CL3.2.2

The parties of the Paris Agreement agreed to keep global warming well below 2°C and assess the necessary greenhouse gas emissions reductions every five years during the global stocktake. Globally, the necessary reductions in greenhouse gases are often derived using the remaining emissions budget concept. However, estimations of this budget vary by a factor of two to three and may hamper efforts to establish ambitions emissions reductions. Here, we propose an adaptive approach that side-step these uncertainties to quantify these global emissions reductions during the successive global stocktake solely based on regularly updated observations of past temperatures, radiative forcing, and emissions statistics. The approach consists of three main steps repeated every five years: (1) determining the anthropogenic warming to date and hence the remaining warming allowed, (2) estimating the remaining CO₂ forcing equivalent (CO₂-fe) emission budget, and (3) proposing a CO₂-fe or CO₂ emission trajectory for the next 5 years. We test this approach using the Bern3D-LPX Earth System Model of Intermediate Complexity and demonstrate that the temperature targets 1.5°C and 2°C can be reached following a smooth emissions pathway. The adaptive nature makes the approach robust against inherent uncertainties in the observational records, climate sensitivity to emissions, and effectiveness of emissions reduction implementations. The approach thus allows developing an emissions trajectory that would iteratively adapt to ultimately meet the agreed temperature goal. The approach also provides a strong alternative to the often-used pre-defined emissions or concentration pathways (such as SSPs), which can result in very different end-of-century temperatures for the same emission or concentration trajectories. Some of these pathways are developed to be consistent with a given warming level (e.g., SSP1-1.9 for 1.5°C), not knowing the actual response of the Earth system to emissions. As opposed to these simulations, simulations from different models using the adaptive approach we propose here would be directly comparable in terms of warming and broader climate impacts but would differ in terms of required emissions. Our approach would hence guide a valuable and highly policy-relevant complementary set of simulations for the next generation of CMIP models resulting in a range of future emission trajectories compatible with a given global warming target.

How to cite: Terhaar, J., Frölicher, T., Aschwanden, M., Friedlingstein, P., and Joos, F.: Adaptive emission reduction approach to reach the Paris Agreement temperature targets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10398, https://doi.org/10.5194/egusphere-egu22-10398, 2022.

India is now the fourth largest emitter of greenhouse gases (GHG) in the world with one of the highest growth-rate of emissions. As a fast-growing major economy, its future emissions trajectory is important for the long-term global goal of restricting the temperature rise to “well below 2 ℃”, compared to pre-industrial levels. In India, emissions from methane (CH₄) and nitrous oxide (N₂O) account for about a quarter of all greenhouse gas emissions. The agriculture sector contributes to over 70% of these non-CO₂ emissions through activities like rice cultivation, livestock rearing (enteric fermentation and manure management) and application of nitrogen fertilizers. On the other hand, the agriculture sector employs two-third of Indian work force. Around 86% farmers fall in the marginal and small (less than 2 hectares) land-holding category and collectively own about 45% of the total agricultural area and around 80% of total cattle. Considering the socio-economic context, reducing emissions from Indian agricultural sector would be a challenge. The subsistence farming, fragmented production and political economy constraints make it difficult to implement the technological and structural interventions to mitigate the non-CO₂ emissions. If India is to achieve net-zero GHG emissions in the latter half of the century, mitigation strategies for the agriculture sector need to balance the climate and sustainable development goals.

In this research, we focus on methane and nitrous oxide emissions from the Indian agricultural activities. Our analysis uses the GAINS model which has been widely applied for assessing the mitigation strategies for non-CO₂ emissions and multiple air pollutants at regional and global scales. We analyse four mitigation scenarios using different combinations of activities and control measures. For the reference and sustainable policy scenarios, we compare the current policies (often lacking any controls) versus maximum feasible reductions through technological and management control measures to inform the Indian and global climate policy debates. The preliminary results suggest that a combination of sustainable agricultural practices and control measures could reduce the CH₄ and N₂O emissions by about 30% by 2050 as compared to the reference scenario. This would also contribute to the reduction of ammonia emissions with considerable co-benefits for local air quality and health.

