CL3.2.1 | Towards a net-zero world and beyond: remaining carbon budgets, ambitious mitigation pathways with or without temperature overshoot, and implications for policy
Towards a net-zero world and beyond: remaining carbon budgets, ambitious mitigation pathways with or without temperature overshoot, and implications for policy
Co-organized by BG8
Convener: Andrew MacDougall | Co-conveners: Peter PfleidererECSECS, Joeri Rogelj, Nadine MengisECSECS, Norman Julius SteinertECSECS, Emily TheokritoffECSECS
Orals
| Mon, 15 Apr, 16:15–18:00 (CEST)
 
Room E2
Posters on site
| Attendance Mon, 15 Apr, 10:45–12:30 (CEST) | Display Mon, 15 Apr, 08:30–12:30
 
Hall X5
Orals |
Mon, 16:15
Mon, 10:45
Achieving the climate goals of the Paris Agreement requires deep greenhouse gas emissions reductions towards a net-zero world. Advancements in mitigation-relevant science should continuously inform the strategies and measures that society pursues to achieve net zero. This session aims to further our understanding of the climate response with particular interest in remaining carbon budgets, emission pathways entailing net-zero targets and overshoot, carbon dioxide removal strategies, the theoretical underpinnings of these concepts, and their policy implications. We invite contributions that use various tools, including fully coupled Earth System Models (ESMs), Integrated Assessment Models (IAMs), or simple climate model emulators.

This year, we have a special focus on risks inherent to overshoot scenarios that have so far been under-researched. Those risks can be related to 1) the feasibility of the large-scale deployment of negative emissions (e.g., carbon dioxide removal) technologies, 2) the potential for long-term irreversible climate impacts even in cases where global warming is reverted, and 3) their implications for climate change (mal)adaptation.

We welcome studies exploring all aspects of climate change and its impacts in response to future ambitious mitigation scenarios. In addition to studies exploring the remaining carbon budget and the TCRE framework, we welcome contributions on the zero emissions commitment (ZEC), effects of different forcings and feedbacks (e.g. permafrost carbon feedback) and non-CO2 forcings (e.g. aerosols, and other non-CO2 greenhouse gases), and climate effects of carbon removal strategies. Additionally, we welcome submissions on the climate response to emission pathway and rate, and the climate-carbon responses to different forcing scenarios or implementations (e.g. SSP scenarios, or idealized scenarios). Contributions from the fields of climate policy and economics focused on applications of carbon budgets, net-zero pathways including residual emission estimates and benefits of early mitigation are also encouraged.

Orals: Mon, 15 Apr | Room E2

Chairpersons: Joeri Rogelj, Peter Pfleiderer, Andrew MacDougall
16:15–16:20
Carbon Budgets, TCRE, and ZEC
16:20–16:30
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EGU24-13603
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solicited
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Virtual presentation
Nathan Gillett

The constant ratio of global warming to cumulative CO2 emissions underpins the use of remaining carbon budgets as policy tools, and the need to reach net zero CO2 emissions to stabilize global mean temperature. One requirement for this proportionality is that the temperature response to a pulse emission of CO2 is independent of the background emissions scenario, and this property has been explained by a balance between the logarithmic dependence of radiative forcing on CO2 concentration, and the saturation of CO2 sinks at higher CO2 levels. Several studies have argued that this proportionality also arises because heat and carbon are mixed into the ocean by similar physical processes, and this argument was echoed in the Intergovernmental Panel on Climate Change Sixth Assessment Report. However, contrary to this hypothesis, atmosphere-ocean fluxes of heat and carbon evolve very differently to each other in abrupt CO2 increase experiments in five earth system models, and changes in the atmosphere, ocean and land carbon pools all contribute to making warming proportional to cumulative emissions. Moreover, an analytical model only exhibits proportional heat and carbon fluxes and proportional warming to cumulative emissions if the land and atmosphere carbon pools are neglected, among other unrealistic assumptions. These results strongly suggest 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.

How to cite: Gillett, N.: What explains the proportionality of global warming to cumulative carbon emissions?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13603, https://doi.org/10.5194/egusphere-egu24-13603, 2024.

16:30–16:40
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EGU24-13448
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Highlight
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On-site presentation
Andrew King, Tilo Ziehn, Matthew Chamberlain, Alexander Borowiak, Josephine Brown, Liam Cassidy, Andrea Dittus, Michael Grose, Nicola Maher, Seungmok Paik, Sarah Perkins-Kirkpatrick, and Aditya Sengupta

Under the Paris Agreement, signatory nations aim to keep global warming well below 2°C above pre-industrial levels and preferably below 1.5°C. This implicitly requires achieving net-zero or net-negative greenhouse gas emissions to ensure long-term global temperature stabilisation or reduction. Despite this requirement, there have been few analyses of stabilised climates and there is a lack of model experiments to address our need for understanding the implications of the Paris Agreement for the Earth system. Here, we describe a new set of experiments using the Australian Community Climate and Earth System Simulator earth system model (ACCESS-ESM-1.5) that enables analysis of climate evolution under net-zero emissions, and we present initial results. Seven 1000-year long simulations were run with global temperatures stabilising at levels in line with the Paris Agreement and at a range of higher global warming levels. We provide a brief overview of the experimental design and show how these model simulations may be used to understand possible net-zero emissions climates. We find major consequences of delayed attainment of global net-zero carbon dioxide emissions for different aspects of the climate system. As the climate stabilises under net-zero emissions, we identify significant and robust changes in temperature and precipitation patterns including continued Southern Ocean warming and reversal of transient mid-latitude drying trends. Regional climate changes under net-zero emissions differ greatly including contrasting trajectories of sea ice extent between the Arctic and Antarctic. While Arctic sea ice extent is projected to stabilise under net-zero emissions, sustained Southern Ocean warming is associated with continued sea ice decline in the Antarctic. We also examine the El Niño-Southern Oscillation (ENSO) and find evidence of reduced amplitude and frequency of ENSO events under climate stabilisation relative to projections under transient warming. An analysis at specific global warming levels shows significant regional changes continue for centuries after emissions cessation. Our findings suggest substantial long-term climate changes are possible even under net-zero emissions pathways. We hope these simulations will be of use to the community and that they motivate further experiments and analyses based on other earth system models.

