CL3.2.1 | Towards a net-zero world: remaining carbon budgets, mitigation pathways and implications for policy
EDI
Towards a net-zero world: remaining carbon budgets, mitigation pathways and implications for policy
Co-organized by BG8
Convener: Andrew MacDougall | Co-conveners: Katarzyna (Kasia) TokarskaECSECS, Joeri Rogelj, Kirsten Zickfeld
Orals
| Fri, 28 Apr, 14:00–15:45 (CEST), 16:15–18:00 (CEST)
 
Room 0.31/32
Posters on site
| Attendance Fri, 28 Apr, 08:30–10:15 (CEST)
 
Hall X5
Orals |
Fri, 14:00
Fri, 08:30
Remaining carbon budgets specify the maximum amount of CO2 that may be emitted while stabilizing warming at a particular level (such as the 1.5°C or 2.0°C target), and are thus of high interest to the public and policymakers. Estimates of the remaining carbon budget come with associated uncertainties, which increase in relative terms as more ambitious targets are being considered, or as emission reductions continue to be delayed. As a result, practical implementation of remaining carbon budgets is challenging.

This session aims to further our understanding of the climate response under various emission scenarios that aim to inform the goals of the Paris Agreement, with particular interest in emission pathways entailing net-zero targets. We invite contributions that use a variety of tools, including fully coupled Earth System Models (ESMs), Integrated Assessment Models (IAMs), or simple climate model emulators, that advance our knowledge of remaining carbon budgets, net-zero targets, and policy implications.

We welcome studies exploring different aspects of climate change in response to future emission scenarios. In addition to studies exploring carbon budgets 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), estimates of the remaining carbon budget to keep warming below a given temperature target, the role of pathway dependence and emission rate, the climate-carbon responses to different emission scenarios (e.g. RCP or SSP scenarios, idealized scenarios, or scenarios designed to reach net-zero emission level), and the behaviour of TCRE in response to artificial carbon dioxide removal from the atmosphere (i.e. CDR or negative emissions). Contributions from the fields of climate policy and economics focused on applications of carbon budgets and benefits of early mitigation are also encouraged.

Orals: Fri, 28 Apr | Room 0.31/32

Chairpersons: Andrew MacDougall, Katarzyna (Kasia) Tokarska
14:00–14:05
14:05–14:25
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EGU23-13480
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solicited
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On-site presentation
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Benjamin Sanderson, Charles Koven, Glen Peters, and Stuart Jenkins

The linear relationship between cumulative emissions and warming has been a consistent feature of climate models, and underpins the concept of a carbon budget and net-zero goal in order to achieve climate stabilisation.  However, research in recent years has identified potential for deviations from this relationship during the net zero transition.  Here, we consider how important such deviations might be for achieving the goals of the Paris Agreement, and whether current metrics of Earth system warming in response to carbon emissions (TCRE, ZEC, RAZE) adequately describe the range of potential warming trajectories which might be experienced in response to different levels of mitigation.  Further, as carbon emissions (hopefully) peak and decline in the coming decades, we examine the prospects for further constraining response parameters as temperatures depart from the linear growth seen over recent decades.  Finally, we consider how the current CMIP experimental protocol could be extended to better define transient response to real world emissions in a net-zero transition.

How to cite: Sanderson, B., Koven, C., Peters, G., and Jenkins, S.: Refining the budget: limits of the cumulative emissions framework and implications for policy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13480, https://doi.org/10.5194/egusphere-egu23-13480, 2023.

14:25–14:35
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EGU23-7143
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ECS
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On-site presentation
Robin Lamboll, Zebedee Nicholls, Chris Smith, Jarmo Kikstra, Edward Byers, and Joeri Rogelj

The remaining carbon budget (RCB), the net amount of carbon dioxide humans can still emit without exceeding a chosen global warming limit, is often used to evaluate political action against the goals of the Paris Agreement. RCB estimates for 1.5C are small, and minor changes in their calculation can therefore result in large relative shifts. Here we evaluate recent RCB assessments by the IPCC and explain differences between them. We present calculation refinements together with robustness checks that increase confidence in RCB estimates. We conclude that the RCB for a 50% chance of keeping warming to 1.5C is around 250 GtCO2 as of January 2023, around 6 years of current CO2 emissions. This estimate changes to 480 and 60 GtCO2 for a 33% and 66% chance, respectively. Key uncertainties affecting RCB estimates are the contribution of non-CO2 emissions, which depends on socioeconomic projections as much as on geophysical uncertainty, and potential warming after net zero is reached. 

How to cite: Lamboll, R., Nicholls, Z., Smith, C., Kikstra, J., Byers, E., and Rogelj, J.: Assessing the size and uncertainty of remaining carbon budgets, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7143, https://doi.org/10.5194/egusphere-egu23-7143, 2023.

14:35–14:45
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EGU23-1240
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ECS
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Highlight
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On-site presentation
Estela Monteiro and Nadine Mengis

The Paris Agreement long-term temperature goal (Paris Agreement LTTG) aims to limit global warming to well below 2ºC, if possible to a maximum of 1.5ºC. To understand how this goal could be accomplished, idealized scenarios have been explored in the past years, with a special focus on pathways for reaching net-zero CO2 emissions. 

Non-CO2 forcing is, however, known to contribute to a decrease in the remaining carbon budgets related to the Paris Agreement LTTG (e.g., Mengis & Matthews, 2020). A full picture regarding this benchmark can therefore only be painted when including the effects of aerosols, non-CO2 greenhouse gases and land use changes. These forcings along with the zero emissions commitment to CO2 will define whether temperature is able to stabilize once CO2 emissions decrease.  

To explore individual effects from anthropogenic non-CO2 forcing agents, their respective contributions to the Paris Agreement LTTG scenarios (Rogelj, et al., 2019) is estimated and put into relation. We will present results primarily on the impacts of aerosols and land use change representation as well as their effects on the carbon cycle and climate by simulating LTTG scenarios using an Earth system model of intermediate complexity (UVic ESCM, version 2.10, Mengis et al., 2020). The climate response in these all forcing net-zero CO2 emission scenarios will provide us with relevant insights concerning allowable emissions for temperature stabilization.

