Towards a net-zero world: remaining carbon budgets, climate response to different emission pathways, and implications for policy

Remaining carbon budgets specify the maximum amount of CO2 that may be emitted to stabilize warming at a particular level (such as the 1.5 °C target), and are thus of high interest to the public and policymakers. Yet, there are many sources of uncertainty which make it challenging to estimate the remaining carbon budget in real world conditions, especially for ambitious mitigation targets.

This session aims to further our understanding of the climate response under different emission scenarios, with particular interest in emission pathways towards net-zero targets, and to advance our knowledge of associated carbon budgets consistent with meeting various levels of warming. We invite contributions that use a variety of tools, including fully coupled Earth System Models, Integrated Assessment Models, or simple climate model emulators.

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, including: the governing mechanisms behind linearity of TCRE and its limitations, 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 reach a given temperature target (for example, the 1.5 °C warming level from the Paris Agreement), the role of pathway dependence and emission rate, the climate-carbon responses to different emission scenarios (e.g. 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.

Co-organized by BG1/ERE1
Convener: Katarzyna TokarskaECSECS | Co-conveners: Andrew MacDougall, Joeri Rogelj, Kirsten Zickfeld
vPICO presentations
| Thu, 29 Apr, 15:30–17:00 (CEST)

vPICO presentations: Thu, 29 Apr

Chairpersons: Katarzyna Tokarska, Andrew MacDougall, Kirsten Zickfeld
Net-zero emisson targets
Joeri Rogelj, Andy Reisinger, Annette Cowie, and Oliver Geden

With the adoption of the Paris Agreement in 2015 the world has decided that warming should be kept well below 2°C while pursuing a limit of 1.5°C above preindustrial levels. The Paris Agreement also sets a net emissions reduction goal: in the second half of the century, the balance of global anthropogenic greenhouse gas emissions and removals should become net zero. Since 2018, in response to the publication of the IPCC Special Report on Global Warming of 1.5°C, a flurry of net zero target announcements has ensued. Many countries, cities, regions, companies, or other organisations have come forward with targets to reach net zero, or become carbon or climate neutral. These labels describe a wide variety of targets, and rarely detailed. Lack of transparency renders it impossible to understand their ultimate contribution towards the global goal. Here we present a set of key criteria that high-quality net zero targets should address. These nine criteria cover emissions, removals, timing, fairness and a long-term vision. Unless net zero targets provide clarity on these nine criteria, we may not know until it is too late whether the collective promise of net zero targets is adequate to meet the global goal of the Paris Agreement.

How to cite: Rogelj, J., Reisinger, A., Cowie, A., and Geden, O.: Towards high-quality net-zero targets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10425,, 2021.

Nadine Mengis and Andreas Oschlies

Article 4 of the Paris Agreement calls for a “balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century”. It is not made explicit if this balance should be achieved for each of the greenhouse gases (GHGs) individually or if some sum of all GHGs is supposed to become net-zero. This confusion translated into several declared climate targets, that range from carbon-neutral, over GHG-neutral to climate-neutral, and sometimes use these terms interchangingly. However, these targets imply different trajectories in terms of single GHG emissions and result in vastly different temperature trajectories.
Here, we show the implications of this confusion concerning declared climate target metrics, using the most commonly used metric of CO2-equivalent emissions. The same trajectory of net-zero-2050 CO2-equivalent emissions, shows vast differences in short term and long-term temperature and carbon cycle responses, depending on the distribution of CO2-equivalent emissions across the different GHGs.
We emphasize that achieving net zero CO2 emissions remains a necessary precondition for long-term temperature stabilization. We also show that methane emissions reduction can have large short term benefits, as it would strongly reduce the short term temperature and thereby increase the natural carbon uptake. Going forward we recommend to aim for more transparency in declared climate goals and suggest aiming to achieve net zero anthropogenic emissions for all GHGs individually.

How to cite: Mengis, N. and Oschlies, A.: The need for clearer climate target definitions - illustrating ambiguities of net zero CO2-eq targets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12998,, 2021.

