The prospects of such a global temperature overshoot raise important questions in relation to Earth system feedbacks under overshoot.
Key questions include: what feedbacks might occur once specific warming targets are exceeded? What is the likelihood of rapid or abrupt change (including tipping points) occurring due to overshoot? What are the consequent risks for society and the natural environment? And, how reversible will these changes be if global mean temperature returns to a lower temperature level at some later date?
Further, it is important to understand the feasibility and side-effects of large-scale deployment of carbon dioxide removal that are necessary to return the Earth system to safer temperatures post-overshoot.
In this session, we welcome abstract submissions on global climate dynamics under peak and decline pathways, on regional to global climate impacts in overshoot scenarios, and mechanisms of non-linearity, particularly the risk of rapid/abrupt Earth system change. We welcome Integrated Assessment, Earth system and impact model experiments focused on overshoot pathways, including investigation of carbon dioxide removal, and realization of warming overshoot pathways with Earth System Models (including idealized pathways such as suggested by TIPMIP). We also invite analysis focussing on consequences in a wide range of sectors, from ocean dynamics to the cryosphere, biodiversity and biosphere changes to human systems and economic consequences of overshoot. Contributions that consider the socio-economic conditions and feasibility of overshoot scenarios, climate effects of large scale carbon dioxide removal, as well as the implications of overshoots for climate change adaptation planning are also strongly encouraged.
Orals: Thu, 1 May | Room 0.49/50
As the global temperature approaches an average of 1.5°C above preindustrial levels - the most ambitious limit set in the 2015 Paris climate agreement - researchers and policymakers are increasingly considering overshoot pathways that aim to mitigate the impacts of transgressing these guardrails. Such overshoot narratives warrant cautious consideration:
Firstly, achieving net-zero goals is challenging as it is. But achieving large-scale net-negative CO2 emissions on a global scale to reverse a temperature or carbon budget transgression will be even more challenging, when considering context-specific feasibility and desirability constraints to the implementation of carbon dioxide removal (CDR) options (see e.g. Borchers et al., 2024).
Secondly, if we would achieve global net-negative CO2 emissions, detecting a temperature overshoots amidst internal climate variability will pose a considerable challenge, that would considerably complicate the monitoring and accordingly the management of such anthropogenic interventions into the climate system.
Thirdly, the financial, social, and governance frameworks required to incentivise, implement and sustain global net-negative CO2 emissions to manipulate the atmospheric CO2 concentration, similar to other forms of climate interventions, are likely unattainable.
Fourth, even if these barriers were overcome, the resulting post-overshoot climate would very likely differ from one where temperature stabilisation is maintained without overshoot (see e.g., Schleussner et al., 2024), with potentially irreversible impacts on ecosystems and climate systems.
Mitigation deterrence - reducing incentives for near-term emissions reductions - represents an considerable risk associated with CDR and overshoot narratives (Carton et al., 2023). It is therefore crucial to approach overshoot research and communication with care, prioritising immediate and effective mitigation strategies to minimise reliance on uncertain and potentially unfeasible overshoot pathways.
Borchers, M., Förster, J., Thrän, D., Beck, S., Thoni, T., Korte, K., et al. (2024). A comprehensive assessment of carbon dioxide removal options for Germany. Earth's Future, 12, e2023EF003986. https://doi.org/10.1029/2023EF003986
Schleussner, CF., Ganti, G., Lejeune, Q. et al. Overconfidence in climate overshoot. Nature 634, 366–373 (2024). https://doi.org/10.1038/s41586-024-08020-9
Carton, W., Hougaard, I.-M., Markusson, N., & Lund, J. F. (2023). Is carbon removal delaying emission reductions? WIREs Climate Change, 14(4), e826. https://doi.org/10.1002/wcc.826
How to cite: Mengis, N.: How Much Attention Should We Give to Overshoot Narratives?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15742, https://doi.org/10.5194/egusphere-egu25-15742, 2025.
The temperature goals of the Paris Agreement are measured as 20-year averages exceeding a pre-industrial baseline. A first single calendar year above 1.5 °C relative to pre-industrial levels is imminent and may have already occurred in 2024, but the implications for the corresponding temperature goal are unclear. Here, we show that, without very stringent climate mitigation, the first year above 1.5 °C occurs within the first 20-year period with an average warming of 1.5 °C. This is due to the ongoing strong anthropogenic multi-decadal warming trend that renders it very unlikely for the temperature of a single year to exceed the average temperature over the coming decades. The results provide an early warning that signals the onset of a period where the climate impacts of a 1.5 °C warmer world will start to emerge, underscoring the urgency of adaptation action. Yet, our findings also indicate that, by rapidly slowing down the warming rate, very stringent near-term mitigation may substantially reduce risks of exceeding the 1.5 °C global warming level soon after the first single year above 1.5 °C has occurred.
