Displays

How to cite: Mengis, N. and Matthews, H. D.: Non-CO2 forcing changes will likely decrease the remaining carbon budget for 1.5°C, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2194, https://doi.org/10.5194/egusphere-egu2020-2194, 2020

The remaining carbon budget quantifies the allowable future CO₂ emissions to keep global mean warming below a desiredlevel. Carbon budget estimates are subject to uncertainty in the Transient Climate Response to Cumulative CO₂ Emissions (TCRE), which measures the warming resulting from a given total amount of CO₂ emitted. Moreover, other sources of uncertainty linked to non-CO₂ emissions have been shown to also strongly affect estimates of the remaining carbon budget. Here we present a new framework that estimates the TCRE using geophysical constraints derived from observations, and integrates the effect of geophysical and socioeconomic pathway uncertainties on the distribution of the remaining carbon budget. We estimate a median TCRE of 0.40 °C and likely range of 0.3 to 0.5 °C (17-83%) per 1000 GtCO₂ emitted. Our 1.5 °C remaining carbon budget has a median value of 710 GtCO₂ from 2020 onwards, with a range of 470 to 960 GtCO₂, (for a 67% to 33% chance of not exceeding the target). Uncertainty in the amount of current warming from non-CO₂ forcing is the dominant geophysical contributor to the spread in both the TCRE and remaining carbon budget estimates. The remaining carbon budget distribution is also strongly affected by current and future mitigation decisions, where the range of non-CO₂forcing across scenarios has the potential to increase or decrease the median 1.5 °C remaining carbon budget by 740 GtCO₂.

How to cite: Matthews, H. D., Tokarska, K., Rogelj, J., Forster, P., Haustein, K., Smith, C., MacDougall, A., Mengis, N., Sippel, S., and Knutti, R.: A new framework for understanding and quantifying uncertainties in the remaining carbon budget, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11278, https://doi.org/10.5194/egusphere-egu2020-11278, 2020

The Transient Climate Response to Cumulative CO₂ Emissions (TCRE) is the proportionality between global temperature change and cumulative CO₂ emissions. The TCRE implies a finite quantity of CO₂ emissions, or carbon budget, consistent with a given temperature change limit. The uncertainty of the TCRE is often assumed be normally distributed, but this assumption has yet to be validated. We calculated the TCRE using a zero-dimensional ocean diffusive model and a Monte-Carlo error propagation (n=10 000 000) randomly drawing from probability density functions of the climate feedback parameter, the land-borne fraction of carbon, effective ocean diffusivity, radiative forcing from an e-fold increase in atmospheric CO₂ concentration, and the ratio of sea to global surface temperature change. The calculated TCRE has a positively skewed distribution, ranging from 1.1-2.9 K EgC^-1 (5-95% confidence), with a mean and median value of 1.9 K EgC^-1 and 1.8 K EgC^-1. The calculated distribution of the TCRE is well described by a log-normal distribution. The CO₂-only carbon budget compatible with 2°C warming is 1 100 PgC, ranging from 700-1 800 PgC (5-95% confidence) estimated using a simplified model of ocean dynamics. Climate sensitivity (climate feedback) is the most influential Earth system parameter on the TCRE, followed by the land-borne fraction of carbon, radiative forcing from an e-fold increase in CO₂, effective ocean diffusivity, and the ratio of sea to global surface temperature change. While the uncertainty of the TCRE is considerable, the use of a log-normal distribution may improve estimations of the TCRE and associated carbon budgets.

How to cite: Spafford, L. and MacDougall, A.: Quantifying the probability distribution function of the Transient Climate Response to Cumulative CO2 Emissions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2200, https://doi.org/10.5194/egusphere-egu2020-2200, 2020

Results from the fully-, biogeochemically-, and radiatively-coupled simulations in which CO₂ increases at a rate of 1% per year (1pctCO2) from its pre-industrial value are analyzed to quantify the magnitude of two feedback parameters which characterize the coupled carbon-climate system. These feedback parameters quantify the response of ocean and terrestrial carbon pools to changes in atmospheric CO₂ concentration and the resulting change in global climate. The results are based on eight comprehensive Earth system models from the fifth Coupled Model Intercomparison Project (CMIP5) and eleven models from the sixth CMIP (CMIP6). The comparison of model results from two CMIP phases shows that, for both land and ocean, the model mean values of the feedback parameters and their multi-model spread has not changed significantly across the two CMIP phases. The absolute values of feedback parameters are lower for land with models that include a representation of nitrogen cycle. The sensitivity of feedback parameters to the three different ways in which they may be calculated is shown and, consistent with existing studies, the most relevant definition is that calculated using results from the fully- and biogeochemically-coupled configurations. Based on these two simulations simplified expressions for the feedback parameters are obtained when the small temperature change in the biogeochemically-coupled simulation is ignored. Decomposition of the terms of these simplified expressions for the feedback parameters allows identification of the reasons for differing responses among ocean and land carbon cycle models.