Alexander Nauels, Carl-Friedrich Schleussner, and Joeri Rogelj

The treatment of non-CO2 greenhouse gases is central for scientific assessments of effective climate change mitigation and climate policy. Radiative forcing of a unit of emitted short-lived gases decays quickly; on the order of a decade for methane, as opposed to centuries for CO2. Metric selection for comparing the climate effect of these emissions with CO2 thereby comes with choices regarding short- vs. long-term priorities to achieve mitigation. The global nature of the well-mixed atmosphere also has implications for the transferability of concepts such as global warming potentials from the global to the national scale.

Here we present the implications of metric choice on global emissions balance and net zero, with a particular emphasis on the consistency with the wider context of the Paris Agreement, both on the global as well as the national level. Stylized scenarios show that interpreting the Paris Agreement emissions goals with metrics different from the IPCC AR5 can lead to inconsistencies with the Agreement’s temperature goal. Furthermore, we illustrate that introducing metrics that depend on historical emissions in a national context raises profound questions of equity and fairness, thereby questioning the applicability of non-constant global warming potentials at any but the global level. We provide suggestions to adequately approach these issues in the context of the Paris Agreement and national policy making.

How to cite: Nauels, A., Schleussner, C.-F., and Rogelj, J.: Greenhouse gas metrics for net zero targets in science and policy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10816,, 2021.

Andreas Geiges, Claire Fyson, Frederic Hans, Louise Jeffery, Silke Mooldijk, Matthew Gidden, Deborah Ramapope, Bill Hare, Claire Stockwell, Sofia Gonzales, and Leonardo Nascimento
In 2020, climate target announcements were dominated by net-zero commitments, including by a number of major emitters. Despite the urgency of more ambitious NDCs in the short term, long-term net-zero targets are important for the transition to global zero emissions. Tracking progress towards and assessing the adequacy of these targets requires an assessment of what they mean for transition pathways and associated emissions trajectories at both national and global levels. 
We present an assessment of net-zero targets of the major emitting countries and their implications for long-term emissions trajectories and warming levels. Based on the work of the Climate Action Tracker, country-specific analyses are aggregated to a global emissions pathway to derive a best estimate for a resulting global warming in 2100. Undertaking this analysis requires assumptions to be made regarding projected emissions and removals from the land-use sector, non-CO2 emissions, and the trajectory of total net emissions after net-zero, which we explain and explore. For example, by computing the cumulative emissions of our aggregated net-zero target emissions pathway, we can compare this pathway with modelled global emissions pathways from the IPCC's SR1.5 Special Report, to draw broad conclusions over what current net-zero commitments might mean for carbon dioxide removal and non-CO2 emissions, and the uncertainties therein.

How to cite: Geiges, A., Fyson, C., Hans, F., Jeffery, L., Mooldijk, S., Gidden, M., Ramapope, D., Hare, B., Stockwell, C., Gonzales, S., and Nascimento, L.: Implications of current net zero targets for long-term emissions pathways and warming levels, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11018,, 2021.

Ilissa Ocko, Tianyi Sun, and Steven Hamburg

The concept of net zero emissions is now a central element of government and business commitments to addressing climate change, with more net zero policies and pledges being rolled out on an almost daily basis. However, of major concern is the limited awareness of how critical the emissions reduction pathway is in achieving desired climate outcomes. The focus of the climate policy community remains on the target date rather than the path to get there, and net zero "by 2050" is considered by many as the required policy characteristic in achieving temperature targets. Ultimately, the rate and magnitude of future warming relies on the amount, type, and timing of greenhouse gas emissions. Based on different combinations of these factors, it is both possible to succeed or fail in achieving temperature goals even if the global community reaches net zero by 2050. For similar reasons, it is also possible to miss the net zero by 2050 target and still succeed in meeting temperature goals. Therefore, it is important to clarify the role of the decarbonization pathway taken and offer recommendations to ensure that net zero pathways succeed in achieving global climate goals. In this analysis, we show how different net zero paths can lead to a range of temperature outcomes, and how we can strengthen the probability of meeting globally agreed upon climate goals by establishing complementary near-term targets. Key components of ensuring success in achieving temperature targets include incorporating a carbon dioxide budget and acting early to reduce methane emissions. Not only do these actions make achieving our goals more likely, but they also make the path forward more affordable and less dependent on technology not yet available at scale. Overall, improved understanding of the role of the path to net zero would create greater flexibility in effectively fulfilling commitments; open opportunities for trading across groups of greenhouse gases with no loss in climate benefits; and make it easier and cheaper to accomplish corporate and government goals.