How to cite: Bevacqua, E., Schleussner, C.-F., and Zscheischler, J.: A year above 1.5 °C signals Earth is most probably within the 20-year period that reaches the Paris Agreement limit, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2849, https://doi.org/10.5194/egusphere-egu25-2849, 2025.
With the global annual mean temperature in 2024 exceeding 1.5°C above preindustrial levels, there is an urgent need to investigate pathways for returning the Earth system to lower temperature levels. In addition to stringent emission reduction, we need portfolios of Carbon Dioxide Removal (CDR) techniques to achieve the net-zero emission target. Therefore, it is crucial to evaluate various land and ocean-based CDRs for their effectiveness, environmental risks, and additional benefits.
This study evaluates the CO₂ sequestration potential and efficacy of two prominent CDR methods—Bioenergy with Carbon Capture and Storage (BECCS) and Ocean Alkalinity Enhancement (OAE)—applied both individually and in combination. Using the Norwegian Earth System Model (NorESM2-LM), simulations were designed with ramped-up CDR deployment, targeting 5.2 million km² of bioenergy feedstock for BECCS and a CaO deployment rate of 2.7 Gt/year for OAE by 2100 across the exclusive economic zones of Europe, the United States, and China. The results reveal a nearly additive carbon removal effect of BECCS and OAE. Over the period 2030-2100, OAEsequestered a total of 7 ppm of CO2 with an accumulated 82.3 Gt CaO, achieving a CDR effectiveness of 0.08 ppm per Gt of CaO, while BECCS removes 23 ppm of CO2, with CDR effectiveness of 3.1 ppm per million km² of bioenergy crops. The combined BECCS-OAE simulation offsets anthropogenic CO₂ emissions of 5.4 Gt/year by 2100—equivalent to over 60% of current global transport sector emissions. However, the combined CDR scenario shows negligible effects on the global annual mean temperature, with no clear response detectable against the high internal variability. This underscores the limitations of current CDR approaches in addressing climate warming over the 21st century and emphasizes the need for substantial emissions reductions, supportive policies and diversified CDR strategies to facilitate a return to lower global temperatures.
How to cite: Sathyanadh, A., Muri, H., Esfandiari1, H., Bourgeois, T., Schwinger, J., Bergman, T., Partanen, A.-I., Debolsky, M., Seifert, M., and Keller, D.: Towards Net Zero: Evaluating Combined Terrestrial and Marine CDR Approaches, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6081, https://doi.org/10.5194/egusphere-egu25-6081, 2025.
Deep uncertainty about the costs and resource limits of carbon dioxide removal (CDR) options challenges the design of robust portfolios. To address this, we identified key uncertainties in CDR pathways and developed the CDR-SPEC model, a mixed-integer linear optimization model for cost-optimal and time-dependent CDR portfolios with endogenous treatment of technology cost dynamics. Within this framework, we sampled the option space to explore the impact of input parametric uncertainty on the composition and performance of CDR portfolios. The resulting database contains detailed information about how varying combinations of uncertainty conditions trigger the implementation of different CDR portfolios. What is missing is an understanding of which factors drive large variability in the outcomes, where outcome is understood as any metric of performance, without making assumptions about their desirability.
To shed light on this, we recur to the concept of entropy, a measure of the uncertainty in a distribution. We use this as a proxy for guiding an exploration strategy that aims at maximising the amount of information gained about a desired outcome, providing a comparative assessment of each uncertain parameter’s contribution to an outcomes distribution. This assessment shows that among all parameters represented in CDR-SPEC, cumulative (i.e., from 2020 to 2100) removal requirements (CRR) drives the largest entropy reductions across a series of outcomes. The interpretability of this result is nevertheless challenged by the multitude of uncertainty dimensions this parameter englobes: it represents both scenario uncertainty (i.e., how much abatement takes place for different greenhouse gases) and climate response uncertainty (i.e., potential additional CDR incurred when considering beyond-median warming outcomes). Unpacking these two dimensions bundled under CRR would allow highlighting the relative impact of key uncertainties in the science that informs CDR-deployment policies.
Representing all three dimensions of uncertainty (i.e., CDR-specific, scenario and climate response uncertainty) requires expanding our understanding of the impacts of different CDR approaches on global temperatures under varying assumptions on how the earth system responds to emissions. This could be achieved by, for a fixed illustrative mitigation pathway and set of CDR-specific parameters, iterating the results from the CDR portfolio analysis in a simple climate emulator until the removals required for climate stabilisation and the removals delivered by the CDR portfolio converge. For many illustrative mitigation pathways and sets of CDR-specific parameters, this results in a database which disentangles all three dimensions of uncertainty mentioned above.
How to cite: Rodriguez Mendez, Q., Fuss, S., and Creutzig, F.: Unpacking uncertainty in Carbon Dioxide Removal requirements, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5917, https://doi.org/10.5194/egusphere-egu25-5917, 2025.