How to cite: King, A., Ziehn, T., Chamberlain, M., Borowiak, A., Brown, J., Cassidy, L., Dittus, A., Grose, M., Maher, N., Paik, S., Perkins-Kirkpatrick, S., and Sengupta, A.: Exploring climate stabilisation at different global warming levels in ACCESS-ESM-1.5, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13448, https://doi.org/10.5194/egusphere-egu24-13448, 2024.

16:40–16:50
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EGU24-19759
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Highlight
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On-site presentation
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Benjamin Sanderson, Chris Smith, Charles Koven, Zeb Nicholls, Norman Steinert, and Marit Sandstad

The concept of a remaining carbon budget associated with global warming levels has underpinned mitigation efforts since the Paris Agreement.  However, as observed temperatures near 1.5 degrees, a number of challenges have emerged for the continued use of carbon budgets to frame mitigation needs.  Firstly, while the transient response to cumulative emissions describes the temperature response to constant emissions - Paris-compatible pathways require deep emissions cuts and potentially extended periods of negative emissions, the temperature outcome of which is complicated by zero emissions commitments and non-CO2 responses.  Understanding of Zero emissions commitments has been thus far been primarily informed by idealised experiments which terminate emissions during an idealised concentration ramp - but these metrics are subject to unrealistic termination shocks and model-specific emissions pathways.  Second, non-CO2 responses remain highly uncertain, and recent satellite observations of global radiative imbalance raise further questions on the adequacy of current modeling platforms to describe the warming which should be expected due to aerosol phaseout. 

Here, we consider how two novel developments impact carbon budgets beyond estimates presented in the IPCC 6th Assessment.  Firstly, we present an ESM ensemble of climate reversibility experiments which provides a more realistic proxy for non-TCRE carbon dynamics during a net zero transition for use in carbon budgets.  Secondly, we consider how the inclusion of recent global mean temperature measurements and CERES top of atmosphere radiative flux measurements would impacts the calibration of simple climate models - with subsequent impacts on both estimates of both TCRE and expected warming due to non-CO2 effects.  Synthesising this information, we provide an updated estimate of carbon budgets and timing with respect to the 1.5 and 2 degree thresholds.

How to cite: Sanderson, B., Smith, C., Koven, C., Nicholls, Z., Steinert, N., and Sandstad, M.: Revising carbon budgets in a 1.5 degree world, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19759, https://doi.org/10.5194/egusphere-egu24-19759, 2024.

Negative Emissions, Overshoot, and Climate Reversibility
16:50–17:00
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EGU24-12476
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Highlight
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On-site presentation
Carl-Friedrich Schleussner

Global emission reduction efforts continue to be insufficient to meet the temperature goal of the Paris Agreement. This makes the systematic exploration of so-called overshoot pathways that draw temperatures back down to safer levels in the long term a priority for science and policy.

I will present major insights from the Horizon 2020 PROVIDE project on overshoot pathways. We find that global and regional climate change in a post-overshoot world would be substantially different from a world that avoided overshoot, bearing profound implications for adaptation needs. Irrespective of the peak warming, we find that achieving declining global temperature remains critical for limiting long-term climate risks including sea-level rise and cryosphere changes. Reversal of warming by deploying carbon dioxide removal (CDR) at scale, however, is not guaranteed. In addition to uncertain technical and sustainability limitations of CDR, we find that a preventive CDR capacity of several hundred gigatonnes might be desirable to hedge against strong Earth system feedbacks that amplify warming. Aiming for temperature decline is thus not a robust strategy to achieve a climate objective, but rather one part of a broader approach towards managing long-term climate risks. It is no replacement for stringent near-term emission reductions to limit risks at peak warming in the first place.

How to cite: Schleussner, C.-F.: Beyond the Peak: What we know and don't know about temperature overshoot, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12476, https://doi.org/10.5194/egusphere-egu24-12476, 2024.

17:00–17:10
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EGU24-12063
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ECS
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On-site presentation
Fabrice Lacroix, Friedrich Burger, Yona Silvy, Regina Rodrigues, Carl F. Schleussner, and Thomas L. Frölicher

Our understanding of impacts and feedbacks associated with temporarily overshooting the Paris Agreement temperature goal - where the 1.5 °C global warming target is exceeded and retraced at a later time period - is currently limited. Such overshoot scenarios are of increasing likelihood and have the potential to be devasting in terms of both their peak impacts and irreversibility, affecting natural and human systems.

Here, we apply the Earth System Model GFDL-ESM2M coupled to the Adaptive Emission Reduction Approach (AERA) in order to perform novel policy-relevant simulations over the 1861 to 2500 period that temporarily overshoot the global warming target of 1.5 °C at various levels of peak global warming (2.0, 2.5 and 3.0 °C), and compare these to a reference scenario that stabilizes at 1.5 °C. We use this framework to isolate features arising from the overshoots, and investigate (1) negative emissions needed to reverse an overshoot and their impacts for cumulative emissions, (2) spatial differences in surface warming and oceanic heat content between overshoot and 1.5°C stabilization case, and (3) impacts that these spatial differences have for precipitation, sea level rise and ocean ecosystem stressors.

Our framework suggests levels of negative carbon emissions of up to 9 Pg C yr-1 to revert the global temperature the most extreme overshoot of 3.0 °C back to 1.5 °C, with less cumulative emissions allowed in the long-term than in the 1.5 °C simulation to maintain global temperature at 1.5°C. We detect long-term high latitude warming of up to 2.1 °C averaged over the North Atlantic and 0.5 °C over the Southern Ocean that persists after the overshoot. We attribute the persistent warming in the high latitudes to the recovery of both Atlantic Meridional Overturning Circulation and Antarctic abyssal overturning, which retrace to even higher levels in the overshoots than in the 1.5 °C stabilization case. These impact the distribution of precipitation, for instance stronger precipitation found in the high latitudes in the overshoots, as wells as the Pacific Walker Cell. The model also shows that due to excess heat storage in the subsurface of low latitudinal oceans, sea level rise does not recover back to 1.5 °C stabilization levels in overshoot scenarios, remaining up to 20 % higher in the strongest overshoot. The persistent long-term changes that the overshoots that we detect imply consequences for regional climates, cryosphere and marine ecosystems lasting for decades or even centuries after the overshoot reversal. 

