Specific topics could include:
- Understanding TCRE and ZEC components, frameworks for investigating the processes and contributions to TCRE, ZEC, and identifying where uncertainty comes from, focus on Land/Ocean processes (e.g., CO2 fertilization, permafrost; ocean co-uptake of heat/CO2)
- Observational or Emergent constraints
- Use of simple models/emulators and model hierarchy.
While international climate policies now focus on limiting global warming to well below 2 °C or pursuing a 1.5 °C level of global warming, the climate modelling community has not provided an experimental design in which all Earth system models (ESMs) converge and stabilize at the same prescribed global warming levels. This gap hampers accurate estimations based on comprehensive ESMs of the carbon emission pathways and budgets needed to meet such agreed warming levels and of the associated climate impacts under temperature stabilization. Here, we apply the Adaptive Emission Reduction Approach (AERA) with ESMs to provide such simulations in which all models converge at 1.5 and 2.0 °C warming levels by adjusting their emissions over time. These emission-driven simulations provide a wide range of emission pathways and resulting atmospheric CO2 projections for a given warming level, uncovering uncertainty ranges that were previously missing in the traditional Coupled Model Intercomparison Project (CMIP) scenarios with prescribed greenhouse gas concentration pathways. Meeting the 1.5 °C warming level requires a 40 % (full model range: 7 % to 76 %) reduction in multi-model mean CO2-forcing-equivalent (CO2-fe) emissions from 2025 to 2030, a 98 % (57 % to 127 %) reduction from 2025 to 2050, and a stabilization at 1.0 (−1.7 to 2.9) PgC yr−1 from 2100 onward after the 1.5 °C global warming level is reached. Meeting the 2.0 °C warming level requires a 47 % (8 % to 92 %) reduction in multi-model mean CO2-fe emissions until 2050 and a stabilization at 1.7 (−1.5 to 2.7) PgC yr−1 from 2100 onward. The on-average positive emissions under stabilized global temperatures are the result of a decreasing transient climate response to cumulative CO2-fe emissions over time under stabilized global warming. This evolution is consistent with a slightly negative zero emissions commitment – initially assumed to be zero – and leads to an increase in the post-2025 CO2-fe emission budget by a factor of 2.2 (−0.8 to 6.9) by 2150 for the 1.5 °C warming level and a factor of 1.4 (0.9 to 2.4) for the 2.0 °C warming level compared to its first estimate in 2025. The median CO2-only carbon budget by 2150, relative to 2020, is 800 GtCO2 for the 1.5 °C warming level and 2250 GtCO2 for the 2.0 °C warming level. These median values exceed the median IPCC AR6 estimates by 60 % for the 1.5 °C warming level and 67 % for 2.0 °C. Some of the differences may be explained by the choice of the mitigation scenario for non-CO2 radiative agents. Overall, this new type of warming-level-based emission-driven simulation offers a more coherent assessment across climate models and opens up a wide range of possibilities for studying both the carbon cycle and climate impacts, such as extreme events, under climate stabilization.
How to cite: Silvy, Y. and Frölicher, T. and the AERA-MIP author team: AERA-MIP: TCRE, emission pathways and remaining budgets compatible with 1.5 and 2 °C global warming stabilization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21870, https://doi.org/10.5194/egusphere-egu25-21870, 2025.