Carbon dioxide removal (CDR) is now commonly considered an unavoidable part of a mitigation portfolio to meet global climate goals, complementing rapid and sustained cuts in existing emissions. However, current mitigation assessments of the potential role of CDR have tended to ignore the uncertainty in the Earth System response to our emissions. Here, we assess the level of “preventive” CDR to hedge against a stronger-than-median Earth System response. Using the C1 (“1.5°C with no or limited overshoot”) set of pathways assessed by the Intergovernmental Panel on Climate Change (IPCC), we show that the potential preventive CDR, in addition to CDR already deployed in these pathways, for a very likely (>= 90%) chance of reaching 1.5°C in 2100 may be 323 - 787 Gt CO2 (interquartile range). This is of a similar order of magnitude, and additional to the pathways’ existing assumed deployment, potentially exacerbating existing concerns over large-scale CDR deployment. We show that scenarios that limit residual emissions, both from long-lived (e.g., CO2 and N2O) and short-lived climate forcers (e.g., CH4), can significantly reduce the scale of required preventive CDR. Ensuring preventive CDR capacity is available at scale, if needed after net-zero, will require additional near-term investments. We cannot know now whether a net zero society will need to utilize it but emphasize that the option must be available to them. Our results suggest the need to rethink the role of so-called “hard-to-abate” emission sectors – limiting the emissions in these sectors in addition to rapid near-term cuts in emissions may be crucial to mitigate the worst climate impacts and avoid unsustainable CDR deployment.
How to cite: Ganti, G., Pelz, S., Klönne, U., Gidden, M., Schleussner, C.-F., and Nicholls, Z.: Preventive carbon dioxide removal under climate response uncertainty, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12619, https://doi.org/10.5194/egusphere-egu25-12619, 2025.
Integrated Assessment Models (IAMs) combine economy, energy, and sometimes land-use modeling approaches and are commonly used to evaluate climate policies under least-cost scenarios. The marginal abatement cost (MAC) curve approach has been commonly used in climate policy analyses to show the carbon price level for a given abatement level, which has also been applied as a way to parameterize the complex behavior of IAMs. Here, we propose a new methodological framework to i) emulate the IAM’s emission reductions in response to carbon price pathways through MAC curves (i.e., IAM emulator) and then ii) extend IAM’s emission pathways (usually given until 2100) to 2300 with the emulator.
As part of the Horizon Europe RESCUE and OptimESM projects, our approach is used to extend the greenhouse gas (GHG) emission pathways from different sources and carbon dioxide removal (CDR) pathways generated by the REMIND-MAgPIE model. A key feature of the approach is that we individually capture the emission reductions associated with CDRs (i.e., afforestation, bioenergy and carbon capture and storage (BECCS), direct air capture with carbon storage (DACCS), industrial CCS, and ocean alkalinity enhancement (OAE)) through MAC curves. Our approach relies on the following simplifying assumptions: i) MAC curves are assumed time-independent over periods, ii) abatement levels are assumed independent across GHGs (CO2, CH4, and N2O), sectors (energy- and non-energy-related emissions), and CDR options, and iii) a uniform carbon price is used across sectors and CDRs, with the GWP100 metric used to fix the price ratios between different GHGs.
We approximated the dynamics of REMIND-MAgPIE with MAC curves, using equation log(f(x)+1)=a*xb+c*xd for sectoral gases and equation f(x)=a*xb+c*xd for CDR options, respectively. f(x) represents the corresponding carbon price level at x, while the variable x represents the abatement level relative to the assumed baseline level, expressed as a percentage for sectoral emissions or the absolute amount of CO2 removed for CDR options. a, b, c, and d are the parameters that are optimized for each case. Additionally, we derived the maximum abatement levels of REMIND-MAgPIE from its simulation results under all carbon budgets, which reflect the limit of, for example, CCS capacity and sectoral mitigation potential. We also calculated for each gas, sector, and CDR option the maximum first and second derivatives of temporal changes in abatement levels to capture the limits of the technological change rate and the socio-economic inertia.
By combining the IAM emulator with a reduced-complexity climate model ACC2, we further derived extended emission pathways beyond 2100 using the least-cost approach for temperature trajectories (1 °C, 1.5 °C, and 2 °C) with overshoots of up to 2 °C. These pathways illustrate various CDR use cases over the coming centuries. Our extended scenarios, generated on the basis of long-term climate-economy interactions, can serve as input to Earth System Models investigating the long-term consequences of climate change mitigation strategies, particularly the implications of CDR deployment and associated Earth system dynamics over centennial timescales.
How to cite: Xiong, W., Tanaka, K., Johansson, D. J. A., Merfort, L., and Bauer, N.: Projecting long-term pathways of greenhouse gas emissions and carbon dioxide removal with an Integrated Assessment Model emulator, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9195, https://doi.org/10.5194/egusphere-egu25-9195, 2025.
