Explore the vast range of titles available.

Specific components of the Earth system may abruptly change their state in response to gradual changes in forcing. This possibility has attracted great scientific interest in recent years, and has been recognized as one of the greatest threats associated with anthropogenic climate change. Examples of such components, called tipping elements, include the Atlantic Meridional Overturning Circulation, the polar ice sheets, the Amazon rainforest, as well as the tropical monsoon systems. The mathematical language to describe abrupt climatic transitions is mainly based on the theory of nonlinear dynamical systems and, in particular, on their bifurcations. Applications of this theory to nonautonomous and stochastically forced systems are a very active field of climate research. The empirical evidence that abrupt transitions have indeed occurred in the past stems exclusively from paleoclimate proxy records. In this review, we explain the basic theory needed to describe critical transitions, summarize the proxy evidence for past abrupt climate transitions in different parts of the Earth system, and examine some candidates for future abrupt transitions in response to ongoing anthropogenic forcing. Predicting such transitions remains difficult and is subject to large uncertainties. Substantial improvements in our understanding of the nonlinear mechanisms underlying abrupt transitions of Earth system components are needed. We argue that such an improved understanding requires combining insights from (a) paleoclimatic records; (b) simulations using a hierarchy of models, from conceptual to comprehensive ones; and (c) time series analysis of recent observation-based data that encode the dynamics of the present-day Earth system components that are potentially prone to tipping.

Interactions between The Atlantic Meridional Overturning Circulation (AMOC) And The Greenland Ice-Sheet (GrIS), both considered major tipping elements in the Earth system, are critical for understanding their future evolution under anthropogenic climate change. As global warming progresses, the potential weakening of the AMOC raises concerns that meltwater from the disintegrating GrIS could trigger a complete AMOC shutdown. Here, we assess the processes and feedback mechanisms that may either accelerate or stabilize these two Earth system components under idealized future scenarios in an ice-sheet coupled Earth system model of intermediate complexity with perturbed parameter ensembles. Our findings indicate that, under a moderate idealized scenario (2×CO2,PI, corresponding to ∼3 °C global mean warming), GrIS meltwater alone is unlikely to trigger an AMOC collapse. However, this risk increases with higher emissions. Notably, the delayed GrIS response to the warming results in peak meltwater fluxes entering the North Atlantic only when the AMOC is already in its recovery phase, Thereby reducing the likelihood of collapse. Additionally, the system is further stabilized by the cooling induced by the thermal bipolar seesaw. This cooling is sufficiently strong that, in the event of a future AMOC collapse, GrIS melting would effectively cease for ∼3 °C warming, and its disintegration would be substantially delayed even under higher warming levels. Nonetheless, rapid CO2 reduction remains essential to prevent irreversible state transitions of both the AMOC and GrIS.

Uncertainties persist in the understanding of the Atlantic meridional overturning circulation and its response to external perturbations such as freshwater or radiative forcing. Abrupt reduction of the Atlantic circulation is considered a climate tipping point that may have been crossed when Earth’s climate was propelled out of the last ice age. However, the evolution of the circulation since the Last Glacial Maximum (22–18 thousand years ago) remains insufficiently constrained due to model and proxy limitations. Here we leverage information from both a compilation of proxy records that track various aspects of the circulation and climate model simulations to constrain the Atlantic circulation over the past 20,000 years. We find a coherent picture of a shallow and weak Atlantic overturning circulation during the Last Glacial Maximum that reconciles apparently conflicting proxy evidence. Model–data comparison of the last deglaciation—starting from this new, multiple constrained glacial state—indicates a muted response during Heinrich Stadial 1 and that water mass geometry did not fully adjust to the strong reduction in overturning circulation during the comparably short Younger Dryas period. This demonstrates that the relationship between freshwater forcing and Atlantic overturning strength is strongly dependent on the climatic and oceanic background state.The Atlantic meridional overturning circulation was shallow and weak during the Last Glacial Maximum, and water masses took time to adjust to circulation shifts during the Last Deglaciation, according to a reassessment of proxy records and model simulations.