How to cite: Lacroix, F., Burger, F., Silvy, Y., Rodrigues, R., Schleussner, C. F., and Frölicher, T. L.: Long-Term Negative Emissions and Irreversibilities following Temporary Overshoots: An Earth System Model Perspective, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12063, https://doi.org/10.5194/egusphere-egu24-12063, 2024.

17:10–17:20
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EGU24-5598
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ECS
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On-site presentation
Aurich Jeltsch-Thömmes, Giang Tran, Sebastian Lienert, David Keller, Andreas Oschlies, and Fortunat Joos

Carbon Dioxide Removal (CDR) is now widely discussed for offsetting residual greenhouse gas emissions or even reversing climate change. For example, all emissions scenarios of the Intergovernmental Panel on Climate Change that meet the “well below 2°C” warming target of the Paris Agreement include CDR. Ocean alkalinity enhancement (OAE) may be one possible CDR where the carbon uptake of the ocean is increased by artificial alkalinity addition. Here, we apply the Bern3D-LPX and the UVic Earth system models of intermediate complexity in observationally-constrained large perturbed parameter ensembles to investigate the effect of massive OAE on modelled carbon reservoirs and fluxes. OAE is assumed to be technically successful and deployed as an additional CDR in the SSP5-3.4 temperature overshoot scenario. 

Trade-offs involving feedbacks with atmospheric CO2 result in a low efficiency of an alkalinity-driven atmospheric CO2 reduction of -0.35 [-0.37 – -0.33] mol C per mol alkalinity addition (skill-weighted mean and 68% c.i.). The alkalinity-driven ocean carbon uptake is partly offset by the release of carbon from the land biosphere and a reduced ocean carbon sink in response to lowered atmospheric CO2 under OAE.
We further apply the Bern3D-LPX ensemble in idealized simulations, in which ΔSAT increases first to ~2°C and then declines to ~1.5°C, to investigate lags in surface air temperature change (ΔSAT). In these simulations, ΔSAT lags the decline in CO
2-forcing by decades, depending on the equilibrium climate sensitivity of the respective ensemble member.
Finally, we use the Bern3D-LPX ensemble simulations and the, in comparison to earlier studies with the Bern3D-LPX model, updated and longer observational records to assess climate metrics such as the transient climate response to emissions, the transient climate response, and the equilibrium climate sensitivity.
 

Our results suggest that massive OAE, if technically and socio-economically achievable, might be able to lower atmospheric CO2 but considering the trade-offs and lags, not emitting carbon is preferable. 

How to cite: Jeltsch-Thömmes, A., Tran, G., Lienert, S., Keller, D., Oschlies, A., and Joos, F.: Carbon dioxide removal: trade-offs and lags in large perturbed parameter simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5598, https://doi.org/10.5194/egusphere-egu24-5598, 2024.

17:20–17:30
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EGU24-20890
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On-site presentation
Nico Bauer, David Keller, Julius Garbe, Kristine Karstens, Franziska Piontek, Werner von Bloh, Wim Thiery, Maria Zeitz, Matthias Mengel, Jessica Strefler, Kirsten Thonicke, and Ricarda Winkelmann

Temperature targets of the Paris Agreement limit global net cumulative emissions to very tight carbon budgets. The possibility to overshoot the budget and offset near-term excess emissions by net-negative emissions is considered economically attractive as it eases near-term mitigation pressure. While potential side effects of carbon removal deployment are discussed extensively, the additional climate risks and the impacts and damages have attracted less attention. We link six models for an integrative analysis of the climatic, environmental and socio-economic consequences of temporarily overshooting a carbon budget consistent with the 1.5 ◦C temperature target along the cause-effect chain from emissions and carbon removals to climate risks and impact. Global climatic indicators such as CO2-concentration and mean temperature closely follow the carbon budget overshoot with mid-century peaks of 50 ppmv and 0.35 ◦C, respectively. Our findings highlight that investigating overshoot scenarios requires temporally and spatially differentiated analysis of climate, environmental and socioeconomic systems. We find persistent and spatially heterogeneous differences in the distribution of carbon across various pools, ocean heat content, sea-level rise as well as economic damages. Moreover, we find that key impacts, including degradation of marine ecosystem, heat wave exposure and economic damages, are more severe in equatorial areas than in higher latitudes, although absolute temperature changes being stronger in higher latitudes. The detrimental effects of a 1.5 ◦C warming and the additional effects due to overshoots are strongest in non-OECD countries (Organization for Economic Cooperation and Development). Constraining the overshoot inflates CO2 prices, thus shifting carbon removal towards early afforestation while reducing the total cumulative deployment only slightly, while mitigation costs increase sharply in developing countries. Thus, scenarios with carbon budget overshoots can reverse global mean temperature increase but imply more persistent and geographically heterogeneous impacts. Overall, the decision about overshooting implies more severe trade-offs between mitigation and impacts in developing countries.

How to cite: Bauer, N., Keller, D., Garbe, J., Karstens, K., Piontek, F., von Bloh, W., Thiery, W., Zeitz, M., Mengel, M., Strefler, J., Thonicke, K., and Winkelmann, R.: Exploring risks and benefits of overshooting a 1.5 ◦C carbon budget over space and time, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20890, https://doi.org/10.5194/egusphere-egu24-20890, 2024.

17:30–17:40
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EGU24-10449
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On-site presentation
Yiannis Moustakis, Tobias Nützel, Hao-Wei Wey, and Julia Pongratz

Even though it has been estimated that country-level commitments on Afforestation/Reforestation (AR) are quite ambitious, amounting globally to 633Mha by 2060, typically modelling studies either apply only moderate levels of AR or are fully idealized, thus not taking into account technoeconomic and biodiversity considerations. Typically, high-end emission trajectories are also employed, yielding strong fertilization of vegetation by elevated CO2 levels and thus enhanced terrestrial carbon stocks, while the CDR potentials over more strongly mitigated pathways remain understudied. This is especially the case for overshoot pathways that are gaining research interest recently, given their relevance for reaching the more ambitious 1.5oC goal.

Here, with the fully coupled MPI-Earth System Model we investigate the mitigation potential of an ambitious yet spatiotemporally plausible AR scenario under an overshoot emission trajectory (SSP5-3.4os). The developed AR scenario employed here is commensurate with country commitments in 2060 and extends to 2100 reaching 935 Mha globally and is constrained by technoeconomic considerations based on a multitude of 1,259 Integrated Assessment Model-generated pathways. To further constrain the scenario, we consider biodiversity and restoration priority maps.