The TIPMIP ESM experiment protocol employs a specified CO2-emission rate, unique to each participating model, to achieve a common linear increase in global mean surface air temperature of ~0.2K/decade for integrations started from a pre-industrial (piControl) run. This is referred to as the ramp-up phase of TIPMIP. At different levels of global warming (GWL) into the ramp-up, models switch to zero CO2 emissions and run in this mode for 500 years. Using CO2 emissions only, it is possible to control the rate of global warming across models and also ensure that models branch into zero-emission runs at the same GWL after the same period and rate of (ramp-up) warming. While the simplicity and commonality of warming across models is a positive feature of the protocol, it is reasonable to ask how representative of real-world global warming the protocol is. We address this question by comparing simulations made with version 1.2 of the UK Earth System Model (UKESM1.2). We compare a 4-member ensemble of UKESM1.2 following the TIPMIP ramp-up protocol (i.e. CO2 emissions only started from a piControl run) against a 4-member ensemble using full CMIP6 historical forcing started from the same piControl. We compare the two ensembles over a 40-year period that approximately covers global warming of 0.2 to 1.0K above pre-industrial values. This corresponds to the interval 1975-2015 for the historical runs, and to years 10 to 50 of the ramp-up. We compare key metrics across the two ensembles, focusing on radiation and energetics, the cryosphere, carbon cycle, and modes of variability, examining both the mean climate and temporal trends across the 40-year period. Initial results suggest the TIPMIP ramp-up compares well to the full historical runs, as well to observations, providing confidence that conclusions drawn from a multi-model assessment of the TIPMIP protocol will be relevant to the real world.
How to cite: Walton, J., Swaminathan, R., Jones, C., Dittus, A., Liddicoat, S., Rumbold, S., and Smith, R.: Assessing the Realism of the TIPMIP ESM idealized Experiment Protocol, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4403, https://doi.org/10.5194/egusphere-egu25-4403, 2025.
Currently implemented emission reduction policies are projected to result in anthropogenic warming of the climate system beyond the 1.5 ºC and 2ºC temperature targets of the Paris Agreement. Consequently, achieving the Agreement objectives may only be possible following a period of ‘overshoot’, where warming temporarily exceeds either of these targets, before later declining and stabilising below them through net-negative CO2 emissions. Whilst previous studies have illustrated that global mean surface temperature rise is reversible under net-negative CO2, it remains unclear whether other human-induced climate impacts will exhibit the same degree of reversibility. This study assesses the irreversibility and hysteresis behaviour of regional temperature extreme frequencies under net-negative CO2 emissions. We analyse the results of eight Earth System Models that have completed the Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) Tier 1 experiment, in which atmospheric CO2 concentrations follow a symmetric 1% per year rise and decline between their preindustrial level and up to quadruple this value. For equivalent global warming levels reached through periods of positive and negative emissions respectively, we observe a high degree of hysteresis and short-term irreversibility in extreme temperature frequency across most land and ocean regions, with the sign of such changes displaying a general hemispheric asymmetry. Whilst much of this behaviour can be attributed to ongoing thermal inertia, our results suggest non-linearities in large-scale climate components, such as the Atlantic Meridional Overturning Circulation and El Niño-Southern Oscillation, may contribute to centennial-scale, irreversible changes in regional temperature extreme frequency under net negative CO2 emissions.
How to cite: Clark, S., King, A., Brown, J., Cassidy, L., and Alastrué de Asenjo, E.: Irreversibility and hysteresis in regional temperature extreme frequency under net-negative CO2 emissions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7548, https://doi.org/10.5194/egusphere-egu25-7548, 2025.
This study aims to analyze the effect of increasing atmospheric CO2 concentrations on the Atlantic Meridional Overturning Circulation (AMOC) and its dependence on North Atlantic deep water formation. We used EC-Earth3-HR, the high-resolution version of the global coupled climate model EC-Earth3, with a spatial resolution of about 0.25 degrees in the ocean and 40 km in the atmosphere. Our configuration has undergone a tuning process, and a multi-centennial spin-up has been performed. The experiments analyzed consist of a pre-industrial control simulation (piControl), a one percent per year increase in CO2 experiment (1pctCO2), branching from year 250 of the piControl simulation, and two experiments with fixed CO2 concentrations (400.9 ppm and 551.5 ppm). These two experiments branch off from points corresponding to global temperature anomalies of around 1°C and 2°C in the 1pctCO2 experiment. Both simulations equilibrate at a higher global warming level. As the climate warms, North Atlantic waters become warmer and fresher, weakening deep convection and deep water formation, which reduces the strength of the AMOC by approximately 10% in the low and 20% in the high fixed-CO2 concentration experiments. Deep water formation is assessed using a novel method based on horizontal volume convergence within the main convective areas. The weakening is primarily driven by the Labrador Sea, followed by the Greenland Sea, while the Irminger Sea sustains the remaining deep water formation. These changes align with variations in AMOC strength and meridional volume transport at 26°N and 45°N. Our study emphasizes the connection between North Atlantic deep water formation and AMOC, offering insights into its expected weakening as CO2 concentrations rise.