How to cite: Ocko, I., Sun, T., and Hamburg, S.: Net-zero greenhouse gas targets: pathway and when reached equally important, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1425,, 2021.

Jiewei Chen, Huijuan Cui, and Yangyang Xu

The Paris Agreement stated a goal to keep global warming to well below 2℃, preferably to  1.5 C above preindustrial levels. To further ensure the implementation of the Paris Agreement, recently, many countries have proposed to achieve carbon neutral between 2050-2070. In this study, we produce a set of carbon neutral scenarios and examined their climate responses using the minimal complexity earth simulator (MiCES). First of all, parameter sensitivity analysis is applied to optimize the parameters for the model using a multi-parameter sensitivity analysis method and output measurement method, which turns out that the 7 parameters related to heat and carbon transferred are most sensitive among all 37 parameters. Uncertainties of the key parameters are further constrained by observed emission and temperature within their uncertainty range, providing reference bounds of parameters with 95% confidence intervals. Then we design ideal emission scenarios with China’s carbon emission peak at 2024,2027,2030 and carbon neutral in 2050,2055,2060,2065,2070 and extrapolated to world’s emission. With improved key parameters’ value, we simulated  climate responses to carbon neutral scenarios. We found that the Paris goal of limiting temperature increase to 1.5 °C above pre-industrial levels will require either negative carbon emission or all greenhouse gases neutral during this century, and the carbon neutral before 2060 proposed by Chinese government will contribute to limiting global temperature increase with the 2 °C level.

How to cite: Chen, J., Cui, H., and Xu, Y.: Climate Simulations to Carbon Neutral with Improved Minimal Complexity Earth Simulator , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5274,, 2021.

TCRE and carbon budgets
Alexandra Avrutin and Philip Goodwin

A central goal of climate science and policy is to establish and follow carbon emissions pathways towards a single metric of changes in the Earth system. Currently, this most often means restricting global mean surface warming to 1.5 and 2 °C, in line with the Paris Climate Agreement. However, anthropogenic emissions do not lead solely to increases in global mean temperature, but also cause other changes to the Earth system. This study aims to quantify carbon emission pathways that are consistent with additional climate targets, and explore the impact of applying these additional climate targets on the future carbon budget. Here, we consider ocean acidification, although eventually multiple additional climate targets could be considered. 

Emission of carbon dioxide leads to ocean acidification, since the ocean is a significant carbon sink in the climate system, absorbing an estimated 16 to 30% of yearly anthropogenic carbon emissions (Friedlingstein et al., 2020). Increased ocean acidification threatens ocean biodiversity, specifically coral reef systems and calcifying organisms, with impacts up the food web. The effects of acidification extend towards human systems, in part due to the impact on fisheries: Narita et al. (2012) estimate that the loss of mollusk production alone due to acidification could cost 100 billion USD globally following a business-as-usual trajectory towards 2100.

Despite the far-reaching damage caused by ocean acidification, there has been little successful effort to explicitly address ocean acidification in climate policy apart from the Paris Agreement warming targets of 1.5 and 2°C (Harrould-Kolieb and Herr, 2012). Although these targets mitigate many elements of dangerous climate change, Schleussner et al. (2016) project that carbon emission pathways consistent with 1.5°C cause 90% of coral reef areas between 66°N and 66°S to be at risk of long-term degradation in all but a single model run.  

Calculating a future carbon budget based on a temperature goal alone is subject to significant uncertainty, largely due to uncertainties in response of the climate system to forcing and natural carbon sequestration. Here, results from a large observation-constrained model ensemble are presented for pathways that achieve multiple climate targets. The uncertainty in the resulting future carbon budget, compared to the budget for temperature-only targets, is discussed. A secondary aim is to establish a pair of mean ocean pH targets that are analogous with the Paris Agreement targets for global mean warming. 