Based on a big ensemble member approach allowing for robust probabilistic analysis, our results demonstrate that ambitious AR can robustly mitigate global temperature in 2100 by 0.2oC, peak temperature by 0.09oC, and reduce temperature overshoot duration by 13 years, while also delaying the land carbon sink-to-source transition by ~10 years, compared to a reference scenario with constant land-use at 2015 levels. Temperature mitigation emerges also at the local scale, where biogeochemically-induced cooling compensates for any biogeophysically-induced local warming.

Overall, ambitious AR should be considered as a useful mitigation tool complementary to drastic emissions reduction even under more strongly mitigated pathways, despite potentially weaker CO2 fertilization.

How to cite: Moustakis, Y., Nützel, T., Wey, H.-W., and Pongratz, J.: Can ambitious forestation mitigate temperature overshoot?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10449, https://doi.org/10.5194/egusphere-egu24-10449, 2024.

17:40–17:50
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EGU24-3169
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ECS
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On-site presentation
Chahan M. Kropf, Kam Lam Leung, and Jamie W. Mc Caughey

Heat stress is a significant threat to human health and well-being, particularly in urban areas, and is expected to worsen in the future due to climate change. This study investigates the impacts of heat stress on human health in Lisbon, Portugal, during both days and nights under climate overshoot scenarios. The study employs a single hazard multi-impact approach to assess the health impacts of heat stress, considering both acute and long-term effects. The results show that the impacts of heat stress on human health are unequally distributed across the population, with some parishes being more affected than others. The study also finds that the impacts of heat stress will increase dramatically under current climate policies. In the daytime, heat stress is primarily driven by heat waves and maximum temperatures, leading to acute effects on human health, such as mortality. These effects are most pronounced in certain parishes and are expected to increase significantly even by 2040. Behavioural adaptation strategies such as adapting working hours have some potential to reduce heat impacts in certain settings. At night, heat stress is primarily driven by minimum daily temperatures, leading to sleep loss and long-term effects on health. Adaptation options for mitigating these impacts might require infrastructure investments. These findings highlight the need for targeted adaptation strategies to address the unequal distribution of heat stress impacts even under climate overshoot.

How to cite: Kropf, C. M., Leung, K. L., and Mc Caughey, J. W.: Heat impacts during days and nights under climate overshoot: a single hazard multi-impact approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3169, https://doi.org/10.5194/egusphere-egu24-3169, 2024.

17:50–18:00
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EGU24-18992
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ECS
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On-site presentation
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Stefanie Rynders, Yevgeny Aksenov, Jörg Schwinger, Timothée Bourgeois, and Chris Jones

It is now expected carbon emissions will follow an overshoot trajectory. A realistic emission-driven overshoot scenario esm-SSP534-over is available from the CMIP6 archive. We analysed the simulations to examine reversibility of the Arctic sea ice cover. Reversibility here means that at the end of the 21st century the sea ice extent is the same as that at the earlier point in the century with the same atmospheric CO2 concentration. Firstly, in an emission driven simulation the system behaves differently on the upward and downward branches of CO2 concentration. We show it is better to use atmospheric CO2 concentration rather than Arctic surface air temperature, as the relation between the two is not linear. Total Arctic sea ice extent shows consistent behaviour in 3 out of 4 models (CNRM-ESM2, MIROC, UKESM1) with a CO2 concentration threshold above which sea ice becomes irreversible. This can be explained by the continued ocean heat transport into the Arctic even though the Atlantic Meridional Overturing Circulation (AMOC) declines. The NorESM model has very different behaviour, sea ice extent is reversible and even overshoots beyond the present-day extent. We suggest this is caused by the known strong AMOC decline in this model. The analysis indicates Arctic air temperature is a result of the changes in sea ice extent rather than the driving factor, as is often assumed, both ultimately controlled by ocean heat transport. From the available simulations we conclude there is large uncertainty in the future Arctic climate state. This uncertainty extends to the future global air temperatures as different models show different inertia on CO2 concentrations, which only materialises in the downward emission branch. This affects many other climate variables with their own time lag. Climate inertia and time delays in the earth system should be investigated further to improve fidelity of future projection. This necessitates the use of emission-driven scenarios instead of concentration-driven ones which do not allow for the full inclusion of internal earth system feedbacks. 

We acknowledge funding from the projects COMFORT (grant agreement no. 820989) and OceanNETs (grant agreement no. 869357) under the European Union’s Horizon 2020 research and innovation programme, and from the EC Horizon Europe project OptimESM “Optimal High Resolution Earth System Models for Exploring Future Climate Changes”, grant 101081193 and UKRI grant 10039429, from the project EPOC, EU grant 101059547 and UKRI grant 10038003. For the EU projects the work reflects only the authors’ view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains.

How to cite: Rynders, S., Aksenov, Y., Schwinger, J., Bourgeois, T., and Jones, C.: On the reversibility of Arctic sea ice loss, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18992, https://doi.org/10.5194/egusphere-egu24-18992, 2024.

Posters on site: Mon, 15 Apr, 10:45–12:30 | Hall X5

Display time: Mon, 15 Apr 08:30–Mon, 15 Apr 12:30
Chairpersons: Nadine Mengis, Norman Julius Steinert, Emily Theokritoff
X5.60
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EGU24-218
Matthew Gidden, Thomas Gasser, Giacomo Grassi, Nicklas Forsell, Iris Janssens, William Lamb, Jan Minx, Zebedee Nicholls, Jan Steinhauser, and Keywan Riahi

Global mitigation pathways play a critical role in informing climate policies and targets that are in line with international climate goals. However, it is not possible to directly compare modelled results with national inventories used to assess progress under the UNFCCC due to differences in how land-based fluxes are accounted for.

National inventories consider carbon flux on managed land using an area-based approach with managed land-areas determined by nations. Emissions scenarios consider a different managed land area and are calibrated against data from detailed global carbon cycle models that account for natural (indirect) and anthropogenic (direct) fluxes separately by design. 