How to cite: Arora, V., Katavouta, A., Williams, R., Jones, C., Brovkin, V., and Friedlingstein, P. and the rest of C4MIP carbon feedbacks analysis team: Carbon-concentration and carbon-climate feedbacks in CMIP6 models, and their comparison to CMIP5 models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6124, https://doi.org/10.5194/egusphere-egu2020-6124, 2020

Climate change is progressing fast and net negative emissions will most likely be needed to achieve ambitious climate targets. To determine the amount of negative emissions needed, it is key to identify the reversibility of our carbon sinks, i.e. to establish how much their of their strength is lost during declining emissions. Specifically, the strength of the ocean carbon sink is likely to decline with ongoing rising emissions and subsequent negative emissions might lead to the ocean reverting into a carbon source.

In light of these challenges, we analyze strength and reversibility of the ocean carbon sink with the Norwegian Earth System Model under an idealized scenario, the 'Climate and carbon reversibility experiment' of CDRMIP. Here, a strong atmospheric CO₂ increase of 1% per year is assumed until CO₂-concentrations have quadrupled, followed by a decrease of 1% per year until pre-industrial concentrations are restored. Our model results indicate that the oceanic CO₂-uptake is not able to keep pace with the atmospheric rise and descent, but shows only a slow increase of oceanic CO₂-uptake and a sudden decrease with the onset of negative emissions. However, the seasonal envelope illustrates that this is not true for all months but that the oceanic CO₂-uptake during austral winter months shows both a strong uptake and high reversibility.

A regional analysis of seasonal characteristics shows that a strong and reversible CO₂-uptake throughout the experiment is only maintained by the biological pump in high latitudes during spring and summer (austral and boreal, respectively). For other months and latitudes, the oceanic CO₂-uptake is weak or even turns into outgassing due to ongoing warming and subsequent sluggish cooling of sea surface temperature. In our model simulation, the inertia of sea surface temperature is the main cause for the irreversibility of the oceanic CO₂-uptake. This result, however, is highly dependent on the amount of CO₂ taken up during rising emissions. In a corresponding simulation without warming, our model’s ocean takes up more CO₂ during rising emissions, leading to dissolved inorganic carbon being the main cause for the irreversability of the oceanic CO₂ sink.

Our study shows that seasonal mechanisms are of high importance when considering the strength of the ocean carbon sink under negative emissions. Regional monthly trajectories visualize different aspects of biological and physical mechanisms, which can be observed early on and help to verify strength and reversibility of the ocean carbon sink.

How to cite: Goris, N., Tjiputra, J., Pilskog, I., and Schwinger, J.: Strength and reversibility of the ocean carbon sink under negative emissions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17643, https://doi.org/10.5194/egusphere-egu2020-17643, 2020

Bioenergy with carbon capture and storage (BECCS) is one of negative-emission technologies that must be applied if we are to achieve the 1.5 °C, or even the 2 °C, warming targets of the Paris Agreement. As a start, existing coal-fired power plants could be retrofitted to co-fire with biofuel from agricultural and forestry residues, but the potential and costs of BECCS are as yet unassessed. Here, we modelled an optimal county-scale network of BECCS in China, by considering: spatial information on biofuel feedstock; power-plant retrofitting to increase the use of biofuel; biofuel transport to power stations and CO₂ transport to geological repositories for carbon storage; and BECCS life-cycle emissions. BECCS at marginal costs of $100 per tonne CO₂-equivalent (t CO_2-eq)^-1 could abate net CO_2-eq emissions by up to  Gt yr^-1, assuming that CO₂ emitted by power plants could be captured at 90% efficiency and accounting for additional emissions of greenhouse gases from the production cycle of BECCS. Because of the huge stock of useable agricultural and forestry residues in China, this carbon price leverages 20 times more mitigation of CO₂ emissions by BECCS in China than in western North America. To cap cumulative emissions over 2011-2030 from China’s power sector at 5% of the global remaining carbon budget for the 2 °C limit since 2011, BECCS would require marginal costs of $ (t CO_2-eq) ^-1, or the equivalent of investing 0.45% of GDP to generate 1.22 PWh yr^-1 of electricity by 2030; this would abate 35% more carbon emissions than the announced nationally determined contribution by China. These results clarify the economics of emission abatement by BECCS in China, suggesting that using the available domestic biofuel feedstock has the potential to make a great contribution to global carbon emission mitigation.

How to cite: Wang, R. and the BECCS group: Capacity of bioenergy with carbon capture and storage in China consistent with the global remaining carbon budget, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2807, https://doi.org/10.5194/egusphere-egu2020-2807, 2020

Earth System Models (ESMs) are designed to project changes in the climate-carbon cycle system over the coming centuries. These models agree that the climate will change significantly under feasible scenarios of future CO₂emissions. However, model projections still cover a wide range for any given scenario, which impedes progress on tackling climate change. Estimates of the Transient Climate Response to Emissions (TCRE), and therefore of remaining carbon budgets, are affected by uncertainties in the response of land and ocean carbon sinks to changes in climate and CO₂, and also by continuing uncertainties in the sensitivity of climate to radiative forcing. Over the last 7 years Emergent Constraints have been proposed on many of the key uncertainties. Emergent constraints use the full range of model behaviours to find relationships between measureable aspects of present and past climates, and future climate projections. This presentation will summarise proposed emergent constraints of relevance to future climate-carbon cycle projections, and discuss the implications for the remaining carbon budgets for stabilisation at 1.5K and 2K.