Cover caption The air bubbles trapped in the Antarctic Vostok and EPICA Dome C ice cores provide composite records of levels of atmospheric carbon dioxide and methane covering the past 650,000 years. Now the record of atmospheric carbon dioxide and methane concentrations has been extended by two more complete glacial cycles to 800,000 years ago. The new data are from the lowest 200 metres of the Dome C core. This ice core went down to just a few metres above bedrock at a depth of 3,260 metres. Two papers report analyses of this deep ice, including the lowest carbon dioxide concentration so far measured in an ice core. Atmospheric carbon dioxide is strongly correlated with Antarctic temperature throughout the eight glacial cycles, but with significantly lower concentrations between 650,000 and 750,000 years before present. The cover shows a strip of ice core from an Antarctic ice core from Berkner Island, this slice from a depth of 120 metres. Photo by Chris Gilbert, British Antarctic Survey. Elsewhere in this issue, we move from climates past to future plans for climate prediction. Changes in past atmospheric carbon dioxide concentrations can be determined by measuring the composition of air trapped in ice cores from Antarctica. So far, the Antarctic Vostok and EPICA Dome C ice cores have provided a composite record of atmospheric carbon dioxide levels over the past 650,000 years 1 , 2 , 3 , 4 . Here we present results of the lowest 200 m of the Dome C ice core, extending the record of atmospheric carbon dioxide concentration by two complete glacial cycles to 800,000 yr before present. From previously published data 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 and the present work, we find that atmospheric carbon dioxide is strongly correlated with Antarctic temperature throughout eight glacial cycles but with significantly lower concentrations between 650,000 and 750,000 yr before present. Carbon dioxide levels are below 180 parts per million by volume (p.p.m.v.) for a period of 3,000 yr during Marine Isotope Stage 16, possibly reflecting more pronounced oceanic carbon storage. We report the lowest carbon dioxide concentration measured in an ice core, which extends the pre-industrial range of carbon dioxide concentrations during the late Quaternary by about 10 p.p.m.v. to 172–300 p.p.m.v.

Carbon dioxide removal (CDR) from the atmosphere is part of all emission scenarios of the IPCC that limit global warming to below 1.5 °C. Here, we investigate hysteresis characteristics in 4× pre-industrial atmospheric CO2 concentration scenarios with exponentially increasing and decreasing CO2 using the Bern3D-LPX Earth system model of intermediate complexity. The equilibrium climate sensitivity (ECS) and the rate of CDR are systematically varied. Hysteresis is quantified as the difference in a variable between the up and down pathway at identical cumulative carbon emissions. Typically, hysteresis increases non-linearly with increasing ECS, while its dependency on the CDR rate varies across variables. Large hysteresis is found for global surface air temperature ( ΔSAT), upper ocean heat content, ocean deoxygenation, and acidification. We find distinct spatial patterns of hysteresis: ΔSAT exhibits strong polar amplification, hysteresis in O2 is both positive and negative depending on the interplay between changes in remineralization of organic matter and ventilation. Due to hysteresis, sustained negative emissions are required to return to and keep a CO2 and warming target, particularly for high climate sensitivities and the large overshoot scenario considered here. Our results suggest, that not emitting carbon in the first place is preferable over carbon dioxide removal, even if technologies would exist to efficiently remove CO2 from the atmosphere and store it away safely.

Most of the policy debate surrounding the actions needed to mitigate and adapt to anthropogenic climate change has been framed by observations of the past 150 years as well as climate and sea-level projections for the twenty-first century. The focus on this 250-year window, however, obscures some of the most profound problems associated with climate change. Here, we argue that the twentieth and twenty-first centuries, a period during which the overwhelming majority of human-caused carbon emissions are likely to occur, need to be placed into a long-term context that includes the past 20 millennia, when the last Ice Age ended and human civilization developed, and the next ten millennia, over which time the projected impacts of anthropogenic climate change will grow and persist. This long-term perspective illustrates that policy decisions made in the next few years to decades will have profound impacts on global climate, ecosystems and human societies — not just for this century, but for the next ten millennia and beyond.

Our world is rapidly changing. Societies are facing an increase in the frequency and intensity of high-impact and extreme weather and climate events. These extremes together with exponential population growth and demographic shifts (e.g., urbanization, increase in coastal populations) are increasing the detrimental societal and economic impact of hazardous weather and climate events. Urbanization and our changing global economy have also increased the need for accurate projections of climate change and improved predictions of disruptive and potentially beneficial weather events on kilometer scales. Technological innovations are also leading to an evolving and growing role of the private sector in the weather and climate enterprise. This article discusses the challenges faced in accelerating advances in weather and climate forecasting and proposes a vision for key actions needed across the private, public, and academic sectors. Actions span (i) utilizing the new observational and computing ecosystems; (ii) strategies to advance Earth system models; (iii) ways to benefit from the growing role of artificial intelligence; (iv) practices to improve the communication of forecast information and decision support in our age of internet and social media; and (v) addressing the need to reduce the relatively large, detrimental impacts of weather and climate on all nations and especially on low-income nations. These actions will be based on a model of improved cooperation between the public, private, and academic sectors. This article represents a concise summary of the white paper on the Future of Weather and Climate Forecasting (2021) put together by the World Meteorological Organizations’ Open Consultative Platform.

