations. If this was the case the associated climate phenomena would become dominated by destructive wave interference that can manifest as a catastrophic breakdown, for example as an extreme runaway of temperatures. We discuss this wave interaction theory result in the context of the precipitation extremes and how these could be altering for the Sahel and SE Asia.

Bruun and Evangelou (2019) Anderson localization and extreme values in chaotic climate dynamics, arXiv:1911.03998.

Bruun, Sheen, Skákala, Evangelou and Collins (2019), Modulation of arid Sahel conditions by earth system modes, Geophysical Research Abstracts.

Bruun, Allen and Smyth (2017) Heartbeat of the Southern Oscillation explains ENSO climatic resonances, JGR Oceans, 122, 6746–6772, doi:10.1002/2017JC012892.

How to cite: Bruun, J., Evangelou, S., Sheen, K., and Collins, M.: Abrupt transitions, wave interactions and precipitation extremes in climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11690, https://doi.org/10.5194/egusphere-egu2020-11690, 2020.

For a wide range of values of the incoming solar radiation, the Earth features at least two attracting states, which correspond to competing climates. The warm climate is analogous to the present one; the snowball climate features global glaciation and conditions that can hardly support life forms. Paleoclimatic evidences suggest that in past our planet flipped between these two states. The main physical mechanism responsible for such instability is the ice-albedo feedback. Following an idea developed by Eckhardt and co. for the investigation of multistable turbulent flows, we study the global instability giving rise to the snowball/warm multistability in the climate system by identifying the climatic Melancholia state, a saddle embedded in the boundary between the two basins of attraction of the stable climates. We then introduce random perturbations as modulations to the intensity of the incoming solar radiation. We observe noise-induced transitions between the competing basins of attractions. In the weak noise limit, large deviation laws define the invariant measure and the statistics of escape times. By empirically constructing the instantons, we show that the Melancholia states are the gateways for the noise-induced transitions in the weak-noise limit. In the region of multistability, in the zero-noise limit, the measure is supported only on one of the competing attractors. For low (high) values of the solar irradiance, the limit measure is the snowball (warm) climate. The changeover between the two regimes corresponds to a first order phase transition in the system. The framework we propose seems of general relevance for the study of complex multistable systems. Finally, we propose a new method for constructing Melancholia states from direct numerical simulations, thus bypassing the need to use the edge-tracking algorithm.

Refs.

V. Lucarini, T. Bodai, Edge States in the Climate System: Exploring Global Instabilities and Critical Transitions, Nonlinearity 30, R32 (2017)

V. Lucarini, T. Bodai, Transitions across Melancholia States in a Climate Model: Reconciling the Deterministic and Stochastic Points of View, Phys. Rev. Lett. 122,158701 (2019)

How to cite: Lucarini, V.: Global Stability Properties of the Climate: Melancholia States, Invariant Measures, and Phase Transitions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9491, https://doi.org/10.5194/egusphere-egu2020-9491, 2020.

The El Niño Southern Oscillation (ENSO) is one of the most prominent interannual climate phenomena. An early and reliable ENSO forecasting remains a crucial goal, due to its serious implications for economy, society, and ecosystem. Despite the development of various dynamical and statistical prediction models in the recent decades, the “spring predictability barrier” (SPB) remains a great challenge for long (over 6-month) lead-time forecasting. To overcome this barrier, here we develop an analysis tool, the System Sample Entropy (SysSampEn), to measure the complexity (disorder) of the system composed of temperature anomaly time series in the Niño 3.4 region. When applying this tool to several near surface air temperature and sea surface temperature datasets, we find that in all datasets a strong positive correlation exists between the magnitude of El Niño and the previous calendar year’s SysSampEn (complexity). We show that this correlation allows to forecast the magnitude of an El Niño with a prediction horizon of 1 year and high accuracy (i.e., Root Mean Square Error = 0.23°C for the average of the individual datasets forecasts). For the 2018 El Niño event, our method forecasts a weak El Niño with a magnitude of 1.11±0.23°C.  Our framework presented here not only facilitates a long–term forecasting of the El Niño magnitude but can potentially also be used as a measure for the complexity of other natural or engineering complex systems.