References: 

Mengis, N., Matthews, H.D. Non-CO2 forcing changes will likely decrease the remaining carbon budget for 1.5 °C. npj Clim Atmos Sci 3, 19 (2020). https://doi.org/10.1038/s41612-020-0123-3

Mengis, N., Keller, D. P., MacDougall, A. H. et al. Evaluation of the University of Victoria Earth System Climate Model version 2.10 (UVic ESCM 2.10). Geosci. Model Dev. 13, 4183–4204 (2020). https://doi.org/10.5194/gmd-13-4183-2020  

Rogelj, J., Huppmann, D., Krey, V. et al. A new scenario logic for the Paris Agreement long-term temperature goal. Nature 573, 357–363 (2019). https://doi.org/10.1038/s41586-019-1541-4

How to cite: Monteiro, E. and Mengis, N.: Does net-zero CO2 stabilize the climate? - On the contributions of the remaining climate forcing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1240, https://doi.org/10.5194/egusphere-egu23-1240, 2023.

14:45–14:55
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EGU23-4447
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On-site presentation
Thomas Frölicher, Jens Terhaar, Mathias Aschwanden, Pierre Friedlingstein, and Fortunat Joos

The parties of the Paris Agreement agreed to keep global warming well below 2°C and pursue efforts to limit it to 1.5°C. A global stocktake is instituted to assess the necessary emissions reductions every 5 years. Here we present an adaptive approach to successively quantify global emissions reductions that allow reaching a temperature target within ±0.2°C, solely based on regularly updated observations of past temperatures, radiative forcing and emissions statistics, and not on climate model projections. Testing this approach using an Earth system model of intermediate complexity demonstrates that defined targets can be reached following a smooth emissions pathway. Its adaptive nature makes the approach robust against inherent uncertainties in observational records, climate sensitivity, effectiveness of emissions reduction implementations and the metric to estimate CO2 equivalent emissions. This approach allows developing emission trajectories for CO2, CH4, N2O and other agents that iteratively adapt to meet a chosen temperature target.

How to cite: Frölicher, T., Terhaar, J., Aschwanden, M., Friedlingstein, P., and Joos, F.: Adaptive emission reduction approach to reach any global warming target, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4447, https://doi.org/10.5194/egusphere-egu23-4447, 2023.

14:55–15:05
15:05–15:15
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EGU23-950
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ECS
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On-site presentation
Nicoletta Brazzola, Anthony Patt, and Jan Wohland

To meet ambitious climate targets, the aviation sector needs to neutralize CO2 emissions and reduce non-CO2 climatic effects. Despite being responsible for approximately two-thirds of aviation’s impacts on the climate, most aviation non-CO2 species are currently excluded from climate mitigation efforts. Here we identify three plausible definitions of climate-neutral aviation that include non-CO2 forcing and assess their implications considering future demand uncertainty, technological innovation, and CO2 removal. We use empirical relationships to translate aviation emissions to climate forcing and a reduced-complexity climate model to assess the impacts of these climate neutrality frameworks, including the needed CO2 removal, on global temperature in the context of the different demand and technology scenarios. We demonstrate that simply neutralizing aviation’s CO2 emissions, if nothing is done to reduce non-CO2 forcing, causes up to 0.4 °C additional warming, thus compromising the 1.5 °C target. We further show that substantial rates of CO2 removal are needed to achieve climate-neutral aviation in scenarios with little mitigation, yet cleaner-flying technologies can drastically reduce them. Our work provides policymakers with consistent definitions of climate-neutral aviation and highlights the beneficial side effects of moving to aircraft types and fuels with lower indirect climate effects.

How to cite: Brazzola, N., Patt, A., and Wohland, J.: Definitions and implications of climate-neutral aviation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-950, https://doi.org/10.5194/egusphere-egu23-950, 2023.

15:15–15:25
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EGU23-5097
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Highlight
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On-site presentation
Tilo Ziehn, Andrew King, Josephine Brown, Liam Cassidy, and Alexander Borowiak

To stop global warming, humanity needs to achieve close to net-zero greenhouse gas emissions. Many countries are committed to reducing greenhouse gas emissions significantly over the next decade and to reaching net-zero emissions by 2050, in line with the Paris Agreement goal of limiting global warming well below 2°C above pre-industrial levels. This can only be achieved through deep decarbonization and removal of carbon dioxide from the atmosphere. However, many questions remain about the long-term implications of stabilising global temperatures at the Paris Agreement goals or missing this target but stabilising the climate at a higher global warming level.

We have run bespoke millennium-length simulations with the Australian Earth System Model, ACCESS-ESM1.5, under net-zero emissions at different global warming levels ranging from about 1.5°C to 3°C. Here, we discuss these simulations and analyse the evolution of temperature, precipitation and carbon budgets. We will present results on the linearity of local climate changes under different stabilised global warming levels and how these compare with local changes in rapidly warming climates. We will also discuss the processes that cause these local non-linearities and raise opportunities for research that these simulations provide.

We must gain a better understanding of potential future climates which evolve under near-zero greenhouse gas emissions. Policymaking is based on achieving net-zero emissions and we hope our work and similar analyses can increase understanding of a changing climate under net-zero scenarios.

How to cite: Ziehn, T., King, A., Brown, J., Cassidy, L., and Borowiak, A.: Investigating the implications of net-zero emissions at different global warming levels, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5097, https://doi.org/10.5194/egusphere-egu23-5097, 2023.

15:25–15:35
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EGU23-14454
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ECS
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On-site presentation
Andrea Dittus and Ed Hawkins

The 2015 Paris Agreement adopted by 192 parties states the goal of limiting global warming to well below 2, preferably 1.5 degrees Celsius above pre-industrial levels. These goals imply an ambition to stay at or below these levels long-term. Evidence is beginning to emerge that regional patterns of change at given global warming levels (GWLs) can be very different between transiently warming through given GWLs and stabilising at those same GWLs.

In this presentation, we explore regional climate change across multiple variables, with a particular focus on regional precipitation change. Using a novel ensemble of six 500-years long fixed concentration simulations across various levels of warming between 1.5 and 5 degrees above pre-industrial with the CMIP6-generation Earth System Model UKESM1.0, we show that precipitation trends opposite in sign to transient climate change projections occur in several regions at the same GWLs. Such differences have important implications for climate change risk assessments and adaptation discussions, which typically only include transient projections. Here, we provide examples where a transient and stabilised climate differ and discuss the possible mechanisms driving these differences. 

How to cite: Dittus, A. and Hawkins, E.: Regional impacts of climate stabilisation across multiple global warming levels, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14454, https://doi.org/10.5194/egusphere-egu23-14454, 2023.

15:35–15:45
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EGU23-7493
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ECS
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On-site presentation
David Hohn, Torge Martin, Nadine Mengis, and Estela Monteiro

In scenarios with abrupt cessation of CO2 emissions (ZECMIP) the committed warming is expected to be 0±0.3K [1]. It is imperative to understand and narrow this uncertainty range because it is similar in size to the remaining allowable warming until 1.5K. We have shown that temperature changes up to 3K are present at regional scales [2]. Furthermore significant differences between models are observed which are important to understand the uncertainties in the zero emissions committed warming.