How to cite: Cox, P.: Emergent Constraints on Climate-Carbon Cycle Projections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19469, https://doi.org/10.5194/egusphere-egu2020-19469, 2020

Consistent and comparable climate assessments of scenarios are critical within the context of IPCC assessment reports. Given the number of scenarios assessed by WG3, the assessment “pipeline” must be almost completely automated. Here, we present the application of a new assessment pipeline which combines state-of-the-art components into a single workflow in order to derive climate outcomes for integrated assessment model (IAM) scenarios assessed by WG3 of the IPCC. A consistent analysis ensures that WG3’s conclusions about the socioeconomic transformations required to maintain a safe climate are based on the best understanding of our planetary boundaries from WG1. For example, if WG1 determines that climate sensitivity is higher than previously considered, then WG3 could incorporate this insight by e.g. considering much smaller remaining carbon budgets for any given temperature target.

The scenario-climate assessment pipeline is comprised of three primary components. First, a consistent harmonization algorithm which maintains critical model characteristics between harmonized and unharmonized scenarios [1] is employed to harmonize emissions trajectories to a common and consistent historical dataset as used in CMIP6 [2]. Next, a scenario’s reported emissions trajectories are analyzed as to the completeness of its species and sectoral coverage. A consistent set of 14 emissions species are expected, aligning with published work within ScenarioMIP and CMIP6 (see ref [2], Table 2). Should any component of this full set of emissions trajectories be absent for a given scenario, an algorithm (e.g., generalised quantile walk [3]) is employed in order to “back-fill” missing species at the native model regional resolution. Finally, full emissions scenarios are analyzed by an Earth System Model emulator, e.g., MAGICC [4].

In this presentation, we explore differences in climate assessments and estimated remaining carbon budgets across various components of the pipeline for available scenarios in the literature. We consider the impact of alternative choices, especially those made in prior assessments by the IPCC (AR5, SR15), including, for example, the historical emissions database used, the effect of harmonization and back-filling, as well as the version and setup of MAGICC used. 

References

[1] Gidden, M.J., Fujimori, S., van den Berg, M., Klein, D., Smith, S.J., van Vuuren, D.P. and Riahi, K., 2018. A methodology and implementation of automated emissions harmonization for use in Integrated Assessment Models. Environmental Modelling & Software, 105, pp.187-200.

[2] Gidden, M. J., Riahi, K., Smith, S. J., Fujimori, S., Luderer, G., Kriegler, E., van Vuuren, D. P., van den Berg, M., Feng, L., Klein, D., Calvin, K., Doelman, J. C., Frank, S., Fricko, O., Harmsen, M., Hasegawa, T., Havlik, P., Hilaire, J., Hoesly, R., Horing, J., Popp, A., Stehfest, E., and Takahashi, K.: Global emissions pathways under different socioeconomic scenarios for use in CMIP6: a dataset of harmonized emissions trajectories through the end of the century, Geosci. Model Dev., 12, 1443-1475, https://doi.org/10.5194/gmd-12-1443-2019, 2019.

[3] Teske, S. et al., Achieving the Paris Climate Agreement Goals. Springer, 2019.

[4] Meinshausen, M., Raper, S.C. and Wigley, T.M., 2011. Emulating coupled atmosphere-ocean and carbon cycle models with a simpler model, MAGICC6–Part 1: Model description and calibration. Atmospheric Chemistry and Physics, 11(4), pp.1417-1456.

How to cite: Gidden, M., Nicholls, Z., Byers, E., Ganti, G., Kikstra, J., Lamboll, R., Meinshausen, M., Riahi, K., and Rogelj, J.: Climate assessment of emissions scenarios for use in WG3 of the IPCC’s Sixth Assessment Report, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18182, https://doi.org/10.5194/egusphere-egu2020-18182, 2020

Many modelling groups have contributed with CMIP6 scenario experiments to the CMIP6 archive. The analysis of CMIP6 future projections has started and first results indicate that CMIP6 projections are warmer than their counterparts from CMIP5. To some extent this is explained with the higher climate sensitivity of many of the new generation of climate models. However, not only have models been updated since CMIP5 but also the forcings have changed from RCPs to SSPs. The new SSPs have been designed to have the same instantaneous radiative forcing at the end of the 21st century. However, we find that in the EC-Earth3 model the effective radiative forcing differs substantially when the GHG concentrations from the SSP are replaced by those from the corresponding RCP with the same nameplate RF. We estimate that for the SSP5-8.5 and SSP2-4.5 scenarios 50% or more of the stronger warming in CMIP6 than CMIP5 for the EC-Earth model can be explained by changes in GHG gas concentrations. Other changes in the forcing datasets such as aerosols only play a minor role for the additional warming. The discrepancy between RCP and SSP forcing datasets needs to be accounted for when comparing CMIP5 and CMIP6 climate projections and should be properly conveyed to the climate impact, adaptation and mitigation communities.