How to cite: Meng, J., Fan, J., Ludescher, J., Agarwala, A., Chen, X., Bunde, A., Kurths, J., and Schellnhuber, H. J.: Complexity-based approach for El Niño magnitude forecasting before the spring predictability barrier, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2378, https://doi.org/10.5194/egusphere-egu2020-2378, 2020.

Global warming generates the possibility of 'abrupt' or 'irreversible' changes, associated with tipping points. Uncertainties are however sometimes invoked as an argument against political action. The Tipping Points in the Earth System (TIPES) project includes a workpackage whose goal is to rationalise the effects of uncertainty on what should be regarded as an 'optimal policy', given the possibility of tipping points.

To this end, we rely on two disciplinary fields. On the one hand, climate models integrate the dynamical principles, which determine the existence of 'tipping points'. On the other hand, formal decision theory defines the concept of optimal policies and allows us to compute them.

The current contribution outlines the implications and hypotheses needed for combining both frameworks. To exemplify this, we use a simple ice sheet model coupled to both carbon and aerosol models. The coupled system provides us with the formal basis to define the notions of control, irreversibility, and commitment. From this basis, we sketch out the mathematical problem of finding an optimal policy, with emphasis on what needs to be defined to pose the problem properly.

How to cite: Martinez Montero, M., Botta, N., Brede, N., and Crucifix, M.: Optimal policies with tipping points and uncertainties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5470, https://doi.org/10.5194/egusphere-egu2020-5470, 2020.

Millennial scale climate variations called Dansgaard-Oeschger cycles occurred frequently during the last glacial, with their central impact on climate in the North Atlantic region. These events are, for example, well captured by the stable oxygen isotope composition in continental ice from Greenland, but also in records from other regions. Recently, it has been shown that a water isotope enabled general circulation model is able to reproduce those millennial-scale oxygen isotope changes from Greenland (Sime et al., 2019). On a global scale, this isotope-enabled model has not been tested in its performance, as stable oxygen isotope records covering those millennial scale variability were so far missing or not systematically compiled.

In the continental realm, speleothems provide an excellent archive to store the oxygen isotope composition in precipitation during those rapid events. Here, we use a newly established speleothem data base (SISAL, Atsawawaranunt et al., 2018) from which we extracted 126 speleothems, growing in some interval during the last glacial period. We established an automated method for identification of the rapid onsets of interstadials. While the applied method seems to be not sensitive enough to capture all warming events due to the diverse characteristics of speleothem data (temporal resolution, growth stops and dating uncertainties) and low signal-to-noise-ratio, we are confident that our method is not detecting variations in stable oxygen isotopes that do not reflect stadial-interstadial transitions. Finally, all found transitions were stacked for individual speleothem records in order to provide a mean stadial-interstadial transition for various continental locations. This data set could be useful for future comparison of isotope enabled model simulations and corresponding observations, and to test their ability in modelling millennial scale variability.

References

Atsawawaranunt, et al. (2018). The SISAL database: A global resource to document oxygen and carbon isotope records from speleothems. Earth System Science Data 10, 1687–1713

Sime, L. C., Hopcroft, P. O., Rhodes, R. H. (2019). Impact of abrupt sea ice loss on Greenland water isotopes during the last glacial period. PNAS 116, 4099-4104.

How to cite: Fohlmeister, J., Bores, N., Marwan, N., Columbu, A., Rehfeld, K., Sekhon, N., Sime, L., and Veiga-Pires, C.: Composite data set of last glacial Dansgaard/Oeschger events obtained from stable oxygen isotopes in speleothems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6894, https://doi.org/10.5194/egusphere-egu2020-6894, 2020.

Warmer atmospheric and oceanic temperatures have led to a six-fold increase in mass loss from Antarctica in the last four decades. It is difficult to predict how the ice sheet will respond to future warming because it is subject to positive feedback mechanisms, which could lead to destabilisation. Observational and modelling work has shown that ice streams in West Antarctica may be undergoing unstable and possibly irreversible retreat due to increased basal melting beneath their ice shelves. Being able to identify and predict stability thresholds in ice streams draining the Antarctic Ice Sheet could help establish early warning indicators of near-future abrupt changes in sea level. 
 