Analysing the ZECMIP simulations of nine ESMs, we identify common climate patterns and notable differences in an idealised zero CO2 emissions scenario. We distinguish between patterns within and outside of natural model climate variability, and will present first results for likely causes in commonalities linked to ocean circulations and predominant climate modes.

 

References:

[1] A. H. MacDougall et al., Is There Warming in the Pipeline? A Multi-Model Analysis of the Zero Emissions Commitment from CO2, Biogeosciences 17, 2987 (2020).

[2] A. H. MacDougall, J. Mallett, D. Hohn, and N. Mengis, Substantial Regional Climate Change Expected Following Cessation of CO2 Emissions, Environmental Research Letters 17, 114046 (2022).

How to cite: Hohn, D., Martin, T., Mengis, N., and Monteiro, E.: Possible causes for regional zero emissions commitment signals, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7493, https://doi.org/10.5194/egusphere-egu23-7493, 2023.

Coffee break
Chairpersons: Andrew MacDougall, Katarzyna (Kasia) Tokarska
16:15–16:20
16:20–16:30
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EGU23-11607
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Highlight
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On-site presentation
Stefan Rahmstorf, Geoffrey Supran, and Naomi Oreskes

The performance of mainstream climate models has received extensive scrutiny. By contrast, climate projections by the fossil fuel industry have never been assessed. Based on recent archival discoveries, we quantitatively evaluate all available global warming projections documented by – and in many cases modeled by – Exxon and ExxonMobil Corp scientists between 1977 and 2003. We find that most of their projections accurately forecast warming consistent with subsequent observations. Their projections were also consistent with, and at least as skillful as, those of independent academic and government models. We find that Exxon and ExxonMobil Corp also correctly rejected the prospect of a coming ice age, accurately predicted when human-caused global warming would first be detected, and reasonably estimated the ‘carbon budget’ for holding warming below 2°C. Our results show that ExxonMobil predicted global warming correctly and that by the 1980s, they already knew how much global warming the company’s products were likely to cause.  

How to cite: Rahmstorf, S., Supran, G., and Oreskes, N.: Assessing ExxonMobil’s global warming projections, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11607, https://doi.org/10.5194/egusphere-egu23-11607, 2023.

16:30–16:40
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EGU23-9011
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Virtual presentation
Charles Koven, Benjamin Sanderson, and Abigail Swann

Two key metrics of the coupled carbon-climate system are used to inform the remaining carbon budget for climate stabilization: the transient climate response to cumulative CO2 emissions (TCRE) and the zero emissions commitment (ZEC), which govern the global temperature response to positive and zero emissions, respectively. We ask whether and how these two metrics describe global temperatures under net negative CO2 emissions, using an idealized scenario defined by a gradual and symmetric reversal from positive to negative emissions. Using a full Earth system model and an ensemble of simple climate model simulations, we show that the two metrics do capture the global temperature dynamics even under large amounts of negative emissions, with ZEC predicting the deviation from symmetric TCRE proportionality under negative emissions. Further, we show that because ZEC begins to influence global temperatures even before reaching net zero, it can be better thought of as a measure of the deviation from the path-independence of the TCRE relationship than as a measure of the committed warming after reaching net zero.

How to cite: Koven, C., Sanderson, B., and Swann, A.: Do the TCRE and ZEC metrics work under net negative CO2 emissions?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9011, https://doi.org/10.5194/egusphere-egu23-9011, 2023.

16:40–16:50
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EGU23-1691
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ECS
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On-site presentation
Hunter Douglas, Dave Frame, and Laura Revell

As more and more countries set net-zero targets and progress is made on decarbonising industries, the prospect of achieving a net-zero (or even net-negative) emissions world is beginning to come into focus. Our current understanding, however, is that the climate will not immediately stabilise everywhere under such conditions. Understanding where and how the climate will change in a net-zero world is important for adaptation planning and goal setting. Climate change emergence techniques are useful for quantifying change relative to what people and ecosystems are accustomed to. However, to date these techniques have been little used to assess climate change at and beyond net-zero emissions. Whether or not aspects of the climate system “de-emerge” and return to within baseline variability remains under-explored. In this work, we use CMIP6 models to quantify climate change emergence in terms of signal-to-noise for annual- and seasonal-average temperature and precipitation, as well as strength and position of the eddy-driven jets.

Applying this framework, we calculate the rate and extent of de-emergence that occurs when carbon dioxide concentrations fall. We first combine results from multiple models participating in the Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) to establish global and regional behaviour for these variables under an idealised rising/falling CO2 scenario. We then apply the same analysis to multiple models’ results for ScenarioMIP emissions pathways with net-negative CO2 emissions (SSP1-1.9, SSP1-2.6, SSP4-3.4, and SSP5-3.4). We find that both temperature and precipitation exhibit partial reversibility on the scale of decades to centuries, albeit with significant hysteresis due to lag effects. These patterns are clearly apparent in the CDRMIP results and less so for the SSPs. There are significant regional differences in the rate and extent of de-emergence, including a strong land-sea contrast. The jet parameters, in contrast, respond quickly to greenhouse gas and other forcings, and so do not exhibit comparable hysteresis. Those models with data extending beyond 2100 allow for better quantification of de-emergence. CO2 peak concentrations and rates of change both influence the stable climate state, though disentangling these factors remains challenging.  

How to cite: Douglas, H., Frame, D., and Revell, L.: Emissions pathways change how abnormal climatic conditions de-emerge beyond net-zero, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1691, https://doi.org/10.5194/egusphere-egu23-1691, 2023.

16:50–17:00
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EGU23-4638
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ECS
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On-site presentation
Koramanghat Unnikrishnan Jayakrishnan and Govindasamy Bala

Anthropogenic emissions of carbon dioxide since the beginning of industrialization have led to an increase in global surface temperature. This rapid increase in global surface temperature is unprecedented over the past 2000 years. The increase in frequency of natural disasters such as extreme rainfall, floods and heatwaves indicate that immediate action is required to prevent further impact of climate change. The Paris agreement targets to keep warming below 2°C above the preindustrial state, while pursuing efforts to limit the increase to 1.5°C . While climate mitigation strategies such as reducing fossil fuel emissions and deforestation are currently implemented, recent studies show that artificially removing atmospheric CO2 (negative emissions) might be necessary to achieve the targets set by the Paris agreement. Therefore, understanding the response of climate system towards artificial removal of atmospheric CO2 or equivalently negative emissions is essential.