How to cite: Wyser, K., Kjellström, E., Koenigk, T., Martins, H., and Döscher, R.: Warmer climate projections in CMIP6: the role of changes in the greenhouse gas concentrations from CMIP5 to CMIP6, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18575, https://doi.org/10.5194/egusphere-egu2020-18575, 2020

We present estimates of carbon budgets for different levels of surface air temperature (SAT) increase from multiple 400-member ensembles of simulations with the MIT Earth System Model of intermediate complexity (MESM). Ensembles were carried out using distributions of climate parameters affecting climate system response to external forcing obtained by comparison of historical simulations with available observations.

First, to evaluate MESM performance, we ran two ensembles: one with MESM forced by 1% per year increase in CO₂ concentrations (and with non-CO₂ greenhouse gases (GHG) at pre-industrial level) and the other with GHG concentrations from the RCP 8.5 scenario. Distributions of climate characteristics describing model response to increasing CO₂ concentrations (e.g. TRC and TCRE) as well as values of carbon budgets of exceeding different SAT levels agree well with published estimates.

Then we ran a number of ensembles with MESM driven by emissions produced by the MIT Economic Projection and Policy Analysis (EPPA) Model. Our results show that under stringent mitigation policy concerning non-CO₂ GHGs, the SAT increase can be kept below 2°C relative to pre-industrial with 66% probability through the end of the 21^st century without negative CO₂ emissions. The SAT increase can also be restricted to 1.5°C with 50% probability if such policy is implemented immediately. If GHG emissions follow the path implied by the Paris Agreement pledges through 2030, then it would require either an unrealistically sharp drop in non-CO₂ GHG emissions or negative CO₂ emissions to stay below 1.5°C. Keeping the temperature increase below a chosen value (1.5°C or 2°C) beyond 2100 will most likely require negative CO₂ emissions in part due to difficulties in restricting agricultural methane emissions.

Further analysis shows that temperature change during the 21^st century is also significantly affected by the assumption concerning decrease of SO₂ emissions from energy-intensive industries. Implementation of technologies resulting in reduction of those emissions will decrease the probability of SAT staying below a given limit by about 7-12%. This will affect the time when negative CO₂ emissions will become necessary to prevent temperature increase.

How to cite: Sokolov, A., Morris, J., and Paltsev, S.: Estimates of carbon budgets consistent with global warming of 1.5-2°C from ensembles of simulations with the MIT Earth System Model of intermediate complexity: the role of non-CO2 GHGs and SO2 emissions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5980, https://doi.org/10.5194/egusphere-egu2020-5980, 2020

The IPCC Special Report on 1.5°C concluded that the maximum level of anthropogenic global warming is “determined by cumulative net global anthropogenic CO2 emissions up to the time of net zero CO2 emissions and the level of non-CO2 radiative forcing” in the decades prior to the time of peak warming. Here we quantify this statement, using CO2-forcing-equivalent (CO2-fe) emissions to calculate remaining carbon budgets without treating available mitigation scenarios as a representative sample of possible futures.

CO2-fe emissions are used to calculate an observationally-constrained estimate of the Transient Climate Response to cumulative Emissions (TCRE) using a large ensemble of historical radaitve forcing timeseries. This observationally-constrained TCRE is used to calculate remaining total CO2-fe budgets from 2018 to 1.5°C, which we compare with results discussed in Chapter 2, SR15. We consider contributions to this total remaining budget from CO2 and non-CO2 sources using both historical observations and the available mitigation scenarios in the IAMC scenario database.

We calculate remaining CO2 budgets for a 33, 50 or 66% chance of limiting peak warming to 1.5°C and use these to assess the extent to which scenarios in the IAMC scenario database are consistent with ambitious mitigation as outlined in the Paris Agreement. We argue that, assuming no change in the definition of observed global warming and no increase in TCRE due to non-linear feedbacks, scenarios currently classified as “lower 2°C-compatible” are consistent with a best-estimate peak warming of 1.5°C.

How to cite: Jenkins, S., Cain, M., Friedlingstein, P., Gillett, N., and Allen, M.: Quantifying non-CO2 contributions to remaining carbon budgets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2452, https://doi.org/10.5194/egusphere-egu2020-2452, 2020

The discovery of a nearly linear relationship between cumulative CO₂ emissions and global mean surface temperature increase has given rise to intensive scientific research to assess the maximum allowable CO₂ emissions compatible with some temperature threshold such as the goals of the Paris Agreement. The recent IPCC Special Report on Global warming of 1.5 °C (SR15) used a novel method to calculate remaining carbon budget for the 1.5 °C warming. The first step was to estimate non-CO₂ warming contribution based on a perturbed parameter ensemble of two simple models to get the allowable CO₂-caused warming. The second step was to use a probability density distribution for Transient Climate Response to cumulative CO₂ emissions (TCRE) to calculate the carbon budget from the CO₂-caused warming. One shortcoming of this method is that it ignores potential correlation between non-CO₂ warming contribution and TCRE. A significant part of the non-CO₂ warming comes from decreasing aerosol forcing, and the present-day aerosol forcing linked with TCRE.