Here, we use the shallow-ice flow model Úa to investigate the stability of an idealised ice stream from the third Marine Ice Sheet Model Intercomparison Project (MISMIP+). Initial results show that a gradual variation in ice viscosity, which corresponds to a change in temperature, causes the ice stream to undergo hysteresis across an overdeepened bed. This hysteresis means there are two tipping points, one for an advance phase and one for a retreat phase, both of which lie off the retrograde sloping bedrock. Beyond these tipping points, changes in ice stream grounding line position are unstable and irreversible. This behaviour is also apparent in wider ice streams although there is a change to the onset of instability and the location of tipping points. Further studies will investigate the additional effects of basal melting on these tipping points.

How to cite: Reed, B., Green, M., Gudmundsson, H., and Jenkins, A.: Identifying Antarctic Ice Sheet Tipping Points, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7874, https://doi.org/10.5194/egusphere-egu2020-7874, 2020.

In the Earth's atmosphere, fast propagating equatorial waves generate slow reversals of the large scale stratospheric winds with a  period of about 28 months. This quasi-biennial oscillation is a spectacular manifestation of wave-mean flow interactions in stratified fluids, with analogues in other planetary atmospheres and laboratory experiments. Recent observations of a disruption of this periodic behavior have been attributed to external perturbations, but the mechanism explaining the disrupted response has remained elusive. We show the existence of secondary bifurcations and a quasiperiodic route to chaos in simplified models of the equatorial atmosphere ranging from the classical Holton-Lindzen-Plumb model to fully nonlinear simulations of stratified fluids. Perturbations of the slow oscillations are widely amplified in the proximity of the secondary bifurcation point. This suggests that intrinsic dynamics may be equally influential as external variability in explaining disruptions of regular wind reversals  [1].

[1] Renaud, A., Nadeau, L. P., & Venaille, A. (2019). Periodicity Disruption of a Model Quasibiennial Oscillation of Equatorial Winds. Physical Review Letters, 122(21), 214504.
 

How to cite: Renaud, A., Nadeau, L.-P., and Venaille, A.: Periodicity disruption of a model quasi-biennial oscillation of equatorial winds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9362, https://doi.org/10.5194/egusphere-egu2020-9362, 2020.

Ice sheets, in particular the Antarctic Ice Sheet (AIS), are considered as potential tipping elements (TEs) of the Earth system. The mechanism underlying tipping is the existence of positive feedbacks leading to self-amplification processes that, once triggered, dominate the dynamics of the system. Positive feedbacks can also lead to hysteresis, with implications for reversibility in the context of long-term future climate change. The main mechanism underlying ice-sheet hysteresis is the positive surface mass balance-elevation feedback.  Marine-based ice sheets, such as the western sector of the AIS, are furthermore subject to specific instability mechanisms that can potentially also lead to hysteresis. Simulations with ice-sheet models have robustly confirmed the presence of different degrees of hysteresis in the evolution of the AIS volume with respect to model parameters and/or climate forcing, suggesting that ice-sheet changes are potentially irreversible on long timescales. Nevertheless, AIS hysteresis is only now becoming a focus of more intensive modeling efforts, including active oceanic forcing in particular. Here, we investigate the hysteresis of the AIS in a three-dimensional hybrid ice-sheet--ice-shelf model with respect to individual atmospheric forcing, ocean forcing and both. The aim is to obtain a probabilistic assessment of the AIS hysteresis and of its critical temperature thresholds by investigating the effect of structural uncertainty, including the representation of ice-sheet dynamics, basal melting and internal feedbacks.

How to cite: Montoya, M., Alvarez-Solas, J., Robinson, A., Blasco, J., Tabone, I., and Moreno, D.: Antarctic ice-sheet hysteresis in a three-dimensional hybrid ice-sheet model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9765, https://doi.org/10.5194/egusphere-egu2020-9765, 2020.

Under global warming, soil temperatures are expected to rise. This increases the specific rate of microbial respiration in the soils which in turn warms the soil, creating a positive feedback process. This leads to the possibility of an instability, known as the compost bomb, in which rapidly warming soils release their soil carbon as CO2 to the atmosphere, accelerating global warming. Models of the compost bomb have exhibited interesting dynamical phenomena: excitability, rate induced tipping and bifurcation induced tipping. We examine models with increasing degrees of sophistication, to help understand the conditions that give rise to the compost bomb. We clarify the role an insulating moss layer plays and demonstrate that it has a 'most dangerous' thickness. We also use JULES, a land surface model, to examine where a compost bomb might occur and what affect other processes such as hydrology might have on the compost bomb.