In this study, using a coupled climate and carbon cycle model, we simulate the response of the climate system to net negative emissions in 9 idealized simulations each having a positive emission phase and an equal and opposite negative emission phase such that the cumulative emissions since preindustrial period is zero in each simulation. We specifically address the following two questions: 1) For the same total emissions in the positive phase, does the timescale of emissions have any impact on the long-term response of the climate system? 2) If the timescale of emissions is the same, what is the sensitivity to the magnitude of total emissions in the positive phase? The results from our nine climate-carbon model simulations will be discussed at the meeting.

How to cite: Jayakrishnan, K. U. and Bala, G.: A model-based estimate of the climate and carbon cycle response to negative CO2 emissions over multi-centennial timescales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4638, https://doi.org/10.5194/egusphere-egu23-4638, 2023.

17:00–17:10
17:10–17:20
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EGU23-11791
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ECS
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On-site presentation
Carla Maria Di Natale, Giang Thanh Tran, David P. Keller, Theresa Schaber, Joonas Merikanto, Tommi Ekholm, and Antti-Ilari Partanen

To meet the Paris Agreement targets, in addition to rapid emission reductions, carbon dioxide needs to be removed from the atmosphere with Negative Emission Technologies (NETs). On one hand, these solutions seem promising; on the other hand, they have significant and poorly estimated uncertainties and risks related to their potential to remove atmospheric carbon and wider impacts on the Earth system. One previously largely unexplored aspect of NETs is whether the uncertainty in NETs and e.g. transient climate response to cumulative CO2 emissions (TCRE) are coupled to any degree, e.g. could some NETs have lower carbon removal potential if TCRE is high. 

We estimate how TCRE and selected NETs’ carbon removal potential are dependent on climate system parameters using Perturbed Parameter Ensemble (PPE) with the University of Victoria Earth System Climate Model (UVic ESCM) and Gaussian Process (GP) emulator. Our aim is to explore and quantify any potential correlation between the carbon removal potential of single NETs and TCRE. The NETs considered are afforestation, reforestation, ocean alkalinization, ocean iron fertilization and direct air capture, which all except the last one depend on the perturbed parameters. 

The parameters of interest are chosen according to their expected impact on the climate and carbon uptake, constrained according to observations, and perturbed based on their prior probability distribution functions (PDFs). Then, to explore the parameter’s space, we use GP emulators to estimate model outputs as surrogate of actual ESM runs, which would be computationally too expensive. The emulators are created for the preindustrial spin-up, historical period, future control scenario and one for each NET scenario. They are trained through 300 simulations, considering 20 perturbed parameters. This analysis yields the correlation between the carbon removal potential of each NET and TCRE, and the contribution of each perturbed parameter to these two metrics. 

The preliminary results from the 300-member ensemble give a mean TCRE of 1.63 °C/1000 PgC, which is consistent with the best estimate of 1.65 °C/1000 PgC reported by the IPCC AR6 WGI (2021). The simulations with a high TCRE also tended to have a high ocean iron fertilization’s potential, meaning that some NETs are potentially more effective in removing atmospheric carbon dioxide if the temperature change per cumulative carbon dioxide emissions is high. Identifying such correlations between TCRE and NETs’ potential allows designing more robust mitigation strategies including portfolios of NETs that hedge against high TCRE. 

How to cite: Di Natale, C. M., Tran, G. T., Keller, D. P., Schaber, T., Merikanto, J., Ekholm, T., and Partanen, A.-I.: The coupled uncertainty in negative emission technologies and transient climate response to cumulative CO2 emissions., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11791, https://doi.org/10.5194/egusphere-egu23-11791, 2023.

17:20–17:30
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EGU23-5881
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Highlight
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On-site presentation
Joonas Merikanto, Theresa Schaber, Antti-Ilari Partanen, and Tommi Ekholm

Climate change mitigation strategies consistent with the Paris Agreement’s temperature targets rely heavily on future carbon dioxide removal (CDR). Although such strategies have drawn considerable critique for long, e.g., that they are ‘betting on negative emissions’, the risks from this betting have not been quantified nor addressed properly. We use a lightweight integrated assessment model SCORE to explore possible scenarios using CDR for limiting global warming to 1.5 °C by 2100. Particularly, we quantify the impacts of relying on CDR when accounting for 1) possible under- and overestimation of the cost, potential, and availability (feasibility) of future CDR and 2) the compound effect with uncertainty in climate sensitivity.

All scenario results unquestionably show that aggressive near-term mitigation is required for limiting warming to 1.5 °C by 2100 for all levels of climate sensitivity, but that some amount of CDR is likely required in the future even if climate sensitivity turns out to be extremely low. If uncertainty in climate sensitivity is disregarded, initial assumptions on the CDR feasibility have only minor effects on the total cumulative mitigation cost. However, taking the uncertainty in climate sensitivity into account changes this conclusion. Wrong assumptions on CDR feasibility can, surprisingly, even lead to lower costs under extreme realizations of climate sensitivity, especially in scenarios where CDR feasibility is underestimated. Assuming low feasibility for CDR eliminates the possibility of sky-rocketing costs associated with overestimating CDR feasibility in combination with a high climate sensitivity. Therefore, a prudent climate policy should assume a low feasibility of CDR to reduce the risk of leaving runaway mitigation costs to future generations.

How to cite: Merikanto, J., Schaber, T., Partanen, A.-I., and Ekholm, T.: Betting on CDR under uncertain climate sensitivity is bad climate policy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5881, https://doi.org/10.5194/egusphere-egu23-5881, 2023.

17:30–17:40
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EGU23-6823
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Highlight
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On-site presentation
Matthew Gidden, Thomas Gasser, Giacomo Grassi, Nicklas Forsell, Iris Janssens, William Lamb, Jan Minx, Zebedee Nicholls, Jan Steinhauser, and Keywan Riahi

Taking stock of global progress towards achieving the Paris Agreement requires measuring aggregate national action against modelled mitigation pathways. A key gap exists, however, in how scientific studies and national inventories account for the role of anthropogenic land-based carbon fluxes, resulting in a 5.5-6.0 GtCO2yr-1 difference between the respective present-day land-use estimates. Modelled pathways mainly include direct human-induced fluxes, while inventories submitted by countries to the UNFCCC (NGHGIs) generally include a wider definition of managed land area as well as the indirect removals on that land caused by environmental changes (e.g., the CO2 fertilization effect). This difference hinders comparability between targets set by countries and scientific benchmarks. 