Here, we revisit the carbon budgets presented in SR15 by taking correlation into account. We analysed the FaIR model simulations used in SR15 individually and found a linear relationship between TCRE and non-CO₂ warming for a given temperature increase. After a slight rescaling to get the revised carbon budget (for 0.5 °C additional warming)  match with SR15 budget (600 Gt CO₂) for the 50th-percentile TCRE value of 0.45 K/1000 Gt CO₂, the 33th-67^th percentile revised range was 380-960 Gt CO₂, whereas SR15 gave narrower range of 440-850 Gt CO₂. The wider range was expected as high TCRE is likely associated with high present-day aerosol forcing and hence with high non-CO₂ contribution in future warming when aerosol forcing is decreasing. We analysed only results of FaIR model, and final SR15 numbers are an average of results based on FaIR and MAGICC. Therefore, this analysis should be repeated also for the MAGICC runs. As a conclusion, our results show that TCRE and non-CO₂ warming contribution should not be considered independent variables when assessing remaining carbon budgets.

How to cite: Partanen, A.-I., Liu, J., Smith, C. J., and Korhonen, H.: Revisiting carbon budgets of IPCC 1.5-degree report by taking into account correlation with non-CO2 warming and TCRE, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19723, https://doi.org/10.5194/egusphere-egu2020-19723, 2020

Achieving the goals of the Paris agreement, or a stabilization of the global climate, requires strong and sustained mitigation of a range of anthropogenic emissions. A key stepping stone in this effort would be a measurable reduction in the current rate of global warming, which has been approximately constant over time since the 1970s, or a statistically significant deviation of the time evolution of observations from a predetermined baseline expectation. The various components that contribute to anthropogenic climate change are however markedly different in their total, present day impact, and time scales from emissions reductions to an expected climate system response. Here, we investigate when a significant change in global mean surface temperature could be expected, relative to an emission pathway consistent with current global policies, for a broad range of long and short-lived climate forcers. By combining reduced complexity and Earth System modelling, we investigate a comprehensive set of idealized emission mitigation choices for the near term, while still taking into account natural variability. As expected, mitigation of anthropogenic emissions of CO₂ stands out as the most efficient, both in the short and longer term, although very strong mitigation is required to have a clear effect. Further, we find that strong mitigation policy targeting black carbon (BC) emissions would have a rapid, discernible effect, but a low net effect in the longer term. Mitigation of CH₄ stands out as an option that combines rapid effects on surface temperature with long term gains.

How to cite: Samset, B. H., Fuglestvedt, J. S., and Lund, M. T.: Curbing our expectations: Global temperature impacts from strong mitigation of individual climate forcers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4573, https://doi.org/10.5194/egusphere-egu2020-4573, 2020

How to cite: Mentzoni, E. F., Johansen, A., Martinsen, A. R., Rypdal, K., and Rypdal, M.: Uncertainty in remaining carbon budgets increases with less ambitious targets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15062, https://doi.org/10.5194/egusphere-egu2020-15062, 2020

Carbon budgets are a policy-relevant tool that provides a cap on global total CO₂ emissions to limit global mean warming at the desired level, for example, to meet the Paris Agreement target. Internal variability due to natural fluctuations of the climate system affects the temperature and carbon uptake on land and in the ocean. However, uncertainties arising from internal variability have not been quantified in the Transient Climate Response on Cumulative Emissions (TCRE) framework and related carbon budgets. Here we show that even though land carbon uptake exhibits the highest internal variability, most of the uncertainty in TCRE and carbon budgets arises from the temperature component, in concentration-driven simulations. Resulting remaining carbon budgets for 1.5 and 2.0 °C temperature targets differ even up to ±10 PgC (± 36.7 GtCO2; 5-95% range), due to internal variability, which is approximately equivalent to one year of global annual CO₂ emissions. Our results suggest that calculating carbon budgets directly from climate models’ output does not introduce significant biases in TCRE and remaining carbon budgets due to internal variability. 

How to cite: Tokarska, K., Gillett, N. P., Arora, V. K., and Séférian, R.: Assessing the role of internal variability in the carbon budgets framework , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5361, https://doi.org/10.5194/egusphere-egu2020-5361, 2020

Even if surface warming could be kept below 2.0°C or 1.5°C by 2100, global sea-level rise will occur for several centuries or even millennia. One possible interpretation of a successful climate policy for the next few decades could be that it should avoid global-warming induced impacts on climate, ecosystems and human societies not only within this century, but also for the next centuries and beyond. Here, we perform a proof-of-concept study to introduce a constraint on SLR as a new climate target and compare the economic impact to that of a corresponding temperature target.

In the 21st yearly session of the Conference of the Parties in Paris in 2015, SLR threats to the Small Island Developing States (SIDS) prompted a commitment to strive for a lower global temperature target goal of limiting surface warming below 1.5°C. However, an SLR target more directly relates to their existential threats. We here substantially augmented the climate model of the optimizing climate-energy-economy model MIND (Model of Investment and Technological Development) from an impulse-response model to a three-layer ocean model with much-improved representation of ocean heat uptake. We introduce a global total SLR model with four components, one due to ocean thermal expansion, one due to Greenland ice-sheet melting, one due to Antarctic ice-sheet melting, and one due to mountain glaciers and ice cap melting. The newly developed integrated-assessment framework has enabled us to investigate, for the first time, a sea-level rise climate target.