How to cite: Clarke, J., Ritchie, P., and Cox, P.: Conditions for the Compost Bomb Instability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9929, https://doi.org/10.5194/egusphere-egu2020-9929, 2020.

Reconstruction of ancient climate variability relies on inference from paleoclimate proxy data. However, such data often suffers from large uncertainties in particular concerning the age assigned to measured proxy values, which makes the derivation of clear conclusions challenging. Especially in the study of abrupt climatic shifts, dating uncertainties in the proxy archives merit increased attention, since they frequently happen to be of the same order of magnitude as the dynamics of interest. Yet, analyses of paleoclimate proxy reconstructions tend to focus on mean values and thereby conceal the full range of uncertainty. In addition, the statistical significance of the reported results is sometimes not or at least not accurately tested. Here we discuss both, methods for rigorous propagation of uncertainties and for hypothesis testing with applications to the Dansgaard-Oeschger (DO) events of the last glacial interval and their varying timings in different proxy variables and archives. We scrutinized the mathematical analysis of different paleoclimate records evidencing the DO events and provide results that take into account the full range of uncertainties. We discuss several possibilities of testing the significance of apparent leads and lags between transitions found in proxy data evidencing DO events within and across different ice core archives from Greenland and Antarctica.

How to cite: Riechers, K., Boers, N., Fohlmeister, J., and Marwan, N.: Hypothesis testing and uncertainty propagation in paleo climate proxy data evidencing abrupt climate shifts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15148, https://doi.org/10.5194/egusphere-egu2020-15148, 2020.

Dansgaard-Oeschger (DO) oscillations are most pronounced millennial-scale abrupt climate changes in glacial periods. Abe-Ouchi et al. have simulated DO oscillations with MIROC4m, a fully coupled atmosphere-ocean general circulation model (AOGCM). In that modelling study, it is elucidated that the bipolar seesaw and the Southern Ocean dynamics may play an important role for the occurrence of DO oscillations. In this poster, we present a simple conceptual model for DO oscillations based on the mechanism proposed by Abe-Ouchi et al. In this simple model, relaxation oscillations arise via Hopf bifurcations in a particular region of its parameter space, which is qualitatively consistent with the MIROC4m AOGCM experiments. In general, the period of oscillations does not grow drastically near Hopf bifurcation point in deterministic dynamical systems (Strogatz, "Nonlinear Dynamics and Chaos", 2014; Peltier and Vettoretti, GRL 2014). However, the oscillation periods (return times) increase near the bifurcation points in MIROC4m AOGCM. This gives a U-shape dependence of return times on the parameters in the AOGCM. We show that, in the simple model, such a U-shape dependence is achieved by an addition of noise into the system (which may represent fast "weather" forcings to slow climate) (cf. Mitsui and Crucifix, Clim. Dyn. 2017). We will also mention tipping point behavior found in this simple model.

How to cite: Mitsui, T., Abe-Ouchi, A., Chan, W.-L., and Sherriff-Tadano, S.: A simple conceptual model for Dansgaard-Oeschger oscillations derived from MIROC4m AOGCM experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22226, https://doi.org/10.5194/egusphere-egu2020-22226, 2020.

The El Niño-Southern Oscillation (ENSO) phenomenon is regarded as a policy-relevant tipping element of the Earth's climate system. It has a prominent planetary-scale influence on climatic variability and it is susceptible to anthropogenic forcing, which could alter irreversibly its dynamics. Changes in frequency and/or amplitude of ENSO would have major implications for terrestrial hydrology and ecosystems. The amount of extreme events such as droughts and floods could vary regionally, as well as their intensities. Here, we use an intermediate complexity climate model, namely the Planet Simulator (PlaSim), to study the potential impact on Earth's climate and its terrestrial ecosystems of changing ENSO dynamics in a couple of experiments. Initially we investigate the global effects of a permanent El Niño, and then we analyse changes in the amplitude of the fluctuation. We found that PlaSim model yields a sensible representation of current large-scale climatological patterns, including ENSO-related variability, as well as realistic estimates of global energy and water budgets. For the permanent El Niño state, there were significant differences in the global distribution of water and energy fluxes that led to asymmetrical effects on vegetation production, which increased in the tropics and decreased in temperate regions. In terrestrial ecosystems of regions such as western North America, the Amazon rainforest, south-eastern Africa and Australia, we found that these El Niño-induced changes could be associated with biome state transitions. Particularly for Australia, we found country-wide aridification as a result of sustained El Niño conditions, which is a potential state in which recent wildfires would be even more dramatic. When the amplitude of the ENSO fluctuation changes, we found that although mean climatological values do not change significantly, extreme values of variables such as temperature and precipitation become more extreme. Our approach aims at recognizing potential threats for terrestrial ecosystems in climate change scenarios in which there are more frequent El Niño phenomena or the intensities of the ENSO phases change. Although it is not enough to prove such effects will be observed, we show a consistent picture and it should raise awareness about conservation of global ecosystems.

