Flat10MIP: an emissions-driven experiment to diagnose the climate response to positive, zero and negative CO 2 emissions

Understanding the climate and carbon cycle response to negative CO2 emissions is important for developing climate mitigation strategies that aim to limit global warming to a specific threshold. In this study, using a coupled climate and carbon cycle model, a novel set of nine stylized simulations are conducted with cumulative emissions of 1,000 GtC, 2,000 GtC, and 5,000 GtC over 150, 250, and 500 years, followed by identical cumulative negative emissions so that the net cumulative emissions are zero. On millennial‐timescales, the climate system returns close to the preindustrial state, independent of the emission and removal pathways. However, the thermal and biogeochemical inertia of the ocean play an important role in determining the climate and carbon cycle response during the emission and removal phases. When zero net emissions are reached, surface air temperature is larger by 0–1°C than the preindustrial state, and the atmospheric CO2 concentration is less by 12–29 ppm. These changes increase with both the magnitude and duration of the emission and removal pulses. In contrast, hysteresis in the relationship between global mean surface temperature and cumulative carbon emissions increases with the magnitude but decreases with the duration of emission and removal pulses. Our study highlights the role of ocean inertia in the asymmetry in climate response to emissions and removals and indicates that an earlier emission reduction implying emission/removal pathways with smaller magnitudes and shorter durations for the positive and negative emission pulses would avoid larger climate and carbon cycle impacts on centennial‐timescales.

Ocean negative carbon emissions: A new UN Decade program

Carbon dioxide (CO2) concentrations have increased in the atmosphere as a direct result of human activity and are at their highest level over the last 2-3 million years, with profound impacts on the Earth system. However, the magnitude and future dynamics of land and ocean carbon sinks are not well understood; therefore, the amount of anthropogenic fossil fuel emissions that remain in the atmosphere (the airborne fraction) is poorly constrained. This work aims to quantify the sources and controls of atmospheric CO2, the fate of anthropogenic CO2 over time, and the trend and robustness of the airborne fraction. We use Hector v3.0, a coupled simple climate and carbon cycle model, with the novel ability to explicitly track carbon as it flows through the Earth system. We use a priori probability distribution functions for key model parameters in a Monte Carlo analysis of 10,000 coupled carbon-climate model runs from 1750 to 2300. Results are filtered for physical realism against historical observations and CMIP6 projection data, and we calculate the relative importance of parameters controlling how much CO2 ends up in the atmosphere. Unsurprisingly, we find that anthropogenic emissions are the dominant source of near- and long-term atmospheric CO2, composing roughly 45% of the atmosphere, which is consistent with observational studies of the airborne fraction. The overwhelming majority of model runs exhibited a negative trend in the airborne fraction from 1960-2020, implying that current-day land and ocean sinks are proportionally taking up more carbon than the atmosphere. Furthermore, when looking at the destination of anthropogenic fossil fuel emissions, only a quarter ends up in the atmosphere while more than half of emissions are taken up by the land sink on centennial timescales. This study evaluates the likelihood of airborne fraction trends and provides insights into the dynamics and destination over time of anthropogenic CO2 in the Earth system.

A central finding of the most recent reports of the Intergovernmental Panel on Climate Change (IPCC) is that net zero CO2 emissions are required for stabilizing CO2-induced global average surface temperature increase. However, given substantial uncertainties of the earth system response after reaching net zero CO2, outcomes of long-term declining, or continuously increasing global temperatures after achieving and maintaining net zero CO2 emissions cannot be excluded. At the same time, nearly all emission reduction pathways  assessed by the IPCC, which are consistent with global climate goals, require large-scale deployment of carbon dioxide removal (CDR) to balance remaining positive emissions and to draw down temperatures after a peak. Using evidence from Earth System Models and Simple Climate Models we show that peaking global mean temperature may potentially require net CO2 removals of the order of several hundred gigatonnes (Gt) sustained over multiple decades in case of strong positive earth system feedbacks. We also estimate the contribution of scenario uncertainty (e.g., reduction in emissions of different greenhouse gases) to such net CO2 removal figures. A precautionary approach to ensure a very high chance of peaking of global mean temperature may therefore require the availability of substantial amounts of CDR. Our findings have direct implications for CDR needs in order to achieve the long-term temperature goal of the Paris Agreement. The potential risk of strong earth system feedback outcomes have to be carefully considered when discussing temperature reversal in case of a (temporary) overshoot above the 1.5°C threshold, in particular as there are well-documented biophysical, technological and sustainability limits to CDR deployment.