How to cite: Navarro Labastida, R. G., Karami, M. P., Koenigk, T., de Boer, A., and Sicard, M.: North Atlantic Ocean Circulation Changes Under Increased CO2 Concentrations Using a High-Resolution Global Coupled Climate Model , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11943, https://doi.org/10.5194/egusphere-egu25-11943, 2025.
The polar ice sheets in both Antarctica and Greenland are considered tipping elements in the Earth system. However, the detailed nature of the underlying feedback mechanisms, hysteresis and irreversibility of ice loss on different temporal and spatial scales is an active research frontier.
Tipping dynamics can be triggered by forcing a system beyond a critical threshold (bifurcation-induced tipping), for instance through an increase in global warming beyond some critical level. Alternatively, tipping can also be initiated through fluctuations close to a critical threshold (noise-induced tipping), or by changing the forcing faster than a critical rate (rate-dependent tipping). These mechanisms have been described in complex systems theory and shown in conceptual modelling approaches, but a systematic insight into such dynamics in process-based models for Earth system components is lacking so far.
Here we explore these different types of tipping dynamics for the polar ice sheets based on simulations with the Parallel Ice Sheet Model, disentangling the complexity of critical transitions in response to anthropogenic climate change at different temporal and spatial scales. Our results underscore the importance of the rate of global warming, its variability, as well as the magnitude and duration of potential overshoots – all of which are decisive for the future evolution and long-term stability of the ice sheets.
How to cite: Winkelmann, R. and Klose, A. K.: Three types of tipping dynamics in ice sheets, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14840, https://doi.org/10.5194/egusphere-egu25-14840, 2025.
The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the future due to increasing greenhouse gas concentrations, but it is still debated whether anthropogenic climate change can induce an irreversible collapse or “tipping” of the AMOC. Meltwater from the Greenland ice sheet has often been invoked as a key mechanism for a potential AMOC tipping, but it is not explicitly represented in most state-of-the-art (CMIP6) climate models, adding further uncertainty to assessing the likelihood of irreversible AMOC change.
Here, we perform ensemble simulations with the CMIP6 model EC-Earth3 to assess the effects of future Greenland ice sheet melt and to probe AMOC reversibility with and without Greenland meltwater. To this end, we force EC-Earth3 with a strong global warming scenario (SSP5-8.5) and a high-end Greenland meltwater estimate from the coupled climate–ice sheet model CESM2-CISM2 until 2300.
We find that, as expected, the addition of Greenland meltwater significantly exacerbates the greenhouse gas-induced AMOC weakening especially after the 21st century, with differences mostly attributable to the Arctic Ocean. However, we find no indication of an abrupt AMOC weakening. We then branch off idealized reversibility experiments in which the meltwater and/or greenhouse gas forcings are reversed. Although the AMOC recovery is slow (around two centuries), meltwater-driven additional AMOC weakening in EC-Earth3 appears to be reversible. Regardless of the added meltwater, the AMOC also recovers in an idealized CO2 ramp-down experiment, even overshooting its present-day strength. While our modeling results show little support for an irreversible AMOC change due to future Greenland ice sheet melt, they do underline the importance of representing meltwater in future projections, including overshoot pathways.
How to cite: Mehling, O., Bellomo, K., Fabiano, F., Devilliers, M., Corti, S., and von Hardenberg, J.: Impacts and reversibility of meltwater-induced future Atlantic Meridional Overturning Circulation changes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9113, https://doi.org/10.5194/egusphere-egu25-9113, 2025.
The Optimal High-Resolution Earth System Models for Exploring Future Climate Changes (OptimESM) project aims at developing the next generation of ESMs, bringing together increased model resolution and process realism. Project partners committed to provide a set of idealised simulations with state-of-the-art Earth System Models (ESMs) that go beyond CMIP6 experiments. In particular, there are new ESM simulations with zero CO2 emissions to represent different global warming levels (GWLs), including previous temperature overshoots. These new simulations take into account the committed warming from a gradually warming climate and thereby differ from the widely used time slots around a point in time when a transient climate simulation passes a given future warming level.
First results from these idealised simulations with EC-Earth3-ESM, a post-CMIP6 configuration of the EC-Earth model family will be presented. We analyse and compare the strength and duration of climate extremes at different GWLs with CLIMIX tools, including a comparison against a pre-industrial control and a historical climate simulation.
How to cite: Wyser, K., Koenigk, T., Yang, S., Guo, C., Wang, S., and Nilsson, C.: Climate extremes at various global warming levels, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9780, https://doi.org/10.5194/egusphere-egu25-9780, 2025.