To disentangle the direct and indirect components of land-based carbon fluxes, we use a reduced complexity climate model with explicit treatment of the land-use sector, OSCAR, one of the models used by the Global Carbon Project. We find the discrepancy between model and NGHGI-based accounting methods globally to be 4.4 ± 1.0 Gt CO2 yr-1 averaged over the 2000-2020 time period, which is in line with existing estimates. We then apply OSCAR to the set of pathways assessed by the IPCC to quantify how this gap evolves over time and estimate how key mitigation benchmarks change.

Across both 1.5°C and 2°C scenarios, LULUCF emissions pathways aligned with NGHGI accounting practices show a strong increase in the total land sink until around mid-century. However, the ‘NGHGI alignment gap’  decreases over this period, converging in the 2050-2060s for 1.5°C scenarios and 2070s-2080s for 2°C scenarios. The convergence is primarily a result of the simulated stabilization and then decrease of the CO2-fertilization effect as well as background climate warming reducing the overall effectiveness of the land sink, which in turn reduces the indirect removals considered by NGHGIs. These dynamics lead to land-based emissions reversing their downward trend in most NGHGI-aligned scenarios by mid-century, and result in the LULUCF sector becoming a net-source of emissions by 2100 in about 25% of both 1.5°C and 2°C scenarios.

Assessing emission pathways using LULUCF definitions from national inventory accounting results in downward revisions to emissions benchmarks derived from scenarios. NGHGI-aligned pathways result in earlier net-zero CO2 emissions by around 2-5 years for both 1.5°C and 2°C scenarios, and 2030 emission reductions relative to 2020 are enhanced by about 5 percentage points for both pathway categories. When incorporating the additional land removals considered by NGHGIs, the assessed cumulative net CO2 emissions to global net-zero CO2 also decreases systematically by 15-18% for both 1.5°C and 2°C scenarios.

We find that increasing removals from direct fluxes in 1.5C scenarios overtake estimated removals using NGHGI conventions in the near term. However, by midcentury, the strengthening of direct removals is balanced by weakening of indirect removals, meaning that, on average, carbon removal on land accounted for using NGHGI conventions in 1.5C scenarios results in about half of the LULUCF removals in current policy scenarios. 

We discuss the implications of our results for future Global Stocktakes and market mechanisms under the Paris Agreement.

How to cite: Gidden, M., Gasser, T., Grassi, G., Forsell, N., Janssens, I., Lamb, W., Minx, J., Nicholls, Z., Steinhauser, J., and Riahi, K.: Aligning climate scenarios to emissions inventories shifts global benchmarks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-218, https://doi.org/10.5194/egusphere-egu24-218, 2024.

X5.61
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EGU24-1645
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ECS
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Ruben Prütz, Joeri Rogelj, Sabine Fuss, Jeff Price, Nicole Forstenhäusler, Rachel Warren, Andrey Lessa Derci Augustynczik, Petr Havlík, and Florian Kraxner

Background: Due to ongoing delays in deep global emission reductions, and as more and more countries set national net-zero CO2 targets, carbon dioxide removal (CDR) is continuously gaining importance and attention. Virtually all Paris-aligned AR6 mitigation pathways imply gigatonne-scale CO2 removal even before mid-century, with further upscaling thereafter. Integrated assessment models, used to explore the solution space, currently primarily rely on removals via bioenergy with carbon capture and storage (BECCS) and afforestation, which require massive amounts of land to meet scenario-implied removal scales. This substantial land demand is expected to have severe consequences for biodiversity, which could limit the sustainable scaling potential of these CDR options. Meanwhile, depending on the mode of implementation, afforestation could theoretically benefit habitat conservation in some cases, easing the immense pressure on biodiversity due to ongoing global warming and deforestation.

Objective: By combining spatially-resolved data on biodiversity refugia with spatial time series data from the Global Biosphere Management Model (GLOBIOM) on bioenergy crop plantations and afforestation under different mitigation scenarios, we estimate and compare land use and warming-related pressure on remaining global biodiversity refugia. We compare different biodiversity recovery assumptions after peak warming, consider the land use pressure of ongoing deforestation, and explore additional warming-related refugia loss when excluding CDR from scenarios.

Preliminary results: We show how scenarios with more ambitious temperature outcomes result in higher land use-related pressure on remaining biodiversity refugia areas as more land-intensive CDR is implied in such pathways. Meanwhile, more decisive climate action, including more CDR, substantially reduces the warming-related loss of remaining biodiversity refugia areas. The underlying biodiversity recovery assumptions strongly impact the degree of warming-related refugia loss with considerably less influence on land use-related implications. Generally, the perceived trends are stronger towards 2100 compared to mid-century.

How to cite: Prütz, R., Rogelj, J., Fuss, S., Price, J., Forstenhäusler, N., Warren, R., Derci Augustynczik, A. L., Havlík, P., and Kraxner, F.: Spatial analysis of CDR implications for global biodiversity refugia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1645, https://doi.org/10.5194/egusphere-egu24-1645, 2024.

X5.62
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EGU24-5361
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ECS
Tom Schürmann, Moritz Adam, and Kira Rehfeld

The biosphere’s first-order response to changing Earth system conditions shifts under future emission pathways. One can anticipate such low-order responses, like rebounding carbon stocks once emissions diminish when forecasting emission budgets. However, the impact of changes in second- and higher-order biosphere variability on emission pathways and the prospective large-scale artificial carbon dioxide removal (CDR) remains unclear. An example of such higher-order responses is the vulnerability of land carbon uptake to more extreme climate forcing. In addition, implementing CDR has notable implications for land use, exerting an influence on spatial and temporal biosphere variability. Thus, constraining the interplay between the biosphere’s variability and the emission pathway could inform future emission accounting.

Here, we leverage state-of-the-art Earth system model simulations to investigate the magnitude and pathway-dependency of interactions between the terrestrial biosphere’s variability and the emission pathway. We characterize biosphere variability under different emission scenarios and with varying degrees of representing CO2 removal. The emission- and concentration-driven simulations cover pathways that reach Paris targets without and with temperature overshoot. CDR is either implicitly represented in emission and land use scenarios or explicitly simulated in the model’s land component to match the respective socio-economic pathway.