The ratio of global mean surface air temperature change to cumulative CO2 emissions, referred to as transient climate response to cumulative CO2 emissions (TCRE), has been shown to be approximately constant on centennial time scales. The mechanisms behind this constancy are not well understood, but previous studies suggest that compensating effects of ocean heat and carbon fluxes, which are governed by the same ocean mixing processes, could be one cause for this approximate constancy. This hypothesis is investigated by forcing different versions of the University of Victoria Earth System Climate Model, which differ in the ocean mixing parameterization, with an idealized scenario of 1% annually increasing atmospheric CO2 until quadrupling of the preindustrial CO2 concentration and constant concentration thereafter. The relationship between surface air warming and cumulative emissions remains close to linear, but the TCRE varies between model versions, spanning the range of 1.2°–2.1°C EgC−1 at the time of CO2 doubling. For all model versions, the TCRE is not constant over time while atmospheric CO2 concentrations increase. It is constant after atmospheric CO2 stabilizes at 1120 ppm, because of compensating changes in temperature sensitivity (temperature change per unit radiative forcing) and cumulative airborne fraction. The TCRE remains approximately constant over time even if temperature sensitivity, determined by ocean heat flux, and cumulative airborne fraction, determined by ocean carbon flux, are taken from different model versions with different ocean mixing settings. This can partially be explained with temperature sensitivity and cumulative airborne fraction following similar trajectories, which suggests ocean heat and carbon fluxes scale approximately linearly with changes in vertical mixing.

<p>Ice sheets are important components of the Earth system that are expected to respond strongly to anthropogenic forcing of climate. The aim of this work is to use numerical climate and ice sheet modelling to identify and understand the millennial-scale interaction between the Antarctic and Greenland Ice Sheets (AIS and GIS) and global climate. An initial modelling effort evaluated the response of ice shelves and ice sheets to future CO2 emission scenarios by quantifying the duration and magnitude of summer melt periods. A temperature threshold based on positive degree days was applied to bias-corrected University of Victoria Earth System Climate Model (UVic ESCM) output spanning 1000 years into the future. The simulations indicated that an increase in summer melting over most of the GIS, the Ross and Ronne-Filchner ice shelves, and large sections of the West Antarctic Ice Sheet (where little present-day ablation occurs) could occur if future emissions are not curtailed. This initial work highlighted the need to assess the dynamic response of ice sheets to climate change. I therefore developed an ice sheet/climate model comprised of the UVic ESCM and the Pennsylvania State University Ice Sheet Model. Coupling these models required development of new techniques, including subgrid-scale energy balance calculations that incorporate a surface air temperature (SAT) model bias correction procedure. In testing the model, I found that climate model SAT bias, meltwater refreezing and albedo variations play an important role in simulated ice sheet evolution, particularly as more of the ice surface experiences melting conditions. The model realistically reproduced the AIS and GIS, and captured the surface mass balance (SMB) distributions for both ice sheets well for the present day, including narrow GIS ablation zones. The newly developed model was used to carry out a suite of experiments designed to assess the behavior of the GIS under elevated-CO2 conditions. A deglacial SMB-based GIS stability threshold was identified between 3-4x preindustrial atmospheric levels (PAL) of CO2. Below the threshold, GIS retreat still occurred but the ice ultimately stabilized in a ‘reduced ice sheet’ configuration, while at CO2 >= 4x PAL CO2, ice retreated to mountain ice caps. Ice sheet inception simulations indicated that above 4x PAL CO2, ice growth was limited, while at 4x PAL CO2 ice was able to reach the eastern Greenland coastline. Between 2-3x PAL CO2, separate ice caps in the southern and eastern mountains coalesced and exported ice onto the lowland plains. Large-scale ice sheet growth was limited until 1-2x PAL CO2. GIS ice loss increased with greater cumulative CO2 emissions in transient simulations. However, the ice sheet was able to briefly overshoot the CO2 stability threshold without experiencing drastic ice retreat due to the long response time of the simulated GIS relative to the rate of deep ocean carbon uptake. Finally, several model experiments were carried out using the coupled model to examine the impact of ocean melt-driven AIS retreat on the oceanic circulation and structure. This retreat produced freshwater fluxes to the Southern Ocean that were of the same magnitude (and initially greater) than the background continental flux, and continued for 3000 years after the initial shift to high-melt conditions. The Ross and Weddell Seas became productive sea ice export regions, which resulted in higher salinities in these seas and very low ocean temperatures. Enhanced sea ice export and melt in the open Southern Ocean contributed to a slight shallowing and weakening of the North Atlantic Deepwater circulation cell, that would reinforce predicted trends expected as a result of future anthropogenic CO2 emissions.</p>