Friedlingstein P. et al., 2020, Earth System Science Data, DOI: 10.5194/essd-12-3269-2020

Narita, D. et al., 2012, Climate Change, DOI: 10.1007/s10584-011-0383-3

Harrould-Kolieb E.R. et al., 2012, Climate Policy, DOI: 10.1080/14693062.2012.620788

Schleussner C-F. et al., 2016, Earth System Dynamics, DOI: 10.1080/14693062.2012.620788

How to cite: Avrutin, A. and Goodwin, P.: Carbon budgets under multiple climate targets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12585,, 2021.

Rachael Treharne, Brendan Rogers, Thomas Gasser, Merritt Turetsky, Erin MacDonald, Carly Phillips, and Sue Natali

Arctic regions are warming more than twice as fast as the global average. This rapid warming is expected to drive a substantial net loss of carbon to the atmosphere, particularly from the thawing of ‘permafrost’, or perennially frozen ground. However, the majority of Earth System Models do not account for permafrost or processes driving the loss of permafrost carbon. In addition, where models do consider permafrost carbon feedbacks, thaw is typically simulated as a gradual, top-down process. This ignores critical, non-linear processes - notably abrupt permafrost thaw, wildfire, and fire-induced permafrost thaw. This means that the potential for a strong positive feedback to future climate change from permafrost regions is not well understood among policy decision-makers. There is therefore an urgent need for a comprehensive and policy-relevant assessment of permafrost carbon feedbacks and their implications for the temperature goals outlined in the Paris Climate Agreement. To address this need, we built upon a reduced complexity Earth System Model and gradual permafrost thaw emulator (Gasser et. al., 2018) by incorporating abrupt thaw, fire emissions, and fire-induced thaw. Using this framework, we assessed the implications of a comprehensive representation of permafrost feedbacks for carbon budgets that constrain warming to 1.5°C and 2°C. We found that combined feedbacks - gradual thaw, abrupt thaw, and fire processes - resulted in a substantial reduction in global carbon budgets to remain below 1.5°C and 2°C.

How to cite: Treharne, R., Rogers, B., Gasser, T., Turetsky, M., MacDonald, E., Phillips, C., and Natali, S.: Cold truths: What does a warmer Arctic mean for carbon budgets consistent with the Paris Agreement?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15535,, 2021.

Ric Williams, Paulo Ceppi, and Anna Katavouta

The controls of a climate metric, the Transient Climate Response to cumulative carbon Emissions (TCRE), are assessed using a suite of Earth system models, 9 CMIP6 and 7 CMIP5, following an annual 1% rise in atmospheric CO2 over 140 years. The TCRE is interpreted in terms of a product of three dependences: (i) a thermal response involving the surface warming dependence on radiative forcing (including the effects of physical climate feedbacks and planetary heat uptake), (ii) a radiative response involving the radiative forcing dependence on changes in atmospheric carbon and (iii) a carbon response involving the airborne fraction (involving terrestrial and ocean carbon uptake). The near constancy of the TCRE is found to result primarily from a compensation between two factors: (i) the thermal response strengthens  in time from more surface warming per radiative forcing due to a strengthening in surface warming from short-wave cloud feedbacks and a declining effectiveness of ocean heat uptake, while  (ii) the radiative response weakens in time due to a saturation in the radiative forcing with increasing atmospheric carbon. This near constancy of the TCRE at least in complex Earth system models appears to be rather fortuitous given the competing effects of physical climate feedbacks, saturation in radiative forcing, changes in ocean heat uptake and changes in terrestrial and ocean carbon uptake.

Intermodel differences in the TCRE are mainly controlled by the thermal response, which arise through large differences in physical climate feedbacks that are only partly compensated by smaller differences in ocean heat uptake. The other contributions to the TCRE from the radiative and carbon responses are of comparable importance to the contribution from the thermal response on timescales of 50 years and longer for our subset of CMIP5 models, and 100 years and longer for our subset of CMIP6 models.