Our results emphasize a key effect of carbon emissions pathways on the future SLR after the 21st century. The shape of carbon emissions pathways will strongly influence future SLR after the 21st century and generally affect SIDS over centuries. To reduce SLR-induced impacts on SIDS, a target is required that not only keeps surface warming below a certain level but also reduces surface warming substantially thereafter. We find that a global SLR target will provide a more sustainable and a lower-cost solution to limit both short-term and long-term climate changes for stakeholders who primarily care about SLR among all global warming impact categories compared to a temperature target with the same SLR by 2200.

We find that the SLR target can provide a temperature overshoot profile through a physical constraint rather than arbitrarily defining an overshoot range of temperature as acceptable. Temperature targets with a limited overshoot have been invoked to make the 2.0° and 1.5°C targets feasible in the context of real-world United Nations climate policy; however, rational constraints on the temperature overshoot have been unclear. SLR targets can be viewed as a reinterpretation of the 2.0° and 1.5°C targets and can provide a rational justification of a certain temperature overshoot for stakeholders who primarily care about SLR. Our present framework with reinterpretation of the widely agreed temperature targets can, in principle, be transferred from SLR targets to impact-related climate targets and can be used to identify a more sustainable path toward meeting the Paris Agreement.

How to cite: Li, C., Held, H., Hokamp, S., and Marotzke, J.: Using global sea-level rise targets to find optimal temperature overshoot profile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13753, https://doi.org/10.5194/egusphere-egu2020-13753, 2020

Unlike historical carbon emissions, which have been driven by economics and politics, climate engineering methods must be scientifically assessed, with consideration as to the type, rate, and total amount implemented. Temperatures reductions from carbon dioxide removal have been found to be proportional to the cumulative amount of carbon removed.  Climate engineering “co-benefits”, such as reduced ocean acidification, may also occur and should be considered when optimising an engineered climate solution. In this study we examine the sensitivities of climate engineering to its implementation, focussing on the effects of its time of onset and rate of carbon capture or enhanced weathering, as well as  background emissions and ocean physics.

We use two simple coupled models– a Gnanadesikan-style coupled atmosphere-ocean model and the intermediate-complexity Earth system model GENIE – with idealised setups for negative emissions through either carbon capture and sequestration, enhanced weathering, or a combination. The inclusion of enhanced weathering provides insight as to how changes in ocean carbonate chemistry may impact climate, both in terms of temperature and pH changes. We have created ensembles in which the timing, rate, background emissions scenario, and model physics of the model vary and use these ensembles to understand how these decisions may impact the efficacy of climate engineering.

We find that the effectiveness of climate engineering is dependent upon the background carbon emissions and the choice of climate engineering. Carbon capture reduces surface average temperature more per PgC captured than enhanced weathering, and both are more effective under low emissions scenarios. Additionally, background emissions determine how the impact of climate engineering is realised: under high emissions, earlier implementation of climate engineering results in faster temperature mitigation, although the end state is independent of the onset. When considering reductions in ocean acidification, we find that the alkalinity flux in our enhanced weathering experiments leads to a higher pH than for carbon capture, as well as the pH signals being less dependent on the timing. Thus, the timing and pathway of the climate engineering is important in terms of the resulting averted warming and acidification, though the final equilibrium is still effectively determined by the cumulative carbon budget.

How to cite: Turner, K., Williams, R. G., Katavouta, A., and Beerling, D. J.: Sensitivity of climate mitigation signals to climate engineering choice and implementation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18859, https://doi.org/10.5194/egusphere-egu2020-18859, 2020

The warming caused by past CO₂ emissions is known to persist for centuries to millennia, even in the absence of additional future emissions. Other non-CO₂ greenhouse gas emission have caused additional historical warming, though the persistence of this non-CO₂ warming varies among gases owing to their different atmospheric lifetimes. Under deep mitigation scenarios or in an idealized scenario of zero future greenhouse gas emissions, the past warming from shorter-lived non-CO₂ gases has been shown to be considerably more reversible than that caused by CO₂ emissions. Here we use an intermediate-complexity global climate model coupled to an atmospheric chemistry module to quantify the warming commitment and its reversibility for individual and groups of non-CO₂ greenhouse gases. We show that warming caused by gases with short atmospheric lifetimes will decrease by more than half its peak value within 30 years following zeroed emissions at present day, with more 80 percent of peak temperature reversed by the end of this century. Despite the fast response of atmospheric temperature to the elimination of non-CO₂ emissions, the ocean responds much more slowly: past ocean warming does not reverse, but rather continues for several centuries after zero emissions. Further consequences are shown for the land carbon pool, which decreases as an approximately linear function of historical non-CO₂ greenhouse gas induced warming. Given that CO₂ and non-CO₂ greenhouse gas emissions share common emission sources, we also explore a set of scenarios where sets of emissions are zeroed according to two broad source categories: (1) fossil fuel combustion, and (2) land-use and agriculture. Using these additional mode runs, we investigate the temperature change that is avoided if all CO₂ and non-CO2 greenhouse gas emissions from a particular source abruptly stops while others are allowed to continue. These results indicate the possibility of land-use change and agriculture activities continuing under deep mitigation scenarios and ambitious climate targets, without leading to exceedance of global climate targets. Though we analyze unlikely scenarios, our work provides baselines from which more realistic mitigation scenarios can be assessed. The reversibility of peak temperature caused by historical non-CO₂ gases is a relevant measure for policy frameworks seeking to limit global warming to ambitious targets, such as the 1.5 ^oC target adopted by the Paris Agreement