How to cite: Duque-Villegas, M., Salazar, J. F., and Rendón, A. M.: Potential state shifts in terrestrial ecosystems related to changes in El Niño-Southern Oscillation dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12346, https://doi.org/10.5194/egusphere-egu2020-12346, 2020.

The equilibrium climate sensitivity (ECS) of a state-of-the-art Earth System Model of intermediate complexity, the Planet Simulator (PlaSim), is determined under three tuned configurations, in which the model is coupled with a simple Mixed Layer (ML) or with the full 3D Large Scale Geostrophic (LSG) ocean model, at two horizontal resolutions, T21 (600 km) and T42 (300 km). Sensitivity experiments with doubled and quadrupled CO₂ were run, using either dynamic or prescribed sea ice. The resulting ECS using dynamic sea ice is 6.3 K for PlaSim-ML T21, 5.4 K for PlaSim-ML T42 and a much smaller 4.2 K for PlaSim-LSG T21. A systematic comparison between simulations with dynamic and prescribed sea ice helps to identify a strong contribution of sea ice to the value of the feedback parameter and of the climate sensitivity. Additionally, Antarctic sea ice is underestimated in PlaSim-LSG leading to a further reduction of ECS when the LSG ocean is used. The ECS of ML experiments is generally large compared with current estimates of equilibrium climate sensitivity in CMIP5 models and other EMICs: a relevant observation is that the choice of the ML horizontal diffusion coefficient, and therefore of the parameterized meridional heat transport and in turn the resulting equator-poles temperature gradient, plays an important role in controlling the ECS of the PlaSim-ML configurations. This observation should be possibly taken into account when evaluating ECS estimates in models with a mixed layer ocean. The configuration of PlaSim with the LSG ocean shows very different AMOC regimes, including 250-year oscillations and a complete shutdown of meridional transport, which depend on the ocean vertical diffusion profile and the CO₂ forcing conditions. These features can be explored in the framework of tipping points: the simplified and parameterized form of the climate system components included in PlaSim makes this model a suitable tool to study the transitions occurring in the Earth system in presence of critical points.

How to cite: Angeloni, M., Palazzi, E., and von Hardenberg, J.: Mechanisms affecting equilibrium climate sensitivity in the PlaSim Earth System Model with different ocean model configurations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16536, https://doi.org/10.5194/egusphere-egu2020-16536, 2020.

Climate models consistently project a weakening of the Atlantic Meridional Overturning Circulation (AMOC) in response to increasing greenhouse gases (GHGs) over the 21st Century. Models also show the potential for multiple equilibria and tipping points of the AMOC, in response to fresh water forcing. However longer term model integrations at increased levels of GHGs suggest that AMOC weakening is transient, with the AMOC recovering to its initial strength after GHGs are stabilised. Hence the ‘traditional’ forcing scenarios of increasing GHGs followed by stabilisation do not appear to induce tipping. But with increased interest in ‘overshoot’ scenarios motivated by the Paris climate agreement, is it possible that there are climate mitigation pathways that do carry a risk of AMOC tipping?

In this study we present a simple AMOC model which captures both the thermal and fresh water forcing associated with GHG increase, and is able to reproduce previous GCM results for both GHG and idealised fresh water (‘hosing’) scenarios. We identify the conditions under which AMOC tipping could occur, and their significance for ‘safe’ climate mitigation pathways.

How to cite: Wood, R.: Can increasing greenhouse gases cause tipping of the AMOC?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19442, https://doi.org/10.5194/egusphere-egu2020-19442, 2020.