How to cite: Williams, R., Ceppi, P., and Katavouta, A.: Controls of the TCRE in Earth system models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2440,, 2021.

Alexander J. Winkler, Ranga B. Myneni, Markus Reichstein, and Victor Brovkin

The prevailing understanding of the carbon-cycle response to anthropogenic CO2 emissions suggests that it depends only on the magnitude of this forcing, not on its timing. However, a recent study (Winkler et al., Earth System Dynamics, 2019) demonstrated that the same magnitude of CO2 forcing causes considerably different responses in various Earth system models when realized following different temporal trajectories. Because the modeling community focuses on concentration-driven runs that do not represent a fully-coupled carbon-cycle-climate continuum, and the experimental setups are mainly limited to exponential forcing timelines, the effect of different temporal trajectories of CO2 emissions in the system is under-explored. Together, this could lead to an incomplete notion of the carbon-cycle response to anthropogenic CO2 emissions.

We use the latest CMIP6 version of the Max-Planck-Institute Earth System Model (MPI-ESM1.2) with a fully-coupled carbon cycle to investigate the effect of emission timing in form of four drastically different pathways. All pathways emit an identical total of 1200 Pg C over 200 years, which is about the IPCC estimate to stay below 2 °K of warming, and the approximate amount needed to double the atmospheric CO2 concentration. The four pathways differ only in their CO2 emission rates, which include a constant, a negative parabolic (ramp-up/ramp-down), a linearly decreasing, and an exponentially increasing emission trajectory. These experiments are idealized, but designed not to exceed the observed maximum emission rates, and thus can be placed in the context of the observed system.

We find that the resulting atmospheric CO2 concentration, after all the carbon has been emitted, can vary as much as 100 ppm between the different pathways. The simulations show that for pathways, where the system is exposed to higher rates of CO2 emissions early in the forcing timeline, there is considerably less excess CO2 in the atmosphere at the end. These pathways also show an airborne fraction approaching zero in the final decades of the simulation. At this point, the carbon sinks have reached a strength that removes more carbon from the atmosphere than is emitted. In contrast, the exponentially increasing pathway with high CO2 emission rates in the last decades of the simulation, the pathway usually studied, shows a fairly stable airborne fraction. We propose a new general framework to estimate the atmospheric growth rate of CO2 not only as a function of the emission rate, but also include the aspect of time the system has been exposed to excess CO2 in the atmosphere. As a result, the transient temperature response is a function not only of the cumulative CO2 emissions, but also of the time the system was exposed to the excess CO2. We also apply this framework to other Earth system models and observational records of CO2 concentration and emissions.

The Earth system is currently in a phase of increasing, nearly exponential CO2 forcing. The impact of excess CO2 exposure time could become apparent as we approach the point of maximum CO2 emission rate, affecting the achievability of the climate targets.

How to cite: Winkler, A. J., Myneni, R. B., Reichstein, M., and Brovkin, V.: The Transient Response of the Carbon-Cycle-Climate Continuum to CO2 Emissions is Pathway Dependent , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15392,, 2021.

Katarzyna Tokarska, Sebastian Sippel, and Reto Knutti

CO2-induced warming is approximately proportional to the total amount of CO2 emitted. This emergent property of the climate system, known as the Transient Climate Response to cumulative CO2 Emissions (TCRE), gave rise to the concept of a remaining carbon budget that specifies a cap on global CO2 emissions in line with reaching a given temperature target, such as those in the Paris Agreement (e.g., Matthews et al. 2020). However, estimating the policy-relevant TCRE metric directly from the observation-based data products remains challenging due to non-CO2 forcing and land-use change emissions present in the real-world climate conditions.

Here, we present preliminary results for applying and comparing different statistical learning methods to determine TCRE (and later, remaining carbon budgets) from: (i) climate models’ output and (ii) the observational data products. First, we make use of a ‘perfect-model’ setting, i.e. using output from physics-based climate models (CMIP5 and CMIP6) under historical forcing (treated as pseudo-observations). This output is used to train different statistical-learning models, and to make predictions of TCRE (which are known from climate model simulations under CO2-only forcing, per experimental design). Next, we use such trained statistical learning models to make TCRE predictions directly from the observation-based data products.