Scenarios assessed in AR6 show that a combination of deep near-term gross emissions reductions and medium-term carbon removal from the atmosphere are needed to reach net-zero and eventually net-negative CO2 emissions to limit warming in line with the Paris Agreement temperature goal. However, scenarios lacked key information needed to estimate land-based removals and to align their LULUCF projections with NGHGIs. Here, we estimate the land-based removals consistent with NGHGIs using a reduced complexity climate model with explicit treatment of the land-use sector, OSCAR, one of the models used by the Global Carbon Project. Of the 1202 pathways that passed IPCC vetting, 914 provide sufficient land-use change data to allow us to fill this information gap and enable alignment between pathways and inventories.

Across both 1.5°C and 2°C scenarios, pathways aligned with NGHGIs 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. 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 deep mitigation scenarios.

Our results do not change any climate outcome or mitigation benchmark produced by the IPCC, but rather provide a translational lens to view those outcomes. We find that net-zero timings on average advance by around 5 years; however, this does not imply that 5 years have been lost in the race to net-zero, but rather that following the reporting conventions for natural sinks results in net-zero being reached 5 years earlier. Understanding how these different accounting frameworks can be mutually interpreted is a fundamental challenge for evaluating progress towards the Paris Agreement, given the reality that direct and indirect carbon removals cannot be estimated separately with direct observations.

We propose three primary ways to address this science-policy gap. First, targets can be formulated separately for gross emission reductions, land-based removals, and technical carbon removals, allowing for nations to clearly define their expected contributions and to measure progress in each domain separately. Second, nations can clarify the nature of their deforestation pledges. Third, modelling teams can provide their assumptions for the NGHGI correction as part of their standard output which future IPCC assessments can use to vet scenarios.

How to cite: Gidden, M., Gasser, T., Grassi, G., Forsell, N., Janssens, I., Lamb, W., Minx, J., Nicholls, Z., Steinhauser, J., and Riahi, K.: Policy implications from aligning IPCC scenarios to national land emissions inventories, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6823, https://doi.org/10.5194/egusphere-egu23-6823, 2023.

17:40–17:50
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EGU23-11083
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ECS
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On-site presentation
Meng-Hui Lin and Ching-Pin Tung

Sustainability transition on climate change, energy systems, and low carbon society is a big issue in 21 century. However, it’s not a linear question in a specific single research community. This study aims to contribute a transdisciplinary research (TDR) framework to support climate change decision-making as wicked problems. To achieve the global goal of 2050 Net Zero, it needs a science-based scenario setting for both adaptation and mitigation while making decisions and climate risk assessment in business and governance. From knowledge to actions, academics and non-academics are encouraged to engage in climate actions at the same time. This study delivers a system-dynamic approach to integrate the environmental, social, and economical components from participants and stakeholders with different backgrounds in TDR to reduce climate risks including resilient adaptation for physical risk and low carbon transition for transition risk. The TDR framework on climate change dynamic decision-making would be demonstrated through a case of the Carbon Neutrality Project of National Taiwan University.

How to cite: Lin, M.-H. and Tung, C.-P.: Transdisciplinary Research Framework on Climate Change Dynamic Decision-making., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11083, https://doi.org/10.5194/egusphere-egu23-11083, 2023.

17:50–18:00

Posters on site: Fri, 28 Apr, 08:30–10:15 | Hall X5

Chairpersons: Andrew MacDougall, Kirsten Zickfeld
X5.165
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EGU23-1664
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ECS
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Highlight
Soong-Ki Kim, Jongsoo Shin, Soon-Il An, Hyo-Jeong Kim, Nari Im, Shang-Ping Xie, Jong-Seong Kug, and Sang-Wook Yeh

Anthropogenic global warming by carbon dioxide emissions may cause irreversible changes in a wide range of climate variables. A comprehensive understanding of this hysteresis effect and its regional patterns is, however, lacking. Here, we use the Community Earth System Model version 1.2 with a CO2 removal scenario to show that surface temperature and precipitation exhibit globally widespread irreversible changes. To explore the climate hysteresis and reversibility on a regional scale, we develop a novel method that quantifies their spatial patterns. Our experiments project that 89% and 58% of the global area experiences irreversible changes in surface temperature and precipitation, respectively. Strong irreversible response of surface temperature is found in the Arctic, Southern Ocean, and North Atlantic Ocean and of precipitation in the global monsoon regions, tropical Pacific, and the Himalayas. The identified global land hotspots of irreversible changes can indicate elevated risks of negative impacts on developing countries.

How to cite: Kim, S.-K., Shin, J., An, S.-I., Kim, H.-J., Im, N., Xie, S.-P., Kug, J.-S., and Yeh, S.-W.: Irreversible changes in surface temperature and precipitation to CO2 forcing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1664, https://doi.org/10.5194/egusphere-egu23-1664, 2023.

X5.166
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EGU23-6073
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ECS
Yona Silvy, Jens Terhaar, Friedrich Burger, Fortunat Joos, Myles Allen, Victor Brovkin, Jonathan Buzan, Goran Georgievski, Fabrice Lacroix, Donghyun Lee, and Thomas Frölicher

Climate policies such as the Paris Agreement are framed in terms of global warming levels. Based on past warming and past CO2 emissions, the amount of future cumulative CO2 emissions allowed to keep global warming at or below a global warming level can be estimated. Yet, global warming scenarios in the successive Coupled Model Intercomparison Projects are framed in terms of prescribed atmospheric CO2 concentration or emissions, yielding a wide range of warming levels per CO2 pathway in response to the different transient climate responses to cumulative emissions in the coupled climate models. Based on these scenarios and the latest model projections, the IPCC Sixth Assessment Report assessed climatic impacts of different warming levels. These impacts are thus evaluated in simulations where the warming targets are passed transiently, at different points in time, and not stabilized, as opposed to how climate agreements are framed.

Here, we propose a new Model Intercomparison Project AERA-MIP building on an adaptive approach - the Adaptive Emissions Reduction Approach - that successively calculates the compatible emissions to stabilize global warming at the required temperature target. Earth System Models (ESMs) are run forward in emission-driven mode, with prescribed, model-specific emissions successively calculated every five years, so that all models reach the same warming target and thereafter stabilize at this warming level. The warming uncertainty is thus side-stepped, while different emissions pathways emerge out of the variety of participating ESMs. The approach is based on the TCRE framework and successively adapting for any changes in the Earth System that might affect global mean surface temperature, including the zero emissions commitment as emissions approach zero.