Mass loss from the Antarctic Ice Sheet is the main source of uncertainty in projections of future sea-level rise, with important implications for coastal regions worldwide. Central to this is the marine ice sheet instability: once a critical threshold, or tipping point, is crossed, ice-internal dynamics can drive a self-amplifying retreat committing a glacier to substantial ice loss that is irreversible at time scales most relevant to human societies. This process might have already been triggered in the Amundsen Sea region, where Pine Island and Thwaites glaciers dominate the current mass loss from Antarctica. However, current modelling and observational techniques have not been able to establish this rigorously, leading to divergent views on the future mass loss of the West Antarctic Ice Sheet. Here we aim at closing this knowledge gap by conducting a systematic investigation of the tipping points of Pine Island Glacier using established early warning indicators that detect critical slowing as a system approaches a tipping point. We are thereby able to identify three distinct tipping points in response to increases in ocean-induced melt. The third and final event, triggered for less than a tripling of melt rates, leads to a retreat of the entire glacier that could initiate a collapse of the West Antarctic Ice Sheet.

How to cite: Rosier, S., Reese, R., Donges, J., De Rydt, J., Gudmundsson, H., and Winkelmann, R.: The tipping points of Pine Island Glacier, West Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18424, https://doi.org/10.5194/egusphere-egu2020-18424, 2020.

Paleoclimatic records show that under glacial boundary conditions the climate has jumped irregularly between two different climate states. These are the stadial and interstadial climates characterized by extremely abrupt climate change, the Dansgaard-Oeschger events. The irregularity and the fact that no known external triggering is present indicate that these are induced by internal noise, so-called n-tipping. The high resolution record of dust from Greenland icecores, which is a proxy of the state of the atmosphere, can be well fitted by a non-linear 1D stochastic process. But in order to do so the noise process needs to be an alpha-stable process, which is characterized by heavy tails violating the central limit theorem.  I will discus how extreme events can influence the transition from one climate state to the other.

How to cite: Ditlevsen, P.: The role of extreme noise in tipping between stable states , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9798, https://doi.org/10.5194/egusphere-egu2020-9798, 2020.

A complex system is a system composed of highly interconnected components in which the collective property of an underlying system cannot be described by dynamical behavior of the individual parts. Typically, complex systems are governed by nonlinear interactions and intricate fluctuations, thus to retrieve dynamics of a system, it is required to characterize and asses interactions between deterministic tendencies and random fluctuations. 

For systems with large numbers of degrees of freedom, interacting across various time scales, deriving time-evolution equations from data is computationally expensive. A possible way to circumvent this problem is to isolate a small number of relatively slow degrees of freedom that may suffice to characterize the underlying dynamics and solve the governing motion equation for the reduced-dimension system in the framework of stochastic differential equations(SDEs).  For some specific example settings, we have studied the performance of three stochastic dimension-reduction methods (Langevin equation(LE), generalized Langevin Equation(GLE) and Empirical Model Reduction(EMR)) to model various synthetic and real-world time series. In this study corresponding numerical simulations of all models have been examined by probability distribution function(PDF) and Autocorrelation function(ACF) of the average simulated time series as statistical benchmarks for assessing the differnt models' performance. 

First we reconstruct the Niño-3 monthly sea surface temperature (SST) indices averages across (5°N–5°S, 150°–90°W) from 1891 to 2015 using the three aforementioned stochastic models. We demonstrate that all these considered models can reproduce the same skewed and heavy-tailed distributions of Niño-3 SST, comparing ACFs, GLE exhibits a tendency towards achieving a higher accuracy than LE and EMR. A particular challenge for deriving the underlying dynamics of complex systems from data is given by situations of abrupt transitions between alternative states. We show how the Kramers-Moyal approach to derive drift and diffusion terms for LEs can help in such situations. A prominent example of such 'Tipping Events' is given by the Dansgaard-Oeschger events during previous glacial intervals. We attempt to obtain the statistical properties of high-resolution, 20yr average, δ¹⁸O and Ca⁺² collected from the same ice core from the NGRIP on the GICC05 time scale. Through extensive analyses of various systems, our results signify that stochastic differential equation models considering memory effects are comparatively better approaches for understanding  complex systems.

How to cite: Hassanibesheli, F., Boers, N., and Kurths, J.: Reconstruction of Complex Dynamical Systems Using Stochastic Differential Equations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4103, https://doi.org/10.5194/egusphere-egu2020-4103, 2020.