How to cite: Patange, O., Purohit, P., Klimont, Z., Garg, A., and Avashia, V.: Mitigation scenarios for methane and nitrous oxide emissions from Indian agriculture sector, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6410, https://doi.org/10.5194/egusphere-egu22-6410, 2022.

The carbon budget concept (TCRE; Transient Climate Response to cumulative carbon Emissions) emerged as a major concept in climate research since the late 2000s. Due to its simplicity, it is intensively utilized in the international policy arena. It is based on the claim that one can derive the global mean temperature increase solely from the knowledge of historical cumulative emissions by observing the linear relationship between the two, regardless of the emission pathway that preceded ('pathway independence').

Here, we ask for the maximally possible deviations from the TCRE ideal across emission scenario space. While there has been an extensive focus on quantifying the carbon budget using highly complex climate models, there seems to be a lesser focus on the pathway independence and possibly related deviations from the budget. Furthermore, few analytical examinations have been presented, for highly stylized settings only. This study contributes to filling that gap, utilizing the energy balance model FAIR. FAIR incorporates climate feedbacks and correctly emulates the temperature response to an emission pulse.

If the carbon budget approach was perfectly valid, the temperature response to an emitted unit of carbon should be a perfect step function. The actual temperature evolution following the emission pulse is reinterpreted as a Green's function and as such, utilized to calculate the total temperature increase at any given point. The novelty in this work is that the emission pathway is not assumed, but generated by maximizing (minimizing) the temperature output.

With the boundary conditions being the fixed total cumulative emissions and the maximal allowed mitigation efforts, two associated pathways are generated with the temperature increase in a given year acting as an objective value. The deviation from the budget is then extracted as a temperature difference between the upper and the lower bound of the optimization process. The results show that the absolute value of the deviation is less than the standard deviation of climate variability, confirming the fundamentals of the carbon budget approach. We also present an analytical upper bound of the deviation from path independence. The result shows that the deviation is a function of the allowed maximum emission slope.

The advantage of this method is that it can utilize the impulse response properties already published for highly complex models. The current limitation of the presented approach lies in the assumption that the pulse response is assumed constant even though the climate changes. The implications of a changing pulse remain to be explored. We see our work as a twofold contribution: (i) to predict maximally possible TCRE deviations from already published impulse response experiments, and (ii), to generate analytic understanding for the driving variables.

How to cite: Avakumović, V., Brovkin, V., and Held, H.: What are the Maximally Possible Deviations from the Carbon Budget Approach? , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4748, https://doi.org/10.5194/egusphere-egu22-4748, 2022.

The IPCC Special Report on 1.5°C (SR1.5)[i] stated “Reaching and sustaining net-zero global anthropogenic CO₂ emissions and declining net non-CO₂ radiative forcing would halt anthropogenic global warming on multi-decadal timescales (high confidence)”, implying that net zero CO₂ emissions and declining non-CO₂ forcing was a sufficient condition for any ongoing global warming to be indistinguishable from natural climate variability on interdecadal timescales. The IPCC 6^th Assessment Report (AR6)[ii] went much further: “limiting human-induced global warming to a specific level requires limiting cumulative CO₂ emissions, reaching at least net zero CO₂ emissions, along with strong reductions in other greenhouse gas emissions”, implying that net-zero CO₂ emissions was a necessary condition for reducing the ongoing rate of global warming to zero. We discuss interpretations of these statements in the context of a policy environment focussed on the coming century, rather than multi-century timescales. We show that two quantities are important in determining the CO₂ emissions and non-CO₂ forcing consistent with halting global warming: the Rate of Adjustment to Constant Forcing (RACF), or the fraction rate of global warming over the decades following forcing stabilisation, and the Rate of Adjustment to Zero Emissions (RAZE), or the RACF minus the centennial rate of CO₂ forcing decline after CO₂ emissions reach net zero. We use results from the Zero Emissions Commitment Model Intercomparison Project to show that the best-estimate value of the RAZE is close to zero, possibly negative at low warming levels, with a range of uncertainty that straddles zero. Hence the evidence currently available suggests only that achieving net zero or net negative CO₂ emissions is as likely as not required to halt CO₂-induced warming on interdecadal timescales. That said, it is virtually certain that any residual emission consistent with no further warming would be an order of magnitude lower than current emission rates and within the uncertainty of CO₂ sources and sinks in the second half of this century.

