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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.