We present preliminary results and insights from the analysis of the ensemble of  Oceanic Reanalysis System 5 (ORAS5), produced by the European Center for Medium Weather Forecast (ECMWF), which reconstruct ocean’s past history from 1979 to 2018, with monthly means temporal and spatial resolution of 0.25° and 75 vertical levels.

We focused on the AMOC, which can be considered as one of the main drivers of  the Earth’s Climate System, and we observed that the strength at 26.5°N presents a shift in the mean of about 5 Sverdrup in the period 1995-2000 which can be considered as a climate tipping point.  

We aim to investigate the causes of this reduction and propose three mechanisms responsible for the observed AMOC volume transport reduction: the Gulf Stream Separation path, changes of the Mediterranean Outflow Water (MOW) and the North Atlantic Deep Water (NADW) formation processes in the Labrador Sea respectively.

The Gulf Stream Separation path is investigated by visualizing the barotropic stream function averaged over two periods, before and after the 1995-2000. In particular it is possible to detect a shift in the direction of the barotropic currents, which is enhanced further by seasonal climatology analysis. In the first period (greater volume transport), patterns are more intense, and the Gulf Stream reach higher latitudes, allowing for a more vigorous deep water formation in the Labrador Sea than in the second period. 

Moreover, we observe the AMOC volume transport reduction at 26.5°N accompanied with a reduction in the heat fluxes over the Labrador Sea. We think this reduction of heat fluxes has a cascade effect on horizontal averages for temperature, salinity, and potential density profiles, which are manifestations of less deep water production in the Labrador Sea, that can ultimately drive the AMOC weakening. 

Finally, the Mediterranean Sea has experienced, in the last decades, a general warming trend, in particular of deep water temperatures since the mid-1980s. It is well known that this warming induce a large variability in the hydrological characteristics of the MOW becoming more likely one key factor driving the AMOC variability observed in ORAS5. In fact, there’s a larger ensemble spread in both the temperature and salinity climatological profiles at 40°N, i. e. in correspondence of the Gibraltar Strait and Gulf of Cadiz.

This analysis highlights the high sensitivity of the MOW to perturbations producing the different ensemble members of ORAS5.

Our hypothesis is that the nonlinear interaction between these three mechanisms  could have a complex feedback on the AMOC variability.

In conclusion, our preliminary results brought out the relevance of the deep water formation process in the Labrador Sea, the MOW and the Gulf Stream path as the main sources of the AMOC variability and stability. Besides,  our analysis points out the need for further studies, e. g. increasing resolution at the Straits (like Gibraltar Strait), investigating correlations with the variability of the subpolar gyre and developing conceptual studies, using Intermediate Complexity Models interpreted under the lens of Dynamical System Theory and Statistical Mechanics.

How to cite: de Toma, V., Yang, C., and Artale, V.: “Climate shift of the Atlantic Meridional Overturning Circulation (AMOC) in Reanalyses (ORAS5): possible causes, and sources of uncertainty”, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3461, https://doi.org/10.5194/egusphere-egu2020-3461, 2020.

Wetlands, which exist in both natural and man-made landscapes, play a critical role in providing various ecosystem services for both ecology and human-being. These services are affected not only by regional hydro-climatic and geologic conditions but also by human activities. On a landscape scale, wetlands form a complex spatial structure by their spatial distribution in a specific geological setting. Consequently, dispersal of inhabiting species between spatially distributed wetlands organizes ecological networks that are consisted of nodes (wetlands) and links (pathways of movement). In this study, we generated and analyzed the ecological networks by introducing deterministic (e.g., threshold distance) or stochastic (e.g., exponential kernel and heavy-tailed model) dispersal models. From these networks, we evaluated structural or functional characteristics including degree, efficiency, and clustering coefficient, all of which are affected by disturbances such as seasonal hydro-climatic conditions that change wetland surface area, and shocks that may remove nodes from the network (e.g., human activities for land development). Specifically, by using the characteristics of the corresponding ecological networks, we analyzed (1) their network robustness by simulating the removal of nodes selected by their degree or area; and (2) the change of variance as the early-warning signal to predict where critical point may occur in global network characteristics affected by disturbances. The results showed that there was not a clear relationship between network robustness and wetland size for node removal. However, when nodes were removed in the order of degree, the network fragmented rapidly. Also, we observed that the variance of network characteristics in the time-series increased in drier hydro-climatic conditions for all the three network models we tested. This result indicates a possibility of using increasing variance as the early-warning signal for detecting a critical transition in network characteristics as the hydro-climatic condition becomes dry. In sum, the observed characteristics of ecological networks are vulnerable to target attack on hubs (structurally important nodes) or drought. Also, the resilience of a wetlandscape can be low after hubs were destroyed or in a dry season causing the fragmentation of habitats. Implications of these results for modeling ecological networks depending on hydrologic systems and influenced by human activities will provide a new decision-making process, especially for restoring and conservation purposes.

