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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.