The proportionality between global temperature change and cumulative CO2 emissions underpins our understanding of how climate will respond to future emissions, and what level of emissions reductions will be needed to stabilize global temperatures. Typically, fossil fuel and land-use CO2 emissions are treated as equivalent drivers of this global temperature response, and emissions reductions from both sources are assumed to contribute similarly to mitigation targets. However, measuring land-use CO2 emissions in the real world is complicated by the difficulty in separating direct emissions (those caused by deforestation and other human land-use activities) from indirect carbon fluxes caused by CO2 fertilization and other land carbon responses to changing climate conditions. Consequently, an emission (or removal) of CO2 from land use activities as measured and reported in national emissions inventories is not equivalent to a land-use emission as defined in modelling studies that have been used to quantify the climate response to cumulative fossil fuel and land-use CO2 emissions. Here we assess the impact of these different land carbon accounting conventions on two key metrics of the climate response to cumulative CO2 emissions: the Transient Climate Response to cumulative CO2 Emissions (TCRE) and the Zero Emissions Commitment (ZEC). Using a spatially-explicit intermediate complexity Earth system model, we quantify these two metrics as a function of (1) fossil fuel CO2 emissions only; (2) fossil fuel + direct land-use CO2 emissions; and (3) fossil fuel + net land-use CO2 fluxes including indirect land carbon sinks. We show that both the magnitude and time-dependence of the TCRE and ZEC metrics is sensitive to the inclusion and definition of land carbon emissions. This finding underscores the need for improved clarity and care in the application of scientific findings to real-world mitigation efforts related to land carbon emissions and removals.
How to cite: Matthews, H. D., Zickfeld, K., MacIsaac, A., and Dickau, M.: Effect of land carbon accounting methods on the climate response to cumulative CO2 emissions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13389, https://doi.org/10.5194/egusphere-egu25-13389, 2025.
With a well-studied potential to remove CO2 from the atmosphere, reforestation is a CO2 removal intervention common to net-zero CO2 pathways, policies, and the voluntary CO2 offset market. However, the relationship between a reforestation-based CO2 removal and temperature change is complicated by the biogeophsyical effects of reforestation on temperature, which have a demonstrated uncertainty across climate models. Furthermore, reforestation is a land-based intervention occurring in specific geographic locations and the relationship between reforestation within a specific locality and global temperature change is not well-defined.
Here we address these concerns by asking whether the TCRE framework - the fundamental metric relating anthropogenic CO2 emissions to global temperature change - and its regional variant can be applied to measure the effect of reforestation-based CO2 removal on global temperature. We conduct idealized net-zero CO2 simulations in a climate model of intermediate complexity (the UVic ESCM) to quantify the reforestation-TCRE across large-scales of reforestation. We measure reforestation-based CO2 removals by assessing both the change in above-ground and the change in above and below-ground CO2 in reforested areas as compared to a counter-factual simulation without reforestation. We further isolate the biogeophyical effects of reforestation to constrain the reforestation-TCRE to only the carbon-effects of reforestation. We expect our results to show that the reforestation-TCRE is not equal and opposite to the TCRE, which is accountable to the biogephsical effects of reforestation and asymmetries between the climate effects of a reforestation-based CO2 removal and an anthropogenic CO2 emission. Despite the short-coming, we expect our results to provide a metric for calculating a direct relationship between reforestation-based CO2 removal and global temperature change that is relatable to net-zero frameworks and potentially reproducible across climate models.
How to cite: MacIsaac, A., Zickfeld, K., Matthews, D., and MacDougall, A.: The reforestation-TCRE: A metric to quantify the effect of reforestation on global temperature, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11301, https://doi.org/10.5194/egusphere-egu25-11301, 2025.
TCRE – the linearity between global warming and cumulative anthropogenic CO2 emissions – underpins the concept of remaining carbon budgets and is critical for designing mitigation policies in line with the Paris Agreement. The future response of carbon sinks to anthropogenic perturbations is a major source of uncertainty in estimates of future TCRE. In particular, the strong Arctic warming is expected to lead to permafrost thaw, exposing the large amounts of soil organic carbon stored in permafrost to decomposition, and eventually releasing CO2 and CH4 to the atmosphere in a positive climate-carbon feedback. On the other hand, CO2 fertilisation and permafrost nitrogen release are likely to enhance vegetation carbon uptake by counteracting negative feedbacks. However, both the amplitude and the timing of the resulting future net carbon balance in permafrost regions remain highly uncertain. In particular, previous studies, using either land surface or intermediate complexity models, have shown no consensus on the strength of the nitrogen-mediated feedback. In addition, future TCRE estimates are based on (fully coupled) Earth system model (ESM) projections. However, only two ESMs in the CMIP6 ensemble represent permafrost carbon and the last IPCC assessment of TCRE used external estimates of permafrost carbon cycle feedbacks. The inclusion of permafrost carbon cycle processes in ESMs is therefore necessary to improve the reliability of future projections and inform policy decisions.