Simulations of the first participating modelling centers already reveal a panel of emissions pathways that successfully stabilize global warming at 1.5ºC and 2ºC. This includes the decline rate from peak emissions, the timing of having to reach net-zero emissions, and the magnitude of negative emissions needed to stabilize the climate. These different emissions pathways result in a range of atmospheric CO2 concentration evolution (350 to 450 ppm at year 2100 in the 1.5°C stabilization scenario) and distribution of anthropogenic carbon in the Earth System components. Unlike concentration-driven projections, these AERA simulations provide an uncertainty range for impacts that are directly affected by atmospheric CO2 concentration such as ocean acidification. The project also includes temporary temperature overshoot simulations using the AERA approach.

How to cite: Silvy, Y., Terhaar, J., Burger, F., Joos, F., Allen, M., Brovkin, V., Buzan, J., Georgievski, G., Lacroix, F., Lee, D., and Frölicher, T.: Emissions pathways compatible with 1.5ºC and 2ºC stabilized warming in fully-coupled Earth System Models: first results from AERA-MIP, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6073, https://doi.org/10.5194/egusphere-egu23-6073, 2023.

X5.167
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EGU23-6566
Kushal Tibrewal, Katsumasa Tanaka, Olivier Boucher, and Philippe Ciais

Countries pledge GHG mitigation goals in form of near-term emissions targets for 2030, long-term net-zero emissions in mid-century (some countries) and targets for methane emission reductions, towards limiting global warming as stipulated in the Paris Agreement. These undergo regular revisions to strengthen the ambition, with the latest set of revisions occurring during COP26 and COP27. As of December 2022, 169 countries have near-term targets, 56 have long-term targets to become carbon neutral and 150 nations have pledged to reduce their methane emissions up to 30% by 2030.  The end-of-century temperature rise is highly sensitive to the pledges but also to actual extent of implementation of these pledges. Using historical emissions from the PRIMAP-Hist dataset, future emissions of CO2, N2O and CH4 are modelled for each country by incorporating their respective near-term, long-term and methane pledges. Emissions for other climate forcers are assumed to follow SSP4-60. These emission profiles are used as input to simple climate model – ACC2 (Tanaka et al., 2021) to estimate the temperature impacts. With the current pledges, the global temperature rise in 2100 is expected to be 2.0 and 2.1 °C, corresponding to the fulfillment of conditional and unconditional near-term pledges, respectively.  We further explore a suite of emissions pathways to identify some key ‘levers’ across the pledges that can strongly influence the projected global temperature in 2100.  We found that these levers can further reduce the temperature by 0.50 °C or increase by 0.25 °C.  The most significant reductions in temperature rise can be achieved by ratcheting up of the current conditional targets by 10%, shifting the net-zero target year to 2050 for all countries currently having a longer-term goal and adding a net-zero target in 2070 for countries with currently no long-term goals. Inclusion of all these levers can increase the likelihood of limiting temperature rise well below 2 °C and bringing it closer to 1.5 °C. Additional, relatively smaller, contributions accrue from inclusion of Russia, China and India in the Global Methane Pledge. On the other hand, failing to meet even the unconditional NDC targets and delaying the current net-zero targets by 10 years contribute to increases in temperature rise of 0.08 and 0.17 °C. Contributions are also evaluated on a country-by-country basis.

Reference

Tanaka K, Boucher O, Ciais P, Johansson DJA, Morfeldt J (2021) Cost-effective implementation of the Paris Agreement using flexible greenhouse gas metrics. Science Advances 7 (22):eabf9020. doi:10.1126/sciadv.abf9020

How to cite: Tibrewal, K., Tanaka, K., Boucher, O., and Ciais, P.: Levers of climate pledges influencing the Paris Agreement target, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6566, https://doi.org/10.5194/egusphere-egu23-6566, 2023.

X5.168
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EGU23-6584
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ECS
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Moritz Adam, Matthias M. May, Thomas Kleinen, Arya Samanta, and Kira Rehfeld

At the current decarbonization rate, we are set on a path towards re-shaping a substantial share of land for carbon dioxide removal (CDR) over the following decades. However, existing Earth system models which could help to quantify the character of resulting CDR side effects and their consequences for the cumulative CO2 removal do not yet resolve dynamic CDR cover in space. Here, we embark on shedding light on this CDR uncertainty space, scrutinizing CDR impacts in spatial simulations with a comprehensive Earth system model. Assuming CDR to be driven by solar irradiation in the style of photovoltaics, our model is the first to simulate an idealized approach to land-based CDR with its physical, biospheric, and land use couplings on a grid box scale. We analyze dynamic CDR simulations for spatial deployment scenarios according to the country-wise burden of past CO2 emissions, to livelihood constraints, and to optimal irradiation conditions. Shared socio-economic pathways drive the overall global CDR use for a range of potential future emission scenarios. Aside from these spatio-temporal scenarios, the simulations also cover different ways of releasing excess energy from the solar-to-carbon conversion, permitting either local cooling through carbon storage, heat dissipation resulting from system losses or co-benefits for energy production. Based on simulation ensembles for the different scenarios, we quantify Earth system impacts of CDR and their consequences for CO2 removal if grid-scale feedbacks are properly resolved. With new spatially resolved CDR representations in Earth system models we will be able to test CDR-induced Earth system dynamics and CDR promises in greater detail than with existing globally forced projections. This spatially explicit modeling strategy could also open a way toward more comprehensive modeling strategies which include consequences for land use decisions on CDR.

How to cite: Adam, M., May, M. M., Kleinen, T., Samanta, A., and Rehfeld, K.: Consequences of the spatial configuration of Carbon Dioxide Removal for its potential to withdraw atmospheric CO2, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6584, https://doi.org/10.5194/egusphere-egu23-6584, 2023.

X5.169
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EGU23-9715
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ECS
Makcim De Sisto and Andrew H. MacDougall

Nutrient limitation is a core regulation on the amount of carbon fixed by terrestrial vegetation. Hence, the addition of nutrients such as nitrogen and phosphorus in land model structures in Earth system models is essential for an accurate representation of the carbon cycle feedback in future climate projections. Thereby, the estimation of the remaining carbon budget is impacted by the regulation of nutrient limitation in terrestrial ecosystems, and yet it is rarely accounted for. Here, we estimate the carbon budget and remaining carbon budget of a nutrient limited Earth system model, using nitrogen and phosphorus cycles to limit vegetation productivity and biomass. We use eight Shared Socioeconomic Pathways scenarios on three distinct model structures: 1) carbon cycle without nutrient limitation, 2) carbon cycle with terrestrial nitrogen limitation and 3) carbon cycle with terrestrial nitrogen and phosphorus limitation. The three model structures were calibrated to historical temperature data, and to capture the uncertainty of the remaining carbon budget, three different climate sensitives were tuned for each model version. Our results show that overall the nutrient limitation reduced the remaining carbon budget for all simulations in comparison with the carbon cycle without nutrient limitation. Between the nitrogen and nitrogen-phosphorus limitation, the latter had the lowest remaining carbon budget. The mean remaining carbon budget from the Shared Socioeconomic Pathways scenarios simulations for the 1.5 °C target in the no nutrient limitation, nitrogen limited and nitrogen-phosphorus limited models obtained were 303±31, 280±40 and 241±28 Pg C respectively. As for the  2 °C target the mean remaining carbon budget were 517±193, 468±175 and 436±163 Pg C for the no nutrient limitation, nitrogen limited and nitrogen-phosphorus limited models respectively. This represents a reduction of 7.5 and 20.1% for the 1.5 °C target and 9.4 and 15.6% for the 2 °C target in the nitrogen and nitrogen-phosphorus limited simulations compared to the no nutrient limitation model. These results show that terrestrial nutrient limitations constitute an important factor to be considered when estimating or interpreting remaining carbon budgets and are an essential uncertainty of carbon budgets in Earth system models.