We also explore interpretability of the applied techniques, to determine where the statistical models are learning from, what the regions of importance are, and the key input features and weights. Explainable AI methods (e.g., McGovern et al. 2019; Molnar 2019; Samek et al. 2019) present a promising way forward in linking data-driven statistical and machine learning methods with traditional physical climate sciences, while leveraging from the large amount of data from the observational data products to provide more robust estimates of, often policy relevant, climate metrics.


Matthews et al. (2020). Opportunities and challenges in using carbon budgets to guide climate policy. Nature Geoscience, 13, 769–779.

McGovern et al. (2019). Making the Black Box More Transparent: Understanding the Physical Implications of Machine Learning, B. Am. Meteorol. Soc., 100, 2175–2199,

Molnar, C. (2019) Interpretable Machine Learning -A Guide for Making Black Box Models Explainable.

Samek, W. et al. (2019) Explainable AI: Interpreting, explaining and visualizing deep learning.

How to cite: Tokarska, K., Sippel, S., and Knutti, R.: Towards data-driven estimates of the transient climate response to cumulative CO2 emissions using interpretable statistical learning methods, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2451,, 2021.

Negative emissions and beyond
Carl-Friedrich Schleussner, Quentin Lejeune, Philippe Ciais, Thomas Gasser, Joeri Rogelj, and Matthias Mengel

Limiting global mean temperature increase to politically agreed temperature limits such as the 1.5°C threshold in the Paris Agreement becomes increasingly challenging. This has given rise to a class of overshoot emissions pathways in the mitigation literature that limit warming to such thresholds only after allowing for a temporary overshoot. However, substantial biogeophysical uncertainties remain regarding the large-scale deployment of Carbon Dioxide Removal technologies required to potentially reverse global warming. Additionally, beyond global mean temperature very little is known about the benefits of declining temperatures on impacts and adaptation needs. Here we will provide an overview of the current state of understanding regarding the reversibility of global warming, as well as impacts and adaptation needs under overshoot pathways.

We highlight the characteristics of the overshoot scenarios from the literature, and especially those that are compatible with identified sustainability limits for Carbon Dioxide Removal deployment. We will compare those characteristics with uncertainties arising from the Earth System’s response which may complicate the efforts to achieve a decrease in Global Mean Temperature after peak warming is reached. This part will include latest results of the permafrost carbon feedback under stylized overshoot scenarios. Eventually, we will summarise the state-of-the-art knowledge and present new results regarding the impacts of overshoot scenarios for non-linear and time-lagged responses such as sea-level rise, permafrost and glaciers. This will allow for a preliminary assessment of the impact and adaptation benefits of early mitigation compatible with a no or low overshoot pathways.

How to cite: Schleussner, C.-F., Lejeune, Q., Ciais, P., Gasser, T., Rogelj, J., and Mengel, M.: Overshooting warming targets – temperature reversibility and implications for impacts, adaptation needs and near-term mitigation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15079,, 2021.

Negar Vakilifard, Katherine Turner, Ric Williams, Philip Holden, Neil Edwards, and David Beerling

The controls of the effective transient climate response (TCRE), defined in terms of the dependence of surface warming since the pre-industrial to the cumulative carbon emission, is explained in terms of climate model experiments for a scenario including positive emissions and then negative emission over a period of 400 years. We employ a pre-calibrated ensemble of GENIE, grid-enabled integrated Earth system model, consisting of 86 members to determine the process of controlling TCRE in both CO2 emissions and drawdown phases. Our results are based on the GENIE simulations with historical forcing from AD 850 including land use change, and the future forcing defined by CO2 emissions and a non-CO2 radiative forcing timeseries. We present the results for the point-source carbon capture and storage (CCS) scenario as a negative emission scenario, following the medium representative concentration pathway (RCP4.5), assuming that the rate of emission drawdown is 2 PgC/yr CO2 for the duration of 100 years. The climate response differs between the periods of positive and negative carbon emissions with a greater ensemble spread during the negative carbon emissions. The controls of the spread in ensemble responses are explained in terms of a combination of thermal processes (involving ocean heat uptake and physical climate feedback), radiative processes (saturation in radiative forcing from CO2 and non-CO2 contributions) and carbon dependences (involving terrestrial and ocean carbon uptake).  