The eruption of Mount Tambora in 1815 cooled the global climate and caused famines due to crop failures in the "year without summer". Such changes to Earth system conditions, triggered by volcanic activity, increase the uncertainty surrounding future climate change. However, the subsequent response of land carbon uptake and its potential implications for future emission pathways are not well-explored. While multi-annual mean responses are predictable and aid emission budget estimations, higher-order variability in land carbon flux and the effects of non-CO2 forcings, like volcanic aerosols, remain uncertain. Aerosols cause variability by altering the Earth’s radiation balance, reducing surface temperature, and modulating precipitation patterns, driving substantial regional climate change. Additionally, state-dependent non-linearities, such as regional sensitivity differences to aerosol forcing, complicate the climate’s response.
This study utilizes advanced Earth system model simulation to compare ensembles with both semi-stochastic and constantly recurring explicit volcanic forcing to examine their effect on land carbon flux variability in an overshoot scenario. Analyzing temperature and carbon flux spectra, along with mean standardized anomalies from global to regional scales, reveals the temporal and spatial structures of variability driven by intermittent volcanic forcing. On the event scale, we detect the system response via autoregressive processes, which allows us to quantify the impacts of individual events on the terrestrial carbon stock. We put these findings into the context of emission scales in future pathways.
We find a connection between increased volcanic forcing and more considerable variability in land carbon uptake, which seems to be exacerbated at lower CO2 forcing. Because of this state-dependency, the effect varies along the overshoot pathway. Additionally, intermittent volcanic forcing affects carbon flux variability, most prominent on decadal timescales and regional proximity of the eruption. Our study indicates that both positive and negative carbon stock impacts are more variable with increasing event magnitude. However, attribution is challenging due to low signal-to-noise ratios and internal climate variability. The results identify vegetation carbon from the equatorial regions as the primary driver of these polar impacts, with minor positive contributions from soil and litter carbon in northern latitudes. While the average impact across the ensemble approaches zero, the cumulative effects of individual simulations can vary up to the order of the annual terrestrial carbon sink. These results hint that future emission pathways should consider a more realistic volcanic forcing when assessing the land carbon stock's transient behaviour. However, they also require validations through model comparisons. Intermittent volcanic forcing could also represent a natural analog to assess the impacts of stratospheric aerosol injection, a geoengineering method to counteract global warming with sulfur aerosols.
How to cite: Schürmann, T., Adam, M., and Rehfeld, K.: Testing the impacts of natural climate variability on land carbon uptake and overshoot emission pathways in an Earth System Model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10557, https://doi.org/10.5194/egusphere-egu25-10557, 2025.
We present the first results of the TIPMIP ESM experiments using the post-CMIP6 model EC-Earth3-ESM. The main objective of the TIPMIP ESM is to study the risks and consequences of potential tipping events in the Earth system, as well as the potential reversibility of triggered events - as a function of e.g., magnitudes and durations of different global warming levels (GWLs) before cooling the climate back to lower GWLs and pre-industrial climate.
Following the TIPMIP ESM protocol, we performed a set of idealized emission-driven simulations, including 1) ramp-up runs with a constant CO2 emission that drives a global mean surface air temperature (GMSAT) warming rate of 0.2 K/decade; 2) stabilization runs with zero CO2 emission at multiple GWLs; and 3) ramp-down runs to the pre-industrial climate with a negative CO2 emission (same magnitude with the ramp-up run) branched after 50 years of stabilization runs.
We present and discuss the simulated responses in the large-scale features of the different components of the earth system in the ramp-up, stabilization, and first test runs of the ramp-down experiments, with a focus on, e.g., GMSAT, AMOC, sea ice, carbon pools/fluxes, and Greenland Ice Sheet. Our findings suggest that some fast climate system components are reversible, e.g., sea ice, but Arctic summer sea ice can show some delays in recovering at high GWLs. AMOC linearly declines during the ramp-up phase, stabilizes during the stabilization phase, and recovers during the ramp-down phase (with an overshoot). Melting in the Greenland Ice Sheet accelerates during the ramp-up phase, its mass loss continues with somewhat slower speeds during stabilization and hardly reverses during the ramp-down phase when branched at high GWLs.
How to cite: Guo, C., Yang, S., Wyser, K., Koenigk, T., van der Linden, E., Drijfhout, S., Tourigny, E., Nieradzik, L., Bernales, J., and Tian, T.: Earth system modeling of idealized overshoot scenarios under the TIPMIP ESM protocol using EC-Earth3-ESM, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13927, https://doi.org/10.5194/egusphere-egu25-13927, 2025.
To reach climate neutrality, emission reduction must be complemented by carbon dioxide removal technologies aiming to sequester atmospheric CO2 and store it in permanent natural reservoirs. The ocean, which already sequesters roughly a quarter of all anthropogenic CO2 emissions annually, can play a crucial role in this effort. By storing carbon in forms that are not readily exchanged with the atmosphere, it acts as a vast and long-lasting carbon reservoir on a human-relevant timeframe.
This potential has spurred growing interest in the development and deployment of ocean-based carbon dioxide removal technologies. One potentially scalable method is ocean alkalinity enhancement (OAE), which is performed by applying alkaline mineral rocks or solutions at the ocean surface to lower its CO2 partial pressure (pCO2) and accelerate CO2 sequestration and storage as bicarbonate and carbonate ions.