How to cite: Allen, M., Jenkins, S., Froelicher, T., and Friedlingstein, P.: Emissions consistent with halting global warming, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4362, https://doi.org/10.5194/egusphere-egu22-4362, 2022.

Upstream regulatory measures to require fossil fuel producers and importers to pay for carbon dioxide capture and disposal, such as the Carbon Removal Obligation (CRO, Bednar et al, 2021) or Carbon Takeback Obligation (CTBO, Jenkins et al, 2021), provide a potentially valuable "backstop" mitigation policy if demand-side measures fail to reduce emissions fast enough to meet climate goals. But what if renewable energy costs fall much faster than envisaged in the current generation of integrated assessment models? Would these upstream measures then result in a substantial investment in carbon capture and storage (CCS, encompassing both direct-air and point-source capture) that is subsequently stranded because it is not needed? We explore the implications of ultra-low renewable energy costs under an idealised global CTBO regime and argue that over-building mitigation capacity is unlikely given current trends and in any case would represent a sensible precautionary investment. The risk of stranding of CCS capacity is directly linked to the long-term cost of both CCS and the extraction costs of fossil fuels, reinforcing the case for delivering CCS through obligations on the fossil fuel extraction industry in order to align incentives and ensure that those who benefit most from continued use of fossil fuels also shoulder the risks associated with the transition.

How to cite: Allen, M., Jenkins, S., Ives, M., and Kuijper, M.: Is there still a case for Carbon Takeback or Carbon Removal Obligations in a world of low renewable energy costs?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4419, https://doi.org/10.5194/egusphere-egu22-4419, 2022.

All Shared Socioeconomic Pathways (SSP) of future climate scenarios that are well below 2 °C warming require the application of carbon dioxide removal (CDR) technologies. While the mitigation potentials of different CDR methods have been proposed, the climate impacts have only been studied to a limited extent with the Earth System Models (ESMs). As part of the CDR Model Intercomparison Project (CDRMIP), we utilize here the land-ocean-atmosphere coupled FOCI-MOPS model to study the potential reversibility and impacts of different proposed CDR methods. FOCI-MOP is an integration of the marine biogeochemical model, Model of Oceanic Pelagic Stoichiometry (MOPS), in the Flexible Ocean and Climate Infrastructure (FOCI) ESM. Two CDR methods are studied under highly-idealized scenarios: a marine-based CDR of ocean alkalinity enhancement, and a land-based CDR of afforestation and reforestation, given their large theoretical mitigation potentials. In both experiments, the CDR methods are applied under the high CO₂ emission scenario (SSP5-8.5). In the experiment of ocean alkalinity enhancement, alkalinity is added to ice-free ocean at a rate of roughly 0.14 petamole per year. In the experiment of afforestation and reforestation, the land use follows the scenario with high levels of afforestation and reforestation (SSP1-2.6). We look into the efficiency and the side-effects of CDR methods. In addition, we investigate whether the hysteresis behavior exists as well as the non-reversible aspects of the applied CDR, including ocean deoxygenation as well as the respective impacts on both terrestrial and marine primary production. Finally, as models are largely different in their structures and representations of terrestrial and marine biogeochemistry, we compare our results to results from other models participating in CDRMIP for assessing the modeling uncertainty. The results presented here are helpful for a more realistic application of CDR portfolio and provide insights on a mitigation pathway toward a net-zero world in the future.