How to cite: Vakilifard, N., Turner, K., Williams, R., Holden, P., Edwards, N., and Beerling, D.: Large ensemble assessment of how the global surface warming response to cumulative carbon differs for negative and positive carbon emissions  , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12718,, 2021.

Ryan Hanna, Ahmed Abdulla, Yangyang Xu, and David Victor

Global emissions of CO2 have been rising at 1–2% per year, and the gap between emissions and what is needed to stop warming at aspirational goals like 1.5ºC is growing. To stabilize warming at 1.5ºC, most studies find that societies must rapidly decarbonize their economy while also removing CO2 previously emitted to the atmosphere. In response to these realities, dozens of national governments, thousands of local administrative governments, and scores of scientists have made formal declarations of a climate crisis that demands a crisis response. In times of crisis, such as war or pandemics, many barriers to policy expenditure and implementation are eclipsed by the need to mobilize aggressively around new missions; and policymaking forged in crisis often reinforces incumbents such as industrial producers. Though highly motivated to slow the climate crisis, governments may struggle to impose costly polices on entrenched interest groups and incumbents, resulting in less mitigation and therefore a greater need for negative emissions.

We model wartime-like crash deployment of CO2 direct air capture (DAC) as a policy response to the climate crisis, calculating (1) the crisis-level financial resources which could be made available for DAC; (2) deployment of DAC plants paired with all combinations of scalable energy supplies and the volumes of CO2 each combination could remove from the atmosphere; and (3) the effects of such a program on atmospheric CO2 concentration and global mean surface temperature.

Government expenditure directed to crises has varied, but on average may be about 5% of national GDP. Thus, we calculate that an emergency DAC program with annual investment of 1.2–1.9% of global GDP (anchored on 5% of US GDP; $1–1.6 trillion) removes 2.2–2.3 GtCO2 yr–1 in 2050, 13–20 GtCO2 yr–1 in 2075, and 570–840 GtCO2 cumulatively over 2025–2100. Though comprising several thousand plants, the DAC program cannot substitute for conventional mitigation: compared to a future in which policy efforts to control emissions follow current trends (SSP2-4.5), DAC substantially hastens the onset of net-zero CO2 emissions (to 2085–2095) and peak warming (to 2090–2095); yet warming still reaches 2.4–2.5ºC in 2100. Only with substantial cuts to emissions (SSP1-2.6) does the DAC program hold temperature rise to 2ºC.

Achieving such massive CO2 removals hinges on near-term investment to boost the future capacity for upscaling. With such prodigious funds, the constraints on DAC deployment in the 2–3 decades following the start of the program are not money but scalability. Early deployments are important because they help drive the technology down its learning curve (indeed, in the long run, initial costs matter less than performance ceilings); they are also important because they increase the potential for future rapid upscaling. Deployment of DAC need not wait for fully decarbonized power grids: we find DAC to be most cost-effective when paired with electricity sources already available today: hydropower and natural gas with renewables; fully renewable systems are more expensive because their low load factors do not allow efficient amortization of capital-intensive DAC plants.

How to cite: Hanna, R., Abdulla, A., Xu, Y., and Victor, D.: Emergency responses to the climate crisis: The case of direct air capture of CO2, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9269,, 2021.

Raphael Apeaning

Achieving a global net-zero emission pathway around mid-century is a critical precondition to limiting global warming to well below 1.5°C. The role of carbon dioxide removal technologies (CDR) such as direct air capture (DAC) and enhanced weathering (EW) have gained centre stage in the discourse of net-zero emission policies. Using an integrated energy-economy-climate modelling tool called GCAM, this study examines the broad sustainability implications of deploying CDR technologies of a global net-zero pathway. Specifically, the applies several Sustainable Development Goals (SDG) indicators as a lens to assesses the synergies and trade-offs associated with upscaling the deployment of DAC and EW technology options.