The ocean's immense ability to store and release heat on centennial to millennial time scales modulates the impacts of climate perturbations. To gain a better understanding of past variations in mean ocean temperature (MOT), a noble gas‐based proxy measured from ancient air in ice cores has been developed. Here we assess non‐temperature effects that may influence the atmospheric noble gas ratios reconstructed from polar ice and how they impact the temperature signal with an intermediate complexity Earth system model. We find that changes in wind speed, sea‐ice extent, and ocean circulation have partially compensating effects on mean‐ocean noble gas saturation, leading to a slight reduction of noble gas undersaturation at the Last Glacial Maximum (LGM). Taking these effects and ice core measurements into account, our model suggests a revised MOT difference between the LGM and pre‐industrial of −2.1 ± 0.7°C that is also in improved agreement with other independent temperature reconstructions. Plain Language Summary Most of the heat added to the climate system by anthropogenic climate change is taken up by the oceans. To better understand how the ocean responds to climate change over hundreds to thousands of years, an indirect measure for the mean ocean temperature (MOT) based on the temperature‐dependent solubility of noble gases has been developed. Noble gas concentrations of the past atmosphere are archived in air bubbles in polar ice cores, which have been used to reconstruct the MOT of the past 20,000 years when Earth's climate was propelled out of the last ice age. However, uncertainties remain regarding critical parameters that are required to derive the correct MOT of the past. Here we make use of an Earth system model that explicitly simulates the noble gases and allows us to assess these parameters in detail under modern and past climate conditions. We find that changes in wind, sea‐ice, and ocean circulation all play important roles in the partitioning of noble gases between the atmosphere and ocean. By taking these effects into account our model suggests a revised best‐estimate MOT cooling of the last ice age to −2.1 ± 0.7°C, which is about 0.5°C warmer than previous estimates. Key Points Revised implementation of noble gases in Bern3D model tuned to observations of saturation anomalies Complex interplay between air‐sea gas exchange, overturning circulation, and noble gas saturation Including saturation effects noble gas‐based mean ocean temperature of the last glacial maximum is 2°C colder than the Holocene

Cover caption The air bubbles trapped in the Antarctic Vostok and EPICA Dome C ice cores provide composite records of levels of atmospheric carbon dioxide and methane covering the past 650,000 years. Now the record of atmospheric carbon dioxide and methane concentrations has been extended by two more complete glacial cycles to 800,000 years ago. The new data are from the lowest 200 metres of the Dome C core. This ice core went down to just a few metres above bedrock at a depth of 3,260 metres. Two papers report analyses of this deep ice, including the lowest carbon dioxide concentration so far measured in an ice core. Atmospheric carbon dioxide is strongly correlated with Antarctic temperature throughout the eight glacial cycles, but with significantly lower concentrations between 650,000 and 750,000 years before present. The cover shows a strip of ice core from an Antarctic ice core from Berkner Island, this slice from a depth of 120 metres. Photo by Chris Gilbert, British Antarctic Survey. Elsewhere in this issue, we move from climates past to future plans for climate prediction. A detailed atmospheric methane record from the EPICA Dome C ice core that extends the history of atmospheric methane to 800,000 years before present is detailed. Spectral analyses indicate that the long-term variability in atmospheric methane levels is dominated by ∼100,000 year glacial–interglacial cycles up to ∼400,000 years ago with an increasing contribution of the precessional component during the four more recent climatic cycles. Atmospheric methane is an important greenhouse gas and a sensitive indicator of climate change and millennial-scale temperature variability 1 . Its concentrations over the past 650,000 years have varied between ∼350 and ∼800 parts per 10 9 by volume (p.p.b.v.) during glacial and interglacial periods, respectively 2 . In comparison, present-day methane levels of ∼1,770 p.p.b.v. have been reported 3 . Insights into the external forcing factors and internal feedbacks controlling atmospheric methane are essential for predicting the methane budget in a warmer world 3 . Here we present a detailed atmospheric methane record from the EPICA Dome C ice core that extends the history of this greenhouse gas to 800,000 yr before present. The average time resolution of the new data is ∼380 yr and permits the identification of orbital and millennial-scale features. Spectral analyses indicate that the long-term variability in atmospheric methane levels is dominated by ∼100,000 yr glacial–interglacial cycles up to ∼400,000 yr ago with an increasing contribution of the precessional component during the four more recent climatic cycles. We suggest that changes in the strength of tropical methane sources and sinks (wetlands, atmospheric oxidation), possibly influenced by changes in monsoon systems and the position of the intertropical convergence zone, controlled the atmospheric methane budget, with an additional source input during major terminations as the retreat of the northern ice sheet allowed higher methane emissions from extending periglacial wetlands. Millennial-scale changes in methane levels identified in our record as being associated with Antarctic isotope maxima events 1 , 4 are indicative of ubiquitous millennial-scale temperature variability during the past eight glacial cycles.

Evaluation and communication of the relative degree of certainty in assessment findings are key cross-cutting issues for the three Working Groups of the Intergovernmental Panel on Climate Change. A goal for the Fifth Assessment Report, which is currently under development, is the application of a common framework with associated calibrated uncertainty language that can be used to characterize findings of the assessment process. A guidance note for authors of the Fifth Assessment Report has been developed that describes this common approach and language, building upon the guidance employed in past Assessment Reports. Here, we introduce the main features of this guidance note, with a focus on how it has been designed for use by author teams. We also provide perspectives on considerations and challenges relevant to the application of this guidance in the contribution of each Working Group to the Fifth Assessment Report. Despite the wide spectrum of disciplines encompassed by the three Working Groups, we expect that the framework of the new uncertainties guidance will enable consistent communication of the degree of certainty in their policy-relevant assessment findings.