For large-scale application, it is crucial to understand the potential earth system feedbacks generated by alkalinity addition, considering both the space and time dimension. Spatially, coastal alkalinity addition was investigated, as it is more feasible from a political and logistical standpoint. Temporally, as seasonality is a fundamental component of the ocean net annual CO2 uptake, attention was given to the changes to the seasonal CO2 flux and ocean pCO2 cycle. Additionally, different background climate scenarios were considered to assess whether varying levels of warming influence seasonal variations of OAE-induced ocean uptake.
OAE was performed at the European coastline using an earth system model in emission-driven mode, with a low and a high climate change forcing (SSP1-2.6 and SSP3-7.0, respectively, following CMIP6 guidelines). No-OAE simulations were performed as baseline reference including climate change forcing. Alkalinity was applied continuously in the form of calcium hydroxide (Ca(OH)2) at the first ocean layer. Between 2025 and 2035, the alkalinity flux was increased linearly until the equivalent of 1Gt yr-1 (equal to 27 Tmol yr-1) was reached, then maintained constant until the year 2100.
Results found that: a) with alkalinity addition, the ocean CO2 seasonal cycle is dampened due to the decreased sensitivity of an alkalinised ocean to CO2 fluctuations; b) the CO2 seasonal flux into the ocean is amplified given the larger pCO2 imbalance at the air-sea interface; c) while the ocean pCO2 seasonal amplitude reduction is stronger under low warming, the CO2 flux seasonal amplification is stronger in the high warming pathway.
This project has received funding from the European Union’s Horizon Europe research and innovation programme under grant agreement no. 101056939 (RESCUE)
How to cite: Ciscato, C., Butenschön, M., Keller, D., Mehendale, N., and Kemena, T.: Impacts of Simulated Coastal Ocean Alkalinity Enhancement on the Seasonal Cycle of CO2 Air-Sea Gas Exchange and ocean pCO2 in European Waters under Low and High Emission Scenarios, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16142, https://doi.org/10.5194/egusphere-egu25-16142, 2025.
Several important climate system tipping points are associated with the thawing Arctic including ice sheet collapse, AMOC shutdown, sea ice loss and permafrost thaw which may be imminent. This prompts examination of interventions to support the vulnerable systems and has raised great debate on potential governance of research into any future deployments. As well as their global impacts these systems also impact the lives of the Arctic Peoples and local ecosystems. The Arctic contains an estimated 25% of global untapped gas reserves and 13% of oil and large amounts of rare earths such as 40% of global palladium. Much of this is beneath hazardous seas, or as with Greenland, largely beneath thick permanent ice. But the Arctic is rapidly losing its ice cover, exposing more land and ice-free ocean, making it an attractive target for resource extraction and a geopolitical pawn.
Unfortunately, resource extraction provides limited sustainable benefits, for the locals, with most profits to the big mining companies. However, ice itself is a global resource that, if valued proportionately to the damage its loss causes via flooding and building coastal protection, as well as ice-albedo and carbon-temperature feedbacks would be worth tens of trillions of dollars per meter over the century. Even avoiding raising sea levels by 1 inch would be worth far more than the yearly $500 m Danish “block grant” to Greenland. This would allow Arctic Peoples more self-determination, while disproportionately benefiting the Global South which has limited adaptive capacity compared with the Developed World. The ethical alternative is to value the frozen Arctic as a global good, perhaps with an analogous system to the REDD+ mechanism applied to conserving the Amazon.
How to cite: Moore, J.: Exploit or empower: pros and cons of researching interventions in the Arctic , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17043, https://doi.org/10.5194/egusphere-egu25-17043, 2025.
With ongoing climate change, multiple stressors including ocean warming, deoxygenation, ocean acidification and limited nutrient availability will lead to large regime shifts within marine ecosystems[1]. Deep-sea ecosystems are adapted to the stable ambient conditions of the deep ocean and are therefore likely highly vulnerable to human impacts and climate change. Future projections show considerable deep-water warming, acidification, and heat accumulation, and moreover, in strong overshoot scenarios, irreversibility is found in various properties in the deep ocean[2]. Here, we compare rates of warming, acidification, and deoxygenation at depth and the seafloor for a range of emission driven idealized overshoot scenarios run with the fully coupled Norwegian Earth System Model version 2 (NorESM2). We discuss the impact that changing ambient conditions have for deep sea ecosystems at the example of Lophelia Pertusa, a common cold-water coral found in the North Atlantic. The continued exposure to calcium carbonate undersaturation and inhibited aerobic activity due to warming and deoxygenation lead to physiologically unsustainable conditions for cold water corals, which could be alleviated by sustained food supply, i.e., increased export production. We therefore conclude by showing different potential habitat extents in relation to environmental stressors under different evolving climates.