How to cite: MacIsaac, A., Matthews, H. D., Mengis, N., and Zickfeld, K.: The greenhouse gas climate commitment and reversibility of peak warking from historical emissions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12750, https://doi.org/10.5194/egusphere-egu2020-12750, 2020

The surface warming response to carbon emissions, defines a climate metric, the Transient Climate Response to cumulative carbon Emissions (TCRE), which is important in estimating how much carbon may be emitted to avoid dangerous climate. The TCRE is diagnosed from a suite of 9 CMIP6 Earth system models following an annual 1% rise in atmospheric CO2 over 140 years.   The TCRE   is nearly constant in time during emissions for these climate models, but its value   differs between individual models. The near constancy of this climate metric is due to a strengthening in the surface warming per unit radiative forcing, involving a weakening in both the climate feedback parameter and   fraction of radiative forcing warming the ocean interior, which are compensated by a weakening in the radiative forcing per unit carbon emission from the radiative forcing saturating with increasing atmospheric CO2. Inter-model differences in the TCRE are mainly controlled by the   surface warming response to radiative forcing with large inter-model differences in physical climate feedbacks dominating over smaller, partly compensating differences in ocean heat uptake. Inter-model differences in the radiative forcing per unit carbon emission   provide smaller inter-model differences in the TCRE, which are mainly due to differences in the ratio of the radiative forcing and change in atmospheric CO2 rather than from differences in the airborne fraction.     Hence, providing tighter constraints in the climate projections for the TCRE during emissions requires improving estimates of the physical climate feedbacks,   the rate of ocean   heat uptake, and how the radiative forcing saturates with atmospheric CO2.

How to cite: Williams, R., Ceppi, P., and Katavouta, A.: Controls of the Transient Climate Response to Emissions: effects of physical feedbacks, heat uptake and saturation of radiative forcing , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8433, https://doi.org/10.5194/egusphere-egu2020-8433, 2020

The majority of emissions scenarios that limit warming to 2°C, and nearly all emission scenarios that do not exceed 1.5°C warming by the year 2100 require negative CO₂ emissions. Negative emission technologies (NETs) in these scenarios are required to offset emissions from sectors that are difficult or costly to decarbonize and to generate global ‘net negative’ emissions, allowing to compensate for earlier emissions and to recover a carbon budget after overshoot. It is commonly assumed that the carbon cycle and climate response to a negative CO₂emission is equal in magnitude and opposite in sign to the response to an equivalent positive CO₂ emission, i.e. that the climate-carbon cycle response is symmetric. This assumption, however, has not been tested for a range of emissions. Here we explore the symmetry in the climate-carbon cycle response by forcing an Earth system model with positive and negative CO₂emission pulses of varying magnitude and applied from different climate states. Our results suggest that an emission of CO₂into the atmosphere is more effective at raising atmospheric CO₂than a CO₂removal is at lowering atmospheric CO₂, indicating that the carbon cycleresponse is asymmetric, particularly for emissions/removals > 100 GtC. The surface air temperature response, on the other hand, is largely symmetric. Our findings suggest that the emission and subsequent removal of a given amount of CO₂ would not result in the same atmospheric CO₂concentration as if the emission were avoided. Furthermore, our results imply using simple models used to estimate negative emission requirements may result in underestimating the amount of negative emissions needed to attain a given CO₂concentration target.

How to cite: Zickfeld, K., Azevedo, D., and Matthews, D.: Asymmetry in the climate-carbon cycle response to positive and negative CO2 emissions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12790, https://doi.org/10.5194/egusphere-egu2020-12790, 2020

To stabilize long-term climate change at well-below 2°C (ideally below 1.5°C) above pre-industrial levels, large and sustained CO₂ emission reductions are needed.  Despite pledges from numerous governments, the world is not on track to achieve the required reductions within the timeframes outlined in the Paris Agreement, and it appears increasingly likely that an overshoot of the 1.5 or 2 °C temperature target will occur.  If this happens, it may be possible to use carbon dioxide removal methods to return atmospheric CO₂ concentrations to lower levels or even to reduce the magnitude of the overshoot, with the hope that lower CO₂ will rapidly lead to lower temperatures and reverse or limit other climate change impacts.  Here we present a multi-model analysis of how the Earth system and climate respond during the CMIP6 CDRMIP cdr-reversibility experiment, an idealized overshoot scenario, where CO₂ increases from a pre-industrial level by 1% yr^-1 until it is 4 times the initial value, then decrease again at 1% yr^-1 until the pre-industrial level is again reached, at which point CO₂ is held constant.  For many modelled quantities climate change appears to eventually be reversible, at least when viewed at the global mean level.  However, at a local level the results suggest some changes may be irreversible, although spatial patterns of change differ considerably between models.  For many variables the response time-scales to the CO₂ increase are very different than to the decrease in CO₂ with a many properties exhibiting long time lags before responding to decreasing CO₂, and much longer again to return to their unperturbed values (if this occurs).