How to cite: De Sisto, M. and MacDougall, A. H.: Estimated remaining carbon budgets under terrestrial nutrient limitation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9715, https://doi.org/10.5194/egusphere-egu23-9715, 2023.

X5.170
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EGU23-10823
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Highlight
Kirsten Zickfeld, Alexander J. MacIsaac, Josep G. Canadell, Chris D. Jones, and H. Damon Matthews

Nature-based climate solutions (NBCSs) refer to actions that seek to protect, restore and better manage natural landscapes to reduce greenhouse gas (GHG) emission or remove CO2 from the atmosphere. NBCSs are integral part of many countries’ roadmaps to reach net zero GHG emissions by mid century in compliance with the Paris Agreement climate goals. Implementation of NBCSs not only affects cycling of CO2 and other GHGs in the Earth system, but impacts the energy balance at the Earth’s surface through biophysical effects including changes in reflectivity (albedo), surface roughness and the water cycle, with effects on surface temperature. Furthermore, storage of the sequestered CO2 in above-ground biomass is often vulnerable to natural and anthropogenic disturbances, with the risk of re-release after a few decades. Yet, frameworks that seek to balance residual GHG emissions with nature-based CO2 removal often only consider the CO2 sequestration potential, and do not take the full climate impacts of NBCSs and the vulnerability of carbon stocks into account. By implementing large-scale reforestation as an example of a NBCS in an Earth system model we show that offsetting fossil-fuel CO2 emissions with nature-based CO2 removals to achieve net zero CO2 emissions could result in additional warming compared to the case where the CO2 emissions are avoided, if biophysical effects are not considered and nature-based CO2 storage is temporary. We provide recommendations for taking into account the full climate impacts of NBCSs in net zero accounting frameworks and lay out directions for future research.

How to cite: Zickfeld, K., MacIsaac, A. J., Canadell, J. G., Jones, C. D., and Matthews, H. D.: The full climate impacts of nature-based climate solutions must be considered to achieve climate goals, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10823, https://doi.org/10.5194/egusphere-egu23-10823, 2023.

X5.171
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EGU23-10836
Youngsun Kim and Seung-Eon Lee

The Republic of Korea submitted its updated Nationally Determined Contribution (NDC) to the United Nations Framework Convention on Climate Change (UNFCCC) Secretariat in December 2021. The updated NDC target is to reduce total national greenhouse gas (GHG) emissions by 40% from the 2018 level, which is 727.6 Mt CO2eq, by 2030. According to the updated NDC, local governments are also required to revise their GHG reduction plans. In addition, local governments should self-inspect the progress and major achievements of the GHG reduction plan every year in accordance with the evaluation guideline of the Ministry of Environment. Of 6 metropolitan cities, Gyeonggi Province shows the highest GHG emissions in the country, which accounts for about 17% of the total national GHG emissions in 2021. Ironically, Goyang City, a basic local government of Gyeonggi Province, was selected as one of the seven best local governments for carbon neutrality in 2021. The City has set a reduction target of 32.8% below BAU by 2030 and prepared a plan to implement reduction targets by sector. Over the last decade, building and transportation sectors have been the major sources of GHG emissions in Goyang City, accounting for approx. 70% of the city’s total GHG emissions. The city promotes zero-energy building (ZEB) for newly constructed buildings and encourages green remodeling for existing buildings in order to reduce GHG emissions in the building sector. It is essential to introduce renewable energy such as solar, geothermal, hydrothermal, etc. for ZEB and green remodeling. In this study, therefore, the potential for solar power generation, which is most easily applicable to the building sector, and GHG reduction were calculated for residential buildings in Goyang City. To calculate the available area for solar power on the roof of residential buildings, spatial data was constructed using high-resolution aerial photographs and the outline of the building roof was extracted through AI training data.

 

Acknowledgements

This research was carried out as a part of KICT Research Program (Data-Centric Checkup Technique of Building Energy Performance) funded by the Ministry of Science and ICT.

How to cite: Kim, Y. and Lee, S.-E.: Estimation of renewable power generation and greenhouse gas reduction potential in the building sector, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10836, https://doi.org/10.5194/egusphere-egu23-10836, 2023.

X5.172
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EGU23-12247
Antti-Ilari Partanen, Taru Palosuo, Tommi Ekholm, Markku Ollikainen, and Hannele Korhonen

Finland has set a legally binding goal of achieving “carbon neutrality” by 2035 and net-negative greenhouse gas emissions thereafter. The scientific background for this goal is based on an interpretation of what a nationally fair share of global carbon budget compatible with 1.5 °C warming is. This national carbon budget includes also non-CO2 greenhouse gas (GHG) emissions and is thus stricter than the original, global CO2-only carbon budget. Finland’s pathway to carbon neutrality relies not only on emission reductions but also on carbon sinks in the land use, land-use change and forestry (LULUCF) sector. The net sink in the LULUCF sector, as estimated in the national greenhouse gas inventory, is interpreted as negative emissions. This assumption is problematic, as part of the LULUCF sink is considered natural sink in the conceptual framework behind the global carbon budget estimates and assuming it entirely anthropogenic leads to underestimation of the net CO2 emissions.  

Here we present an analysis and revision of the Finnish net greenhouse gas emission pathway with two major improvements. First, we extend the carbon budget framework to nationally allowed warming to be able to account explicitly also for national non-CO2 GHG emissions. The global allowed warming until 2050 from future GHG emissions is calculated as the sum of the remaining warming to 1.5 °C, the global decreasing warming impact of past non-CO2 GHG emissions by 2050, and future warming due to reduced aerosols by 2050. We use the fair share used previously for allocating national carbon budget to calculate national allowed warming contribution until 2050 and subtract non-CO2 GHG contribution based on a Finnish carbon neutrality scenario and simulations with a simple climate model FaIR 2.1. The remaining allowed warming is then used to calculate the national CO2-only carbon budget. 