We acknowledge the project TipESM “Exploring Tipping Points and Their Impacts Using Earth System Models”. TipESM is funded by the European Union. Grant Agreement number: 101137673. DOI: 10.3030/101137673.
[1] Heinze et al., 2020, The quiet crossing of tipping points, PNAS, 118(9)
[2] Schwinger et al., 2022, Emit now, mitigate later? Earth system reversibility under overshoots of different magnitudes and durations, Earth Syst. Dynam., 13, 1641–1665
How to cite: Fröb, F., Bourgeouis, T., Goris, N., and Schwinger, J.: Deep sea and seafloor ecosystem response to net-zero and negative emissions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17370, https://doi.org/10.5194/egusphere-egu25-17370, 2025.
The objective of the Paris Agreement is to obtain commitments from signing parties to reduce greenhouse gas (GHG) emissions to levels consistent with a global warming well below 2 °C above pre-industrial levels and to pursue further efforts to limit it to 1.5 °C. To achieve such ambitious goals, countries should collectively aim to reach a global peak in GHG emissions as soon as possible and to steadily decrease their emissions after that, to ideally accomplish a climate neutral world by the middle of this century. However, it soon became clear that the efforts being committed by signing parties were not enough to achieve such ambitious goals. As a result, the world is currently on track to warming levels that will either temporarily or permanently exceed the agreed temperature targets. Carbon Dioxide Removal (CDR) can complement the phase-out of fossil fuels, supporting net emission reduction towards the achievement of net-zero and, in case of temperature overshoot, net-negative targets. To reach such goals, while limiting the severity of side-effects, a broad set of CDR options - a CDR portfolio - will be necessary.
The RESCUE project aims to develop realistic scenarios that incorporate a mix of large-scale implementations of technology- and ecosystem-based CDR. These scenarios are based on the current state-of-the-art knowledge regarding various aspects, including the limits and constraints of CDR on both land and ocean. The actual effectiveness of these CDR portfolios depends on a series of mechanisms and feedback loops linked to a changing climate.
Currently, the only tools that allow us to understand, assess, and quantify the processes that might affect the effectiveness of large-scale CDR interventions are Earth System Models (ESMs). However, even state-of-the-art ESMs are not yet equipped to represent the variety of CDR approaches being proposed. The RESCUE project is developing these representations in five European ESMs. The rationale behind these developments is to advance beyond the approach used in CMIP6, where negative emissions were prescribed from socioeconomic models as part of a given scenario. Inline with the CMIP7 emissions-driven experimental design focus, CDR interventions in RESCUE are provided to the ESMs as activities (e.g., area and carbon capture and storage efficiency of bioenergy plantations or alkalinity additions to the ocean) with the potential to alter net land and ocean CO2 fluxes. However, the ESM itself quantifies the actual effectiveness, taking into account known climate interactions and feedbacks. Here, we describe the overall design of these CDR representations, necessary assumptions and future directions for their improvement.
How to cite: Bernardello, R., Schwinger, J., Butenschön, M., Nieradzik, L., Tourigny, E., Miller, P., Peano, D., Wårlind, D., Gupta, S., Bischof, S., Bourgeois, T., Mengis, N., Pongratz, J., Teckentrup, L., Merfort, L., Bauer, N., Gidden, M., Gasser, T., and Tanaka, K.: Representing carbon dioxide removal in Earth System Models: towards an activity-driven framework., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17555, https://doi.org/10.5194/egusphere-egu25-17555, 2025.
Over the last few years, there has been increasing interest in the long-term climate stabilisation response that we might expect when net-zero emissions of greenhouse gases are achieved. These studies often explore the stabilisation response across multiple global warming levels (GWLs). Several studies have now shown that regional patterns of change at given GWLs can be very different between transiently warming through a GWL and stabilising at that same GWL.
In a recent study with the UK Earth System Model 1.0, we showed that stabilising the external forcings and running the model forward for 500 years at various GWLs can stop the decline of southern European summer precipitation and reverse the sign of the trend. In northern Europe, the wetting trend is more substantial, and precipitation projections in UKESM1.0 overshoot the pre-industrial baseline in the second century after stabilisation (Dittus et al. 2024).
In this presentation, we explore the mechanisms contributing to this spring and summertime increase in precipitation in the stabilisation simulations with UKESM1, relative to the transient projections from ScenarioMIP. We show that the frequency of different atmospheric circulation types is changing during the 500 years of stabilisation, and also highlight the important role of the land surface and soil moisture feedbacks onto the hydrological cycle.
Dittus, A. J., Collins, M., Sutton, R., & Hawkins, E. (2024). Reversal of projected European summer precipitation decline in a stabilizing climate. Geophysical Research Letters, 51, e2023GL107448. https://doi.org/10.1029/2023GL107448
How to cite: Dittus, A. and Hawkins, E.: Partial reversal of European summer precipitation decline in stabilisation scenarios: where does the moisture come from? , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19149, https://doi.org/10.5194/egusphere-egu25-19149, 2025.