Based on the CMIP6 version of the Institut Pierre-Simon Laplace ESM, we developed IPSL-Perm-LandN, a new ESM that includes an explicit land nitrogen cycle and key permafrost physical and biogeochemical processes. Under future increasing atmospheric CO2 concentrations, the permafrost region remains a carbon sink in IPSL-Perm-LandN despite significant soil carbon losses due to permafrost thaw. In particular, we show a strong negative feedback arising from permafrost nitrogen release, which supports a large land carbon uptake and prevents the carbon-climate feedback parameter γ from increasing (negatively) by more than 10 PgC.°C-1. However, this is likely to be overestimated by our representation of soil nitrogen dynamics and plant nitrogen uptake. Our findings highlight the importance of better constraining the nitrogen cycle in permafrost regions and better representing permafrost carbon processes in ESMs to reduce the uncertainty in TCRE and remaining carbon budgets.
How to cite: Gaillard, R., Cadule, P., Peylin, P., Vuichard, N., and Guenet, B.: Implications of permafrost carbon cycle feedbacks for TCRE: evidence from Earth system modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8578, https://doi.org/10.5194/egusphere-egu25-8578, 2025.
While previous research has extensively explored the effects of rising CO2 levels, the response of the climate and carbon cycle to reductions in CO2 remains less understood. In this study, we are going to uncover the asymmetric carbon-climate responses and underlying processes under different emission pathways, including decreasing and negative CO2 emissions.
Based on the Max Planck Institute Earth System Model (MPI-ESM1-2-LR), we have run a large ensemble of simulations incorporating an interactive carbon cycle under different future scenarios to quantify variations in atmospheric CO2 growth, along with carbon sinks in response to changing emissions. We found asynchronous changes in the atmospheric CO2 and emissions driven by carbon sinks, and the ocean and land become CO2 sources after ~2100 under negative emissions. While the climate responses to cumulative emissions along increasing pathways overlap, the responses along decreasing pathways are asymmetric and show uncertainties in the presence of internal climate variability.
Further idealized flat10 simulations with constant positive and negative CO2 emissions allow us to quantify the response of the carbon sink and climate under deep decarbonization. The climate and carbon cycle is irreversible even under the accumulation of zero emissions, featuring a lower global temperature and atmospheric CO2 concentration. An asymmetric response in the carbon uptake and release, and the ocean storage of carbon and heat intervene in the transient responses of climate to the cumulative CO2 emissions.
By leveraging these simulations under diverse scenarios, we seek to enhance our understanding of the transient climate response, providing insights into the potential impacts of emission reduction strategies and the role of negative emissions in climate mitigation.
How to cite: Li, H., Ramme, L., Li, C., and Ilyina, T.: Asymmetric carbon-climate responses to cumulative emissions under different CO2 pathways, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16501, https://doi.org/10.5194/egusphere-egu25-16501, 2025.