How to cite: Wey, H., Kemena, T., Keller, D., and Oschlies, A.: Coupled Model Simulations of Carbon Dioxide Removal via Ocean Alkalinity Enhancement and Large-scale Afforestation and Reforestation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7662, https://doi.org/10.5194/egusphere-egu22-7662, 2022.

There is growing recognition that meeting the climate objectives of the Paris Agreement will require the world to achieve net-zero carbon dioxide emissions around or before mid-century. Nature-based climate solutions (NbCS), which aim to preserve and enhance carbon storage in terrestrial or aquatic ecosystems, are increasingly being evoked as a potential contributor to net-zero emissions targets. However, there is a risk that any carbon that we succeed in storing in land-based systems could be subsequently lost back to the atmosphere as a result of either climate-related or human-caused disturbances such as wildfire or deforestation. Here we quantify the climate effect of NbCS in a scenario where land-based carbon storage is enhanced over the next several decades, and this stored carbon is then returned to the atmosphere during the second half of this century. We show that temporary carbon sequestration has the potential to decrease the peak temperature increase, but only if implemented alongside an ambitious mitigation scenario where fossil fuel CO₂ emissions were decreased to net-zero during the time that NbCS-sequestered carbon remained stored. We also demonstrate the importance of non-CO₂ climate effects of NbCS implementation; decreases in surface albedo that result from temporary reforestation, for example, have the potential to counter almost half of the climate effect of carbon sequestration. Our results suggest that there is some climate benefit associated with NbCS, even if the carbon storage is temporary, but only if implemented as a complement (and not an alternative) to ambitious fossil fuel CO₂ emissions reductions.

How to cite: Matthews, H. D., Zickfeld, K., Dickau, M., MacIsaac, A., Mathesius, S., Nzotungicimpaye, C.-M., and Luers, A.: Temporary nature-based carbon removal can lower peak warming in a well-below 2°C scenario, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8836, https://doi.org/10.5194/egusphere-egu22-8836, 2022.

The exercise of translating a global carbon budget into other policy-relevant metrics, for instance, global and regional emission pathways over time is, unavoidably laden with value judgements, raising questions around inter- and intra-generational equity. As net zero targets are increasingly adopted by countries around the world, clarifying their adequacy from a perspective of fairness is essential. Given significant delays in reducing emissions globally, achieving net zero emissions will require the deployment of carbon dioxide removal (CDR) technologies. Recent studies have started to apply equity-based indicators to assess how emission removal obligations could be shared between countries contrasting the resulting distribution of CDR deployment with cost-optimal distributions produced by Integrated Assessment Models. The choice of framework used to share CDR between countries in Paris Agreement compatible pathways - whether based on principles of equity or a least-cost approach - has implications for how these pathways are used to inform CDR governance and policy. This includes how they are used to evaluate targets for achieving net zero (and even net negative) emissions and the CDR assumptions that underlie them, as well as to assess which CDR technologies should be developed and how they should be financed. 

Here we will explore the principles of equity and justice that can be considered relevant to CDR deployment in the context of the Paris Agreement. Drawing examples from recent analysis (Fyson et al. 2020, Lee et al. 2021), we will look at how such principles could be applied quantitatively to evaluate national targets and policies. In doing so we will highlight the importance of applying an equity and justice lens when developing Paris Agreement compatible emission reduction and removal strategies.

References

Fyson, C. L., Baur, S., Gidden, M. & Schleussner, C. F. Fair-share carbon dioxide removal increases major emitter responsibility. Nat. Clim. Chang. 10, 836–841 (2020).

Lee, K., Fyson, C. & Schleussner, C. Fair distributions of carbon dioxide removal obligations and implications for effective national net-zero targets. Environ. Res. Lett. 16, (2021).

How to cite: Fyson, C., Ganti, G., and Schleussner, C.-F.: Sharing the burden of carbon dioxide removal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11780, https://doi.org/10.5194/egusphere-egu22-11780, 2022.