Based on the best techno-economic performance estimates of DAC and EW technologies, the results show that these technologies can provide about 10.2GtCO2/year of negative emission by 2065. The upscaling of these CDR technologies can substantially reduce the short-to-medium term mitigation cost by about 54.3 %. This policy cost reduction has potential to ameliorate the adverse economic impact of a net-zero pathway by enhancing the SDG targets of No Poverty (i.e. SDG 1) & Decent work and economic growth (i.e. SDG 8). The results also reveal that CDR technologies can reduce the global temperature overshoot by 0.2°C (i.e. SDG 13 (Climate Change)). Further, these CDR solutions can substitute the demand of bioenergy, which in turn leads to major gains in the reduction of cropland (i.e. SDG 15 (Life on Land)).

The enormous transformations in the global energy system to meet the high energy demand for CDR technologies can lead to substantial trade-offs. For instance, the result points to a 27.3% increase in fossil fuel use, nuclear fuel, and carbon sequestration. This trend is counter to the SDG of “responsible consumption and production” (i.e. SDG 12). Also, due to the high capital cost associated with CDR technologies, deploying these technologies at scale has the potential to exacerbate the average cost of energy. Relatively, the increase in energy prices can have adverse effect affordability of energy (i.e. SDG (Affordable and clean energy)). Finally, the result shows other potential security trade-offs in the food and water sectors. Overall, the results provide instructive policy insights about the importance of designing strategies that balance the short and long-term costs and risks of net-zero and CDR technologies.

How to cite: Apeaning, R.: The sustainable development implications of carbon removal technologies in the context of net-zero climate pathway. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14034,, 2021.

Florian Kapmeier, John Sterman, Lori Siegel, Sibel Eker, Tom Fiddaman, Jack Homer, Juliette Rooney-Varga, and Andrew Jones

Integrated assessment models (IAMs) have advanced scientific understanding of mitigation pathways, yet most are not directly accessible to policymakers and other leaders. Here we describe En-ROADS, a publicly available, fully documented online policy simulation model designed to complement IAMs in public outreach. En-ROADS represents the energy-economy-climate system at a globally aggregated level. It has been carefully grounded in the best available science and is calibrated to fit historical data and projections from multiple IAMs across all SSP scenarios. Through an intuitive interface, users choose assumptions, policies, and actions to mitigate GHG emissions, receiving immediate feedback on likely energy, emissions, and climate pathways through 2100, enabling users to explore policies and uncertainties for themselves.



How to cite: Kapmeier, F., Sterman, J., Siegel, L., Eker, S., Fiddaman, T., Homer, J., Rooney-Varga, J., and Jones, A.: En-ROADS: A Global Energy and Climate Simulator to Support Strategic Thinking and Public Outreach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7608,, 2021.

Susanne Baur, Alexander Nauels, and Carl-Friedrich Schleussner

A growing body of literature investigates the effects of Solar Radiation Modification (SRM) on global and regional climates. Previous studies on SRM have mainly focused on potentials and side effects of deployment without addressing plausible avenues of a subsequent phase-out. This would require large-scale carbon dioxide removal (CDR). Here, we look at SRM deployment lengths to keep global temperature increase to 1.5°C under three emissions scenarios that follow current climate policies until 2100 and are continued with varying assumptions about the magnitude of net-negative CDR (-11.5, -10 and -5 GtCO2yr-1). Our results show that there would be a lock-in of around 245 - 315 years of continuous SRM engagement. During peak deployment in 2125 around 2.80 Wm-2 would have to be compensated by SRM, a number at the upper end of currently estimated maximum SRM potential in climate model environments. In total, around 976 - 1344 GtCO2would need to be removed from the atmosphere via CDR. We find only minor effects of SRM on carbon fluxes a few decades after cessation. Our study shows that even if SRM is combined with high CDR, SRM would come with very long legacies of deployment, implying centuries of costs, cumulative risks and all negative side effects of SRM and CDR combined. 

How to cite: Baur, S., Nauels, A., and Schleussner, C.-F.: Solar Radiation Modification: a multi-century commitment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9673,, 2021.