Remaining carbon budgets consistent with limiting global warming below certain temperature thresholds are estimated from the transient climate response to emissions (TCRE) and the zero emissions commitment (ZEC). TCRE is the amount of warming per unit of cumulative carbon dioxide emissions, while ZEC is the amount of warming that would occur following a complete cessation of emissions. IPCC AR 6 (Canadell et al, 2023, Chapter 5) concluded with medium confidence that TCRE is nearly constant with time and independent of the rate of emissions (or emissions pathway) and therefore it is a good predictor of CO2-induced warming after emissions reductions, although some studies (MacDougal 2017; Seshadri 2017) have pointed towards pathway dependence at very high and very low emissions rates. In all studies there is the implicit assumption that the cumulative fraction of carbon taken up by the terrestrial biosphere is constant, and that the climate feedback parameter and ocean heat uptake efficacy do not change in time. Using a suite of emissions-driven Earth system model simulations, we explore the impact of immediately halting CO2 emissions under different levels of global warming. We show that the climate system undergoes state shifts when AMOC weakens substantially due to forcing and/or internal variability, and only then significant cooling occurs following CO2 emissions cessation but with considerable consequences for regional climates. We identify ranges of non-zero likelihood for AMOC collapse and the associated global warming levels, emissions thresholds and a possible mechanism linked to high latitude sea ice transport variability. We demonstrate that TCRE, ZEC and ocean heat uptake efficiency are state dependent and, while fast feedbacks control ZEC when mitigation occurs at lower emissions levels, AMOC weakening becomes the leading driver setting ZEC at higher emissions levels. Even the most ambitious mitigation of climate change would be ineffective if action is delayed and the climate system is too close to a tipping point of the Atlantic Meridional Overturning Circulation, since there would be significant differences in the committed warming.
How to cite: Romanou, A.: TCRE, ZEC and ocean heat uptake efficacy response to AMOC, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2921, https://doi.org/10.5194/egusphere-egu25-2921, 2025.
The Zero Emissions Commitment (ZEC) is understood to be the result of two evolving processes in the time after net zero: cooling due to carbon uptake by the land and ocean, and warming due to decreasing heat uptake by the ocean. The balance between these warming and cooling effects is what determines whether we can expect additional global temperature change after emissions stop, or whether zero emissions marks the point of temperature stabilisation. But are there observables prior to net zero that can predict which way this balance will fall?
Using the simple climate model MAGICC, we find ZEC to be a function of a handful of variables in the years leading up to net zero: global surface temperature, carbon uptake, ocean heat uptake and effective radiative forcing. This simple regression performs well for predicting additional global temperature change 50 years after net zero, with reasonable predictability 100, 200 and 1000 years after emissions stop. We find that higher warming at net zero increases the probability of a positive ZEC. We test the predictability of this model in FAIR, and assess the agreement with ESM and EMIC results from ZECMIP. We investigate the potential for constraining ZEC using this model and observables available today.
How to cite: Palazzo Corner, S., Rogelj, J., Nicholls, Z., Jones, C., and Smith, C.: Knowing what we know now: predicting ZEC with observables in a simple climate model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12149, https://doi.org/10.5194/egusphere-egu25-12149, 2025.
Climate models reveal a range of global surface temperature responses after net zero, generally a slight cooling, but sometimes a slight continued warming. This post emission response is affected by a range of processes including carbon uptake by the land and ocean, planetary heat uptake and time-varying climate feedback. To reveal their relative importance, a normalised framework is set out for the Zero Emissions Commitment (ZEC), connecting the change in surface temperature (normalised by the change at net zero) to changes in the atmospheric carbon inventory, radiative forcing, planetary heat uptake and climate feedback. Whether the temperature decreases or continues to rise after net zero is controlled by opposing contributions from (i) a weakening in radiative forcing due to a decrease in atmospheric carbon from the uptake by the land and ocean carbon sinks versus (ii) a strengthening in the surface warming due to a decline in ocean heat uptake and sometimes augmented by time-varying climate feedbacks. Inter-model differences in the post emission temperature response for the ZEC Model Intercomparison Project scenario are primarily determined by differences in the ocean uptake of heat and the land uptake of carbon, followed by differences in the ocean uptake of carbon and time-varying climate feedbacks.
How to cite: Williams, R., Goodwin, P., Ceppi, P., Jones, C., and MacDougall, A.: A normalised framework for the Zero Emission Commitment: competing controls by thermal and carbon processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19007, https://doi.org/10.5194/egusphere-egu25-19007, 2025.