How to cite: Kim, B., Lee, H., Lee, K., and Park, J.: Testing early-warning signals for the transition of ecological network properties in wetland complex, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6422, https://doi.org/10.5194/egusphere-egu2020-6422, 2020.

Many dynamical processes in Earth Sciences are the product of many interacting components and have often limited predictability, not least because they can exhibit regime transitions (e.g. tipping points).To quantify complexity, entropy measures such as the Shannon entropy of the value distribution are widely used. Amongst other more sophisticated ideas, a number of entropy measures based on recurrence plots have been suggested. Because different structures, e.g. diagonal lines, of the recurrence plot are used for the estimation of probabilities, these entropy measures represent different aspects of the analyzed system and, thus, behave differently. In the past, this fact has led to difficulties in interpreting and understanding those measures. We review the definitions, the motivation and interpretation of these entropy measures, compare their differences and discuss some of the pitfalls when using them.

Finally, we illustrate their potential in an application on paleoclimate time series. Using the presented entropy measures, changes and transitions in the climate dynamics in the past can be identified and interpreted.

How to cite: Kraemer, K. H., Marwan, N., Wiesner, K., and Kurths, J.: Recurrence Plot based entropies and their ability to detect transitions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10861, https://doi.org/10.5194/egusphere-egu2020-10861, 2020.

The magnitude of the annual biological production in the Baltic proper is determined by the phosphorus (P) concentration C in the surface layer in winter. C is proportional to the total P supply TPS to the water column. TPS has three components; the land-based supply LPS; the ocean supply OPS; and the internal supply IPS from anoxic bottoms. The OPS is minor. The land-based P source, LPS, culminated in the 1980s and at present it has about the same value as in the early 1950s. Despite this, C still increases, and the present time C is at least 3 times higher than C in the 1950s. This runaway evolution of the Baltic proper P content demonstrates that the evolution of C cannot be explained only by the evolution of the external sources LPS and OPS. The runaway behaviour suggests that there is a positive feedback between the state C and the supply TPS. It is shown that the internal P-supply IPS provides such a positive feedback via its dependence on the area of anoxic bottoms Aanox, because IPS is proportional to Aanox and Aanox is proportional to C so that IPS is proportional to C. The internal supply IPS thus increases with C if there are anoxic bottoms. Anoxic bottoms start to occur when C passes the threshold value Ct which occurs when TPS passes the threshold value TPSt. This happened in the Baltic proper at the end of the 1950s. A time-dependent P model describes the evolution of C in the Baltic proper from 1950 to the present quite well.

How to cite: Stigebrandt, A.: The Baltic proper eutrophication - a runaway system that passed the tipping point at the end of the 1950s, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22229, https://doi.org/10.5194/egusphere-egu2020-22229, 2020.

The marine ice sheet instability may have already been initiated in several glaciers in West Antarctica. Hence controlling global temperatures is unlikely to be an effective way of preventing considerable sea level rise. This limits both the utility of greenhouse gas mitigation and solar radiation geoengineering as control mechanisms. Instead we evaluate various other options such as allowing ice shelves to thicken by reducing bottom melting, or slowing ice streams by drying their beds. We consider the engineering limitations, costs, and practical consequences of various designs and how a ladder of implementation might be climbed with regard to learning from Greenland and small-scale field trials. The governance, ethics, legality and societal implications for the local indigenous and global South are also discussed.

How to cite: Moore, J., Wolovick, M., Keefer, B., and Levers, O.: Designing interventions in the ice sheet/sea level system , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3255, https://doi.org/10.5194/egusphere-egu2020-3255, 2020.