To understand the structure of modeled biosphere variability under the different pathways and test the consistency of the joint model system, we investigate regional events like a vegetation expansion event in the Northern Sahara. Here, the objective is to examine the interplay between terrestrial carbon fluxes and CDR utilization in detail on a smaller scale, later expanding to the global level. The following research focuses on identifying and quantifying shifts in biosphere variability over time, their interplay with CDR measures, and their effects on global carbon stocks. We test the model’s sensitivity to design choices (how and where CDR is represented) and pathway (emission target, timing, and duration of temperature overshoot). To this end, we aim to infer the margin of error biosphere variability could cause in emission accounting. Our results will help to evaluate the significance of varying biospheric carbon fluxes for future emission stock-taking in the context of CDR.

How to cite: Schürmann, T., Adam, M., and Rehfeld, K.: Implications of biosphere variability for future emission budgets and carbon dioxide removal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5361, https://doi.org/10.5194/egusphere-egu24-5361, 2024.

X5.63
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EGU24-5522
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Highlight
William Collins

The decrease in carbon dioxide concentrations following a zeroing of emissions leads to a cooling that approximately balances the hidden warming from past emissions, due to the similarity of the timescales of climate response and carbon cycle response. But what are the climate implications of zero emissions of chemically-reactive gases such as nitrous oxide, halocarbons and methane with response timescales that don’t align with those of the climate system?

In this work we invert the analytical formulae used by the IPCC to represent the evolution of climate, to derive the time evolution of radiative forcing needed to stabilise temperatures. We find that stabilising the warming attributable to any gas requires decreases in radiative forcing that depend on the past history of that gas (more rapid historical ramp-up requires stronger future mitigation). We show that for reactive gases the analytically-derived radiative forcing decreases are most closely matched by step-like cuts in emissions, but that even for long-lived gasses such as nitrous oxide the emissions cuts do not need to be 100%. N2O emission cuts of 60-80% are sufficient to stabilise its temperature contribution - depending on the previous emission history.

It has been suggested that a more ambitious goal is to mitigate reactive gases sufficiently that their contribution to temperatures reduces rather than stabilises. We show that the above methodology can equally be applied to a declining temperature profile and so were are able to quantify the cuts in reactive gas emissions consistent with achieving desired cooling goals.

How to cite: Collins, W.: What does Net Zero mean for reactive gases?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5522, https://doi.org/10.5194/egusphere-egu24-5522, 2024.

X5.64
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EGU24-7147
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ECS
Hyun Seung-Hwon and An Soon-Il

In this study, we analyze the results of a multiple ensemble experiment using a single model (CESM2) for the Zero Emission Commitment (ZEC) scenario, where atmospheric CO2 emissions are initially increased as in a warming scenario and then reduced to zero. We found a significant increase in the ensemble spread of global temperature during the Zero Emission period following the warming phase. 
Ensembles which initially have the relatively higher salinity in the North Atlantic during the early Zero Emission period show the higher North Atlantic temperatures and salinity, along with less Arctic sea ice distribution, in later (ZEC) periods. Conversely, ensembles with initially lower salinity displayed opposite characteristics. We propose that the initial conditions of the Zero Emission period are associated to long-period internal variability that occurred during the previous period of positive CO2 emission fluxes (the warming period). The increase in ensemble spread in the Northern Atlantic is due to the the Atlantic Meridional Overturning Circulation (AMOC) salinity feedback becoming elongated due to strong ocean stratification. This suggests a prolonged period for this feedback mechanism, associated with the internal variability in AMOC.

How to cite: Seung-Hwon, H. and Soon-Il, A.: The variability determining the initial conditions for ensemble spread in the ZEC (CESM2) scenario, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7147, https://doi.org/10.5194/egusphere-egu24-7147, 2024.

X5.65
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EGU24-7826
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ECS
Emily Theokritoff, Burcu Yesil, Inga Menke, Mariam Saleh Khan, Inês Gomes Marques, Tiago Capela Lourenço, Hugo Pires Costa, and Carl-Friedrich Schleussner

As climate change intensifies, it is essential to take a wide range of climate scenarios and their consequential impacts into account for adaptation planning. Overshoot scenarios, during which global warming will temporarily exceed the 1.5°C Paris Agreement target before it is brought down again in the following decades, are increasingly likely under current emissions trajectories. They would result in complex risks such as limits to adaptation and irreversible impacts and stress the need to prepare long-term adaptation plans under deep uncertainty.

Here, we introduce the latest version of the Overshooting Proofing Methodology, a self-assessment tool designed to guide adaptation planners and policy-makers to integrate overshoot risks into planning processes, and present novel insights from its application with key stakeholders at city and regional levels. We also reflect on how adaptation pathways can allow to adequately plan a sequence of adaptation measures over time based on information collected through this tool. Its initial implementation in selected cities/regions reflects its applicability in varied climatic settings together with a range of climate related challenges. This work provides insights on key data gaps, capacity building needs and avenues for future adaptation planning, policy-making and research.

How to cite: Theokritoff, E., Yesil, B., Menke, I., Saleh Khan, M., Gomes Marques, I., Capela Lourenço, T., Pires Costa, H., and Schleussner, C.-F.: How can overshoot risks be included in long-term adaptation planning?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7826, https://doi.org/10.5194/egusphere-egu24-7826, 2024.

X5.66
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EGU24-11759
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ECS
Mitchell Dickau and H. Damon Matthews

Current policies have global mean temperature (GMT) on track to surpass the temperature thresholds agreed upon in the Paris Agreement. Therefore, if the goals of the Paris Agreement are to be met, there is an increasing likelihood of temporarily overshooting the 1.5°C or well-below 2.0°C thresholds. Using an intermediate complexity global Earth system model, we explore the climate implications of temperature overshoot using ≈100 pairs of multi-gas emissions scenarios from the ENGAGE project, with peak temperatures from ≈1.5°C to ≈2.2°C. For each pair of scenarios, the first is constrained by the remaining carbon budget (RCB) in 2100, which allows for the possibility of overshoot, while the second is constrained by the same RCB irrespective of a time horizon and acts as a baseline scenario. The comparisons of the pairs of scenarios demonstrate that the climate changes that occur at a given GMT are path dependent. In this presentation, we show how the impacts of overshoot vary depending on: 1) peak temperature, 2) the degree of overshoot, 3) the duration of overshoot, and 4) the amount of warming caused by CO2 vs. non-CO2 emissions. Our study expands on the literature by investigating the climate implications of temperature overshoot in an ensemble of ≈200 multi-gas scenarios with a range of temperature targets using a spatially explicit Earth system model.