The Zero Emissions Commitment (ZEC) describes the climate response following the cessation of emissions and is critical for understanding long-term climate projections and remaining carbon budgets. Using simulations from the UK Earth System Model (UKESM), we explore ZEC behaviour across stabilized warming levels (WLs) following the protocol developed for TIPMIP. UKESM simulations reveal a strong dependence of ZEC on WL: while ZEC is near zero for WL <=2K, it becomes increasingly positive at higher WLs. This behaviour underscores the importance of disentangling the contributions of the different underlying processes to understand the mechanisms driving ZEC variability.
To explore the drivers of this behaviour, we analyse ZEC across a range of WLs focusing on both the thermal response and carbon cycle dynamics. We find that changes in physical feedbacks dominate the WL dependence of ZEC. However, the carbon cycle response still exhibits notable dynamics: land carbon uptake saturates after a few decades, while ocean uptake persists for centuries, shifting the balance between land and ocean contributions over time. While the climate response is approximately linear during the ramp-up phase, we hypothesize that ZEC is influenced by both the magnitude and duration of warming, reflecting a dependence on the system’s distance from equilibrium. These results highlight the critical role of WL-dependent responses in shaping long-term climate commitment and provide new insights into the mechanisms driving the variation in ZEC across scenarios.
How to cite: Gibbs, L., Wiltshire, A., Jones, C., Jones, C., Liddicoat, S., Williams, R., Andrews, T., Robertson, E., Dittus, A., Swaminathan, R., DeMora, L., Walton, J., Ceppi, P., and Kuhlbrodt, T.: Understanding the Mechanisms Behind Zero Emissions Commitment (ZEC) at Different Warming Levels, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18370, https://doi.org/10.5194/egusphere-egu25-18370, 2025.
Achieving global temperature stabilisation requires net-zero CO₂ emissions, a goal widely recognised within the scientific community. However, a critical and contested question remains: will the Earth's climate continue to warm due to thermal and biogeochemical inertia even after emissions cease? This phenomenon, known as Zero Emissions Commitment (ZEC), has been estimated to likely be 0.0 ºC with a multi-model spread of 0.3°C. Considering its magnitude, ZEC may represent a significant fraction of the remaining warming before the 1.5°C threshold is reached.
In an attempt to constrain uncertainties in ZEC estimates, this study presents findings from a large ensemble of simulations conducted using the University of Victoria Earth System Climate Model (UVic ESCM v2.10). The ensemble design systematically varies model parameters within observationally constrained ranges, targeting processes identified as having the largest potential influence on ZEC (Palazzo-Corner et al., 2023). These parameters include carbon cycle feedbacks, ocean heat uptake, and CO2 fertilisation effects, which are represented with appropriate and acceptable levels of complexity within the UVic ESCM.
In line with the CMIP7 emissions-driven experimental design focus, we employ the esm-flat10-zec as well as esm-flat20-zec, which uses a constant emission rate of 10 PgC/yr and 20 PgC/yr, respectively (Sanderson et al., 2024), with varying cumulative emission budgets. This approach allows for the exploration of ZEC parameter uncertainty under varying emission rates and carbon budgets, increasing our process-based understanding of the metric.
References
Sanderson, B. M. et al. The need for carbon-emissions-driven climate projections in CMIP7. Geoscientific Model Development 17, 8141–8172 (2024).
Palazzo Corner, S. et al. The Zero Emissions Commitment and climate stabilization. Frontiers in Science 1, 1170744 (2023).
How to cite: Hohn, D., Tran, G., De Sisto, M., and Mengis, N.: Constraining uncertainties of the Zero Emissions Commitment with a large ensemble of UVic 2.10 climate model simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15995, https://doi.org/10.5194/egusphere-egu25-15995, 2025.
The relationship between CO2-induced warming and global mean temperature, known as the Transient Climate Response (TCR) to cumulative CO2 emissions (TCRE), is anemergent property of the Earth system. It allows to derive allowable CO2 emissions, or carbon budget, for a given anthropogenic warming threshold, such as the Paris Agreement warming target. The assessment of the TCRE in IPCC AR6 makes use of the theoretical framework as proposed by Jones and Friedlingstein (2020), which separate TCRE in two major drivers: the Transient Climate Response (TCR) and the airborne fraction (AF) of anthropogenic CO2 emissions. While published works have allowed to account for a constrained TCR range in the assessment of the TCRE, estimated of AF results only from unconstrained multi-model outputs.