How to cite: Keller, D., Lenton, A., Scott, V., and Vaughan, N. and the Modelling groups who contributed to the carbon dioxide removal model intercomparison project and CDRMIP steering committee members: Is Climate Change Reversible? CDRMIP simulations of the Earth system response to a massive CO2 increase and decrease (emissions followed by negative emissions)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9435, https://doi.org/10.5194/egusphere-egu2020-9435, 2020

In order to reach the reduced carbon emission targets proposed by the Paris agreement one of the widely proposed decarbonizing strategies, referred to as negative emissions technologies (NETs), is the production and combustion of second-generation bioenergy crops in conjunction with carbon capture and storage (BECCS). The international research on NETs has grown rapidly and publications have ranged in scope from reviewing potential and assessing feasibility to technological maturity and discussions on deployment opportunities. However, concerns have been increasingly raised that ungrounded optimism in NETs potential could result in delayed reductions in gross CO₂ emission, with consequent high-risk of overshooting global temperature targets. Negative emissions as a consequence of BECCS are achieved when the CO₂ absorbed from the atmosphere during the growth cycle of biomass is released in combustion and energy production and then captured and stored indefinitely. The simplistic vision of BECCS is that one ton of CO₂ captured in the growth of biomass would equate to one ton of CO₂ sequestered geologically- which we can regard as a carbon efficiency of 1. However, biomass crops are not carbon neutral as GHG emissions are associated with the cultivation of biomass.  Furthermore, throughout the BECCS value chain carbon ‘leaks’. Some life cycle analyses of the entire value chain for a BECCS crop to final carbon storage in the ground have shown leakage of CO₂ to be greater than the CO₂ captured at the point of combustion and thus it has low carbon efficiency. The deployment of BECCS is ultimately reliant on the availability of sufficient, sustainably sourced, biomass for an active CCS industry operating at scale and a favourable policy and commercial environment to incentivise these investments. It has been suggested that the theoretical global demand for biomass for BECCS could range from 50 EJ/yr up to more than 300 EJ/yr, although the technical and economic potential will be significantly less and will be dependent on uncertain social preferences and economic forces. The two most important factors determining this supply are land availability and land productivity. These factors are in turn determined by competing uses of land and a myriad of environmental and economic considerations. It is suggested that removing 3.3 GtC/year with BECCS could annually require between 360 and 2400 Mha of marginal land. The upper bounds correspond to 3x the world’s harvested land for cereal production. The conclusion is that estimates of biomass availability for the future depends on the evolution of a multitude of social, political, and economic factors including land tenure and regulation, trade, and technology. Consequently, the assumptions, in future climate scenarios, that high rates of NETs can be achieved across many countries and land types is not yet demonstrated.

How to cite: Jones, M.: Can biomass supply meet the demands of BECCS?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5461, https://doi.org/10.5194/egusphere-egu2020-5461, 2020

Insect outbreaks have a substantial effect on forest carbon cycle, including turning forests from carbon sink to carbon source. However, there is still lack of evaluation of the impact of insect disturbance on forests carbon cycle across different forest types at the global scale. Hence, we conducted a multi-site analysis to compare ecosystem CO2 fluxes’ change after an insect outbreak by compiling flux data from the literature or flux database. The final database consists of 21 site-years of eddy covariance data from 17 forest sites among diversity of forest types, namely temperate forest, mangrove, larch forest, etc. Our research showed that insect outbreak had significant negative effects on GEP that GEP reduces -186 gCm-2y-1 on average, as well as NEP reducing -146 gCm-2y-1 while had no significant positive effect on Re. Additionally, similar conclusion was gained when analyzing the recovery procedure that GEP increases with time at the slope of 67.6 gCm-2y-2 and NEP increases at slightly lower rate of 49.2 gCm-2y-2. But there is no evidence that Re will increase. As insects cause more severe damage to forests, all three carbon variables share negative correlation that GEP drops at the slope of -6.7 gCm-2y-1 and NEP also decreases in relatively similar rate at 66.3 gCm-2y-1 with Re decreasing by -3.4 gCm-2y-1. Also, it was found that girdling experiments has different impact on carbon budget from normal insect outbreaks while drought could dampen the damage to GEP followed with a greater recovery rate.

How to cite: Chen, L.: The effect of insect disturbance on forest carbon flux: a multisite analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8038, https://doi.org/10.5194/egusphere-egu2020-8038, 2020