How to cite: Dickau, M. and Matthews, H. D.: Investigating the path dependence of climate changes in a 200-member ensemble of overshoot scenarios, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11759, https://doi.org/10.5194/egusphere-egu24-11759, 2024.

X5.67
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EGU24-16039
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ECS
Peter Pfleiderer, Carl-Friedrich Schleussner, and Jana Sillmann

Global warming levels are politically relevant targets, and therefore, in public discussion and in climate science, these global warming levels are often taken as a reference for climate states. While the focus on global warming levels is a useful simplification in many cases, it becomes misleading when looking at temperature overshoot (or stabilization) scenarios. In temperature overshoot scenarios, greenhouse gas concentrations are eventually reduced leading to a decrease in global mean temperatures. In such scenarios, lagged effects, feedback mechanisms, and tipping points can result in considerably different climate states after the overshoot as compared to before at the same global warming level.

Here we assess to what extent changes in regional climate signals are reversed in the period after peak warming when global mean temperature decreases. We analyze a multi-model ensemble of CMIP6 simulations of two overshoot scenarios, SSP5-34-OS and SSP119. In many regions, climate signals are decoupled from global mean temperatures in the decades after peak warming, leading to differences in regional climate signals between before and after the overshoot at the same global warming level.

More dedicated climate simulations of overshoot scenarios would be required to better evaluate how long the influence of the overshoot would affect regional climate signals and to better understand the mechanisms behind these changes. The presented overview of regional climate signals in overshoot scenarios until 2100 already suggests that considerable implications of temperature overshoots for climate impacts are to be expected and that these implications need to be considered for adaptation planning and policy making.

How to cite: Pfleiderer, P., Schleussner, C.-F., and Sillmann, J.: Regional climate signals in overshoot scenarios, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16039, https://doi.org/10.5194/egusphere-egu24-16039, 2024.

X5.68
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EGU24-16992
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ECS
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David Hohn, Estela Monteiro, Giang Tran, and Nadine Mengis

A stabilisation of global temperature can be achieved by reducing anthropogenic CO2 emissions to zero. Delayed warming or cooling, called zero emissions commitment (ZEC), can still occur after emissions have stopped. The magnitude of ZEC has been estimated to be 0±0.3 degrees Celsius, based on multi-model means. The individual models, however, show a wide range of responses from the climate system to the cessation of emissions, furthering the uncertainties regarding future temperature developments.

Therefore, it is crucial to improve our knowledge of the ZEC uncertainty range in multiple aspects. This study contributes to a better understanding of the leading drivers of uncertainty of ZEC by analyzing a perturbed parameter ensemble of key dynamics of ZEC in ambitious mitigation scenarios. Using an Earth system model of intermediate complexity (UVic ESCM), we quantify how model parameters affect ZEC estimates for zero emissions preceded either by idealised constant emissions (20 and 10 PgC/yr) or by net-negative emissions scenarios. Finally, we analyze how the efficiency of Earth system processes relevant to ZEC, like carbon burial and heat uptake, can vary over different timescales after cessation of emissions.

How to cite: Hohn, D., Monteiro, E., Tran, G., and Mengis, N.: Closing in on Zero Emissions Commitment (ZEC) uncertainty, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16992, https://doi.org/10.5194/egusphere-egu24-16992, 2024.

X5.69
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EGU24-17012
Andrea Dittus, Nicola Maher, Andrew King, and Aditya Sengupta

The El Niño Southern Oscillation (ENSO) in the tropical Pacific is the main mode of inter-annual climate variability and a key driver of regional climate across much of the globe. Future changes in its behaviour are highly policy-relevant as they would have large impacts across many regions and significantly affect ecosystems and livelihoods. In this presentation, we explore how ENSO variability evolves in multi-century experiments under fixed atmospheric concentrations of greenhouse gases, where global mean surface temperatures are slowly stabilising.
We show how ENSO variability and its teleconnections change in a range of climate models and experimental designs. Idealised projections under fixed atmospheric concentrations of greenhouse gases across multiple levels of global warming, from 1.5°C to 5°C, are evaluated for the UK Earth System Model 1 alongside abrupt forcing experiments with the Community Earth System Model 1. We also include closely related experimental designs, such as emission-driven stabilisation experiments with ACCESS-ESM-1.5. The differences in how ENSO and its teleconnections respond to further warming in long, multi-century experiments under constant or slowly declining forcing conditions are compared and contrasted to the expected ENSO changes in rapidly warming, transient climate change projections. 

These differences are important to understand in the context of ambitious mitigation scenarios that aim to stabilise global temperatures at, or below, the Paris Agreement temperature targets. Preliminary results suggest that future ENSO variability is model dependent, but withing a single model framework independent of the level at which warming is stabilised at. 

How to cite: Dittus, A., Maher, N., King, A., and Sengupta, A.: Pacific climate variability and its regional impacts in warmer, stabilised climates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17012, https://doi.org/10.5194/egusphere-egu24-17012, 2024.

X5.70
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EGU24-17377
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Highlight
Debbie Rosen, Lawrence Jackson, Piers Forster, and Carl-Friedrich Schleussner

With the human induced increase in global temperatures continuing, the question if and how we might exceed the 1.5°C warming level enshrined in the long-term temperature goal of the Paris Agreement has received increased public and scientific interest. Identifying the level of human induced warming at any given year is subject to a range of uncertainty including from short-term natural variability. A single year, or even several consecutive years, above 1.5°C thus does not imply that the human induced warming level is reached but does provide an early warning of the risk of crossing that threshold.

Here we find that under an emission pathway following current policies, a single year above 1.5°C might imply that a crossing of the global warming threshold could materialise within 11 years thereafter (66% or likely range). For a three (5) year consecutive average, this time window decreases to 5 (2) years. If 1.5°C is reached in 2024, according to our analysis it would mark an unusual event (about 1-in-25 years) under a current policy scenario that reaches 1.5°C around 2040 (central estimate). We find that stringent emission reductions in the near-term can increase the chances of never crossing 1.5°C. Under a scenario of stringent emission decline, an exceedance of 1.5°C in one or several years may be observed without the long-term warming level ever being breached.

The occurrence of a single year at or above 1.5°C should therefore be taken as a final warning for the need for very stringent near term emission reductions to keep the Paris Agreement long-term limit within reach.