The present work applies a novel methodology based on Bayesian statistics to integrate multiple lines of historical evidences to constrain future AF. Bayesinas statistics allows to exploits the time-varying relationship between the total anthropogenic emissions of CO2 and AF over the historical period and propagate this relationship in the future to constrain the AF range at CO2 doubling. The narrower very likely range for AF at CO2 doubling 41-59% (50% as Best estimates) results in a constrained very likely range for the TCRE, 1-2.1 K EgC-1 (1.5 as Best estimates). This constrained range is about 20% smaller than the latest assessed range for the TCRE and shines light on how novel observations and monitoring of anthropogenic emissions and airborne fraction of CO2 could results in even stronger constrain on TCRE estimates in the near future.
How to cite: Séférian, R., Ribes, A., and Qasmi, S.: Toward an observational constraints on the Transient Climate Response to Cumulative Emission, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12981, https://doi.org/10.5194/egusphere-egu25-12981, 2025.
One of the defining challenges of our century is to limit global warming. Reducing anthropogenic carbon dioxide emissions to net zero has been understood to be a central measure in achieving this climate goal. Still, after achieving net zero CO2 emissions, the climate system could show a delayed temperature response. This temperature response is called Zero Emissions Commitment (ZEC) and has been estimated to be approximately 0±0.3 K in the ZECMIP multi-model mean (Jones et al., 2019; MacDougall et al., 2020). Understanding and constraining ZEC remains relevant, especially when considering the remaining carbon budget for reaching ambitious climate targets. However, individual climate models show a high level of uncertainty in the ZEC response.
ZEC is closely related to the carbon cycle, the planetary heat uptake and their respective distance to their equilibrium states at the point of net zero. Therefore, we investigate how the pre-industrial state and responsiveness of these processes to anthropogenic climate change relate to their ZEC response in Earth system models simulating the ZECMIP experiments. We aim to characterise the models' ZEC response as a function of the chosen, observable climate variables (e.g., overturning strength at 26° N, global carbon project (Friedlingstein et al., 2023) carbon fluxes, or ocean heat content of the upper 700 m), that will then later serve as basis for observationally constrained ZEC estimates. We will show first preliminary results and invite feedback on the study design.
References
Friedlingstein, P. et al. (2023). “Global Carbon Budget 2023” Earth System Science Data 15 (12): 5301–69. https://doi.org/10.5194/essd-15-5301-2023.
Jones, C.D. et al. (2019). “The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) Contribution to C4MIP: Quantifying Committed Climate Changes Following Zero Carbon Emissions” Geoscientific Model Development 12 (10): 4375–85. https://doi.org/10.5194/gmd-12-4375-2019.
MacDougall, A.H. et al. (2020). “Is There Warming in the Pipeline? A Multi-Model Analysis of the Zero Emissions Commitment from CO2” Biogeosciences 17 (11): 2987–3016. https://doi.org/10.5194/bg-17-2987-2020.
How to cite: Rahm, T., Hohn, D., and Mengis, N.: After 'Net Zero': Tracing uncertainties in the Zero Emissions Commitment signal, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8653, https://doi.org/10.5194/egusphere-egu25-8653, 2025.
Simple models have been used in a variety of ways to summarize Earth system processes related to global warming and its mitigation. This poster will provide a pedagogical survey of the many uses to which these models have been put, focusing especially their role in understanding the regime of prolonged low carbon dioxide (CO2) emissions following emissions peak and decline. We will show that the low emissions regime is exactly where path independence between global warming and cumulative CO2 emission breaks down, giving rise to the possibility of a substantial zero emissions commitment (ZEC). Using a few different simple models, the factors affecting the ZEC in low emissions scenarios and its relation to path dependence will be described, and we will furthermore examine the observability of respective model parameters affecting the magnitude of ZEC.