 The second improvement is to consider the recent advancements in disentangling natural and anthropogenic carbon fluxes in the LULUCF sector. National results from a recent global study indicate that the Finnish LULUCF sector has been a carbon sink due to the natural sink induced by CO2 fertilization and climate change. The large natural sink is expected to decrease especially in the most stringent global emission reduction scenarios.  

 The preliminary results indicate that Finland’s currently planned pathway is not compatible with its national fair share of allowed warming compatible with the 1.5-degree target, and more stringent emission reductions coupled with strengthening of the land sink and other forms of negative emissions are very likely needed.

How to cite: Partanen, A.-I., Palosuo, T., Ekholm, T., Ollikainen, M., and Korhonen, H.: Net greenhouse gas emission pathway for Finland based on fair share of allowed warming, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12247, https://doi.org/10.5194/egusphere-egu23-12247, 2023.

X5.173
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EGU23-15161
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ECS
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Michael Windisch, Florian Humpenoeder, Leon Merfort, Nicolas Bauer, Jan Philipp Dietrich, Hermann Lotze-Campen, Sonia Seneviratne, and Alexander Popp

Carbon dioxide removal (CDR) can support mitigation efforts and help to limit the footprint of the hardest-to-abate sectors. Forests are one of the most cost-effective solutions to provide this CDR service at scale. Therefore, reforestation has become a major pillar supporting climate targets in scenarios and action plans such as the Nationally Determined Contributions. In addition, forests provide an unassisted aid to climate mitigation, removing a quarter of annual emissions as part of the terrestrial carbon cycle. As a result, today’s mitigation pathways have become a bet on the perpetual growth and permanence of the forest’s carbon storage. However, recent studies are raising doubt about the impeccable future productivity of forests we came to depend on. Forest resilience, especially in biomass hotspots like the Amazon, is in decline. An unexpected carbon stock loss becomes more likely as almost a quarter of primary forests reach critical resilience thresholds. Further, forest disturbances by fire, windfall, and pests become more widespread under changing climatic conditions. Moreover, nutrient limitation might regionally negate positive feedbacks we had hoped for, like CO2 fertilization and prolonged growing seasons. We use the integrated assessment model REMIND-MAgPIE to explore 1.5°C and 2°C mitigation scenarios assuming a range of forest disturbance levels and response timings. Here we show that forest disturbances call for more stringent mitigation targets in all sectors to maintain climate goals. Postponing action instead of preparation risks spiraling costs. Reacting only five years after the disturbance is introduced to the scenario doubles the GDP cost of mitigation action under the same disturbance level. In addition, twice the carbon price is required to reach the same climate goal in 2050. We conclude that even disturbed forests can provide carbon removal services. However, the promise of forest CDR may not be misused to delay decarbonization. Over-relying on forest CDR heightens the risk of unplanned future emissions and leaves us with few options to cope with it.

How to cite: Windisch, M., Humpenoeder, F., Merfort, L., Bauer, N., Dietrich, J. P., Lotze-Campen, H., Seneviratne, S., and Popp, A.: Defending climate targets under threat of forest carbon impermanence, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15161, https://doi.org/10.5194/egusphere-egu23-15161, 2023.

X5.174
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EGU23-16356
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ECS
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Paul R. Price, Barry McMullin, and Aideen O'Dochartaigh

Overshoot of the global 1.5ºC long term temperature goal is likely soon after 2030, so high emitting nations are liable to exceed their fair share of remaining warming to 1.5ºC well before 2030. Net zero globally and for high emitters will occur in overshoot, therefore the meaningful goal is a net negative world until 1.5ºC is reached. In addition to radical near-term reductions in fossil fuel and land CO2 emissions, limiting and returning from overshoot will require substantial warming reduction (negative emissions), via some combination of methane mitigation and carbon dioxide removal (CDR), and limits on excessive agricultural N2O resulting from inefficient reactive nitrogen usage. Therefore, for developed nations and their decision-makers, rapid assessment of the warming impact from primary greenhouse gases for alternative society-wide policy pathway options relative to a fair share of remaining warming to 1.5ºC is required on a clearly defined equity basis. This research applies such a “Paris Test” through: a ‘micro climate model’ GWP* assessment of IPCC 1.5ºC scenarios undertaken to establish a remaining global CO2 warming equivalent (CO2we) budget, aggregated for [CO2+N2O+CH4], to 1.5ºC from 2015; allocation of this budget on a global equal per capita and national population basis to set out 2015 remaining national ‘carbon’ quotas, as of 2015; and, a case study (Ireland) of alternative multi-gas national scenarios to compare aggregate society-wide cumulative CO2we outcomes relative to meeting the 1.5ºC national carbon quota well before 2100. Other equitable budget allocation principles are possible, but this case shows the importance of justifying the reference year choice, and other normative and quantitative assumptions, on a clearly defined “common but differentiated responsibility” basis. The study shows the benefits of such a rapid Paris Test national mitigation policy assessment methodology. Its outputs clarify the considerable difference for developed nations between overshoot net zero, commonly referred to as “no additional warming”, and quota net zero, the Paris Agreement aligned goal, which requires early and substantial CH4 emissions rate reduction as well as CDR. The common use of GWP100 CO2e in mitigation analyses is shown to undervalue the importance of early, deep, and sustained annual CH4 emission rate reduction toward reducing inequitable long-term reliance on uncertain and costly large scale CDR. If the 1.5ºC goal is to be met, by limiting overshoot magnitude and quickly returning to a Paris-consistent net zero quota level, then urgent, substantial and sustained action by developed nations – to radically reduce their fossil fuel use and deforestation responsibility, and to limit nitrogen flows to intensive animal agriculture – will be required at policy ambition levels far greater than those considered ‘technically feasible’ in IPCC mitigation assessments. To meet society-wide, 1.5ºC fair share, national multi-gas quotas, so-called ‘hard-to-abate’ sectors, such as aviation and ruminant agriculture, likely have to be abated substantially and directly within developed nations through policy-directed regulation. This research confirms that the window of options for fair share 1.5ºC climate action in developed nations is closing very rapidly.

How to cite: Price, P. R., McMullin, B., and O'Dochartaigh, A.: Towards a net negative world: applying a rapid “Paris Test” to multi-gas national policy scenarios to assess and enable fair share 1.5ºC achievement, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16356, https://doi.org/10.5194/egusphere-egu23-16356, 2023.