How to cite: Rosen, D., Jackson, L., Forster, P., and Schleussner, C.-F.: Early Warning of Crossing the 1.5°C Global Temperature Change Threshold, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17377, https://doi.org/10.5194/egusphere-egu24-17377, 2024.

X5.71
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EGU24-17881
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ECS
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Christine Kaufhold, Matteo Willeit, Stefanie Talento, and Andrey Ganopolski

Given the large ‘very likely’ range of equilibrium climate sensitivity (ECS, 2 to 5 K), as reported by the Intergovernmental Panel on Climate Change (IPCC) and additional carbon cycle feedbacks, we investigate whether the current Earth system has the potential to significantly deviate from pre-industrial levels in the long-term towards a “hothouse” state. We use the fast Earth system model CLIMBER-X to generate an ensemble of simulations for the next millennium with interactive CO2 and CH4 for ECS values between 2 and 5 K, and force our simulations using the extended low-to-intermediate emission scenarios of SSP1-2.6, SSP4-3.4, and SSP2-4.5. These scenarios are normally associated with peak global warming levels of 1.5, 2, and 3°C respectively for a standard ECS of approximately 3 K.

In simulations using an ECS of 5 K, we observe that the global mean temperature increase would more than double compared to the standard ECS of 3 K. Roughly half of this warming is propelled by positive carbon cycle feedbacks in the different scenarios, with equal contributions from both CO2 and CH4. In the SSP2-4.5 “middle of the road” scenario, we find that a high ECS could see global mean temperatures which exceed 7 °C within the next millennium, with some regions experiencing temperature increases up to 20 °C via polar amplification. If we consider unavoidable residual carbon emissions of less than 10% of our present-day value, we find that the CO2 concentration in the atmosphere can be sustained, thereby resulting in a continuous temperature rise until the year 3000 A.D. unless carbon is sequestered. Prolonged periods of high temperatures, as seen in this study, could lead to the breaching of critical thresholds within the Earth system, like the stability of the Greenland and Antarctic ice sheets for example. As high ECS values cannot be disregarded as implausible at the present time, these results hint hint that we could be on track towards an extreme “hothouse” climate in the long-term if there is no carbon removal.

How to cite: Kaufhold, C., Willeit, M., Talento, S., and Ganopolski, A.: Uncertainties in climate sensitivity and residual carbon emissions permit for a hothouse climate ahead, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17881, https://doi.org/10.5194/egusphere-egu24-17881, 2024.

X5.72
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EGU24-17980
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ECS
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Nadine Mengis

Although largely scientifically robust, there are elements within the politically-negotiated Paris Agreement where defined methods deviate from the best available assessment of what is physically required to achieve its set goals. Fundamentally, one such deviation is about the net zero greenhouse gas (GHG) emissions goal: current interpretations and applications thereof in nationally determined contributions deviate from what is actually required to halt human-induced global warming.

Here I show that, while attempting to be comprehensive, most of the nationally declared climate goals are unspecific if not misleading, do not actually deliver on temperature stabilisation and have a problematic treatment of future carbon removal (CDR) expectations. On the one hand, the net zero CO2-eq goals overemphasise the need for CDR deployment to reach climate targets, since net-negative CO2 emissions are required to compensate for non-CO2 GHGs. On the other hand, accounting for natural sinks as CDR within national net-zero goals overestimates current and future CDR potentials and mitigation actions, and will not actually deliver anthropogenic net zero CO2.

I accordingly propose to re-orient national climate action towards Specific, Measurable, Achievable, Relevant and Time-bound (SMART) goals, such as a net zero fossil fuel CO2 emissions target by mid-century.

How to cite: Mengis, N.: Let’s be SMART about climate goals, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17980, https://doi.org/10.5194/egusphere-egu24-17980, 2024.

X5.73
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EGU24-18510
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ECS
Tobias Nützel, Sabine Mathesius, Jens Krause, Sabine Egerer, Conor Ó Beoláin, Daniel Bampoh, Stefanie Falk, Dieter Gerten, Wolfgang Obermeier, and Julia Pongratz

Virtually all future scenarios in the IPCC AR6 keeping climate change well below 2°C include carbon dioxide removal (CDR), often leading to large transformations of global land surface and land use. Re-/afforestation (AR) and bioenergy with carbon capture and storage (BECCS) are the two most prominent CDR measures in those scenarios. The temporal evolution of carbon uptake and storage is very different between bioenergy plants, which are annually harvested to (ideally) permanent storage, and forests, which sequester carbon for decades on site but can be affected by disturbances. Additionally, while AR dominates current CDR deployment as tree seedlings and saplings can be planted right away, BECCS requires further processing and storage infrastructure leading to longer establishment time scales. Thus, BECCS covers only a tiny fraction of existing and announced amounts of CDR. Hence, depending on whether CDR is intended to support rapid, deep reductions of net emissions in the near term (as in the Nationally Determined Contributions of parties to the Paris Agreement) or to counterbalance residual emissions or even reach net negative emissions in the longer term, either AR or BECCS could be more effective. This will also vary across world regions. 

We compare the temporal dynamics of carbon storage efficiency between AR and BECCS with three state-of-the-art terrestrial biosphere models (JSBACH, LPJmL, LPJ-GUESS). We use a global, highly stylized setup where a fixed share per pixel of current agricultural land is replaced by forests or bioenergy plants, respectively. We analyze the effectiveness of the two CDR methods over time and in different world regions depending on the temporal CDR target. Furthermore, we quantify how the temporal dynamics are affected by the chosen start year of CDR (2015, 2030, 2050), background climate and CO2 concentrations (SSP1-2.6, SSP3-7.0),  natural disturbances and assumptions on management and plant parametrizations in the underlying vegetation models. We specifically consider temporal dynamics on current agricultural areas adjacent to biodiversity hotspots, since these could also become relevant for achieving ecosystem restoration targets. There, CDR through restoration of naturally occurring forests or grasslands with support from local communities can bring synergies for multiple ecosystem services, while premature deployment of AR in non-forest areas or crop-based BECCS would likely decrease biodiversity.

How to cite: Nützel, T., Mathesius, S., Krause, J., Egerer, S., Ó Beoláin, C., Bampoh, D., Falk, S., Gerten, D., Obermeier, W., and Pongratz, J.: Temporal dynamics of terrestrial carbon dioxide removal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18510, https://doi.org/10.5194/egusphere-egu24-18510, 2024.