How to cite: Seshadri, A. K.: The role of simple models in understanding the low CO2 emissions regime and the ZEC, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10543, https://doi.org/10.5194/egusphere-egu25-10543, 2025.
Complex models of the Earth system are increasingly able to represent processes that make up the carbon-climate system, but a variety of simple climate models (SCMs) use parameterized representations of the Earth system, which make them easily deployed tools for climate mitigation assessment and accessible tools for conceptual understanding. However, SCMs vary in their approach to simplifying the Earth system, especially in their representation of the carbon cycle. We examine how two distinct carbon cycle structures within one SCM, FaIR, produce differing constrained projections of future climate under idealized decarbonization. We find that differences in carbon cycle structure lead to differences in the timescales of carbon uptake, which do not directly lead to or explain differences in warming under the same decarbonization emissions scenario. Differences in the metrics of warming are instead primarily explained by assumptions about climate feedbacks and non-carbon cycle forcing, which are parameterized separately from carbon cycling. When we introduce a physically-motivated link reflecting the connection between ocean circulation and energy balance, we see a change in the set of climate feedbacks necessary to explain our observed carbon-climate system. The result is a shift in TCRE, ZEC, and consequent necessary mitigation.
How to cite: Shum, G., Swann, A., Frierson, D., and Koven, C.: Linking carbon cycling to climate feedbacks in a simple climate model for decarbonization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-366, https://doi.org/10.5194/egusphere-egu25-366, 2025.
Understanding the mechanisms governing the evolution of the ocean’s anthropogenic carbon reservoir is critical for assessing its role in the global carbon cycle and susceptibility of the ocean carbon sink to climate change. Anthropogenic carbon, primarily from fossil fuel burning, interacts with and alters the natural carbon cycle, increasing the vulnerability of surface waters to natural carbon leaks. To address these dynamics, we quantify the mechanisms affecting oceanic anthropogenic carbon, including ocean circulation, biological production, and carbonate chemistry, using the Max Planck Institute Earth System Model. By disentangling the multi-factors through separating the evolutions of natural carbon—pre-industrial oceanic carbon pools—and anthropogenic carbon, we aim to develop a clearer and more comprehensive understanding of the ocean carbon cycle. Utilizing idealized emissions-driven simulations, we assess the sensitivity of the ocean carbon sink under varying emission pathways, such as increasing and decreasing CO2 emissions. This mechanistic understanding is crucial to understanding the vulnerability of the ocean carbon sink and monitoring the carbon budget. By linking these insights to the Transient Climate Response to cumulative CO2 Emissions (TCRE), this study contributes to a framework for evaluating carbon cycle feedback under diverse emission pathways.
How to cite: Choi, H.-J., Liu, B., and Ilyina, T.: Understanding the mechanisms driving the ocean’s anthropogenic carbon reservoir under changing emissions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10573, https://doi.org/10.5194/egusphere-egu25-10573, 2025.
The ocean accumulates carbon and heat under anthropogenic CO2 emissions and global warming. In net-negative emissions scenarios, where more CO2 is extracted from the atmosphere than emitted, we expect global cooling. Little is known about how the ocean will release heat and carbon under such a scenario. Here we use an Earth system model of intermediate complexity and show results of an idealized climate change scenario that, following global warming forced by an atmospheric CO2 increase of 1% per year and CO2 doubling at year 70, subsequently features decreasing atmospheric CO2 at a rate of -0.1% per year, implying sustained net-negative emissions. After four hundred years of net-negative emissions and gradual global cooling, abrupt reemergence of heat from the ocean interior leads to a global mean surface temperature increase of several tenths of degrees that lasts for more than a century. The ocean heat "burp" originates in heat that has previously accumulated under global warming in the Southern Ocean at depths and emerges to the ocean surface via deep convection. Surprisingly, this heat burp is largely devoid of CO2. This is because changes in ocean circulation affect heat more than carbon, with an additional muting effect of CO2 loss due to particularities of sea water carbon chemistry. As the ocean heat loss causes a global mean surface temperature increase that is independent of atmospheric CO2 concentrations or emissions, it presents a mechanism that introduces a break down of the quasi-linear relationship of the TCRE.
How to cite: Frenger, I., Frey, S., Oschlies, A., Getzlaff, J., Martin, T., and Koeve, W.: Southern Ocean heat burp in a cooling world, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15339, https://doi.org/10.5194/egusphere-egu25-15339, 2025.
Posters virtual: Mon, 28 Apr, 14:00–15:45 | vPoster spot 5
EGU25-6364 | Posters virtual | VPS5
The double emergence of TCREMon, 28 Apr, 14:00–15:45 (CEST) | vP5.8
The TCRE relationship underlies the necessity of net zero emissions for climate stabilization and the utility of carbon budgets as a policy tool. TCRE emerges near universally from Earth system models, and is consistent with observations. However, recent work has systematically dismantled the leading hypothesis explaining the phenomenon, concluding “that this proportionality is not amenable to a simple physical explanation, but rather arises because of the complex interplay of multiple physical and biogeochemical processes.'' (Gillett, 2023). Here we set two intermediate complexity Earth system models (EMICs) to abiotic states, then turn on broad components of Earth's biogeochemical cycles one at a time to see which combination of processes cause TCRE to emerge.
We find that TCRE emerges when ocean alkalinity is set to observed values, without life on land. TCRE likewise emerges independently when the terrestrial biosphere is turned on, with the ocean in an abiotic low alkalinity state. Idealized experiments with the EMICs show that TCRE occurs for configurations of the Earth system where characteristic timescales of carbon absorption and heat absorption are nearly the same. Our results suggest that the emergence of TCRE does in-fact rely on a simple physical mechanism, but why the living components of Earth system are matching the characteristic timescale of carbon absorption to that of heat remains mysterious.
Gillett, N.P.: Warming proportional to cumulative carbon emissions not explained by heat and carbon sharing mixing processes. Nature Communications 14(1), 6466 (2023)
How to cite: MacDougall, A. and MacIsaac, A.: The double emergence of TCRE, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6364, https://doi.org/10.5194/egusphere-egu25-6364, 2025.
EGU25-11584 | Posters virtual | VPS5
Role of Earth system processes in the Transient Climate Response to cumulative EmissionsMon, 28 Apr, 14:00–15:45 (CEST) | vP5.9
Estimates of remaining carbon emissions budgets to limit global warming to 1.5°C or 2°C rely on the near-linear relationship between the change in global mean temperature and total CO2 emitted since the pre-industrial era. This relationship is known as the Transient Climate Response to cumulative Emissions (TCRE). Previous estimates of TCRE are derived from Earth System Models (ESMs) which are known to lack key processes that affect warming and therefore diagnosed CO2 emissions. Here we use the UK Earth System Model to quantify, for the first time, the impact on TCRE of including six Earth system processes in isolation (results in parenthesis): fire-vegetation interactions (TCRE increased 14.6%); nitrogen limitation of vegetation (+9.7%); diffuse radiation effects on vegetation (+8.5%); changes in vegetation distribution (-1.5%); climate impacts from wetland methane emissions (+5.1%) and from biogenic volatile organic compounds (-1.4%). From these results we recalculate the TCRE of 11 ESMs of the 6th Coupled Model Intercomparison Project (CMIP6) as though each included all six processes. Averaged over the 11 models, TCRE increased by 23.7%, reducing by 19% the associated remaining carbon budget to both 1.5°C and 2°C.
How to cite: Liddicoat, S., Jones, C., Mercado, L., Robertson, E., Sitch, S., and Wiltshire, A.: Role of Earth system processes in the Transient Climate Response to cumulative Emissions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11584, https://doi.org/10.5194/egusphere-egu25-11584, 2025.