This work includes calculations of the cumulative CO2 emissions of two comparable cars—the VW Golf VII—one with a combustion engine and the other with an electric motor. Calculation of CO2 emissions was performed, taking into account the stages of production, utilization and use of the above-mentioned vehicles. For the use phase, it was assumed that the total mileage of the car will be 150,000 km over 10 years. For the electric vehicle, calculations were made assuming five different sources of electricity (from coal only, from natural gas only, from PV and wind turbines, an average mix of European power sources and an average mix of Polish power sources; W1–W5 designations, respectively). For individual sources of electricity, cumulative CO2 emissions were taken into account, that is, resulting both from the production of electricity and the use of the resources (for example, technical service per 1 kWh of electricity produced). The obtained results of the analysis show that for the adopted assumptions regarding operation, for variants W2–W5, the use of an electric car results in lower cumulative CO2 emission than a the use of a combustion car. For a combustion car, the value was 37,000 kg-CO2, and for an electric car, depending on the variant, the value was 43, 31, 16, 23 and 34 thousand kg-CO2 for variants W1 to W5, respectively. Based on the emissions results obtained for individual stages of the use of selected vehicles, a comparative analysis of cumulative CO2 emissions was performed. The purpose of this analysis was to determine whether the replacement of an existing combustion car (that has already been manufactured; therefore, this part of the analysis does not include CO2 emissions in the production stage) with a new electric car, which has to be manufactured, therefore associated with additional CO2 emissions, would reduce cumulative CO2 emissions. Considering three adopted average annual car mileages (3000, 7500 and 15,000 km) and the previously described power options (W1–W5), we sought an answer as to whether the use of a new electric car would be burdened with lower cumulative CO2 emissions. In this case we assumed an analysis time of 15 years. For the worst variant from the point of view of CO2 emissions (W1, electricity from coal power sources only), further use of a combustion car is associated with lower cumulative CO2 emissions than the purchase of a new electric car over the entire analyzed period of 15 years. In turn, for the most advantageous variant (W3, electricity from PV or wind power sources) with an annual mileage of 3000 km, the purchase of a new electric car results in higher cumulative CO2 emissions throughout the analyzed period, whereas for an annual milage of 7500 or 15,000 km, replacing the car with an electric car “pays back” in terms of cumulative CO2 emissions after 8.5 or 4 years, respectively.

<p>The role of Land Cover and Land Management (LCLM) changes in shaping the climate has garnered increasing interest, particularly in light of its potential for climate adaptation and mitigation. Earth System Models (ESMs), however, have hitherto handled LCLM-climate interactions as a unidirectional process, lacking explicit treatment of LCLM-Climate feedbacks. Consequences of these feedbacks nevertheless hold social relevance, affecting agricultural systems, food provision and prices. Furthermore, LCLM can be linked to extreme climate events such as heat waves and drought, which in turn carry economic costs through loss in worker productivity. It is thus essential to integrate LCLM processes and their feedbacks into ESMs, in order to build consistent storylines for future development pathways that take into account their potential for adaptation and mitigation. Moreover, to ensure robustness in the detected LCLM signals, such integration should be done over a range of ESMs.</p><p>Emulators represent a computationally cheap but effective way of approximating ESMs. Here we outline an emulator approach to represent LCLM-Climate feedbacks based on a framework developed by Beusch et al. (2019). This framework provides spatially explicit data by translating annual global mean temperatures into local temperatures and can be extended to use for other relevant variables. The emulator is developed as part of the LAnd MAnagement for CLImate Mitigation and Adaptation (LAMACLIMA) project, and is trained on dedicated ESM simulations that isolate the effects of key land management practices focussed on by LAMACLIMA: irrigation, de/reforestation and wood. Variables considered include temperature, evapotranspiration, runoff, crop yields, carbon storage and heat stress. Besides providing spatially explicit representation of these variables, the emulator also allows flexibility in prescribing land-use scenarios under which their responses are explored.</p><p>Beusch, L. Gudmundsson, and S. I. Seneviratne: Emulating Earth System Model Temperatures: from Global Mean Temperature Trajectories to Grid-point Level Realizations on Land, doi: 10.5194/esd-2019-34, 2019 (accepted for ESD).</p>

Achievability of the Paris Agreement targets in the EU: demand-side reduction potentials in a carbon budget perspective

Based on the pattern of citrus tree/stropharia mushrooms intercropping, returning-straw was used as the raw material for the stropharia mushrooms, and an in-situ experiment was conducted to monitor soil CO2 emissions under different dosage of straw application during the stropharia growth period. Soil CO2 emissions and the influencing factors were analyzed under different treatments of cultivated (HSM, ASM, and DSM) and uncultivated stropharia mushrooms (HS, AS, and DS). The mushroom yield and soil carbon emission efficiency (CEE) were used to provide a theoretical basis for improving the use of land under citrus orchards. The results showed that:① Straw return increased the cumulative CO2 emissions compared with the control system (conventional planting, CK) and cumulative CO2 emissions increased with the dosage of straw application. Cumulative CO2 emissions from soil treated with cultivated stropharia mushrooms were higher than those from soil treated with uncultivated stropharia mushrooms, in the order of DSM (52.09 t·hm-2) > ASM (41.10 t·hm-2) > HSM (33.20 t·hm-2) > DS (27.15 t·hm-2) > AS (25.34 t·hm-2) > HS (18.94 t·hm-2) > CK (12.16 t·hm-2). Cumulative CO2 emissions under the DSM treatment significantly increased by 328.37% compared with CK. ② For the treatment of cultivated stropharia mushrooms, peak soil CO2 emissions occurred during the period of mycelium growth. The highest cumulative CO2 emissions during this period were obtained under the DSM treatment and accounted for 43.27% of the total cumulative emissions. This was followed by ASM and HSM which accounted for 42.63% and 40.57% of emissions, respectively. ③ Cultivated stropharia mushrooms reduced the temperature sensitivity coefficient (Q10). The soil temperature (5 cm depth) had a significant effect on the soil CO2 emission rate (P<0.01) but soil moisture did not (P>0.05). Soil temperature explained 27% to 71% of the variation in soil CO2 emissions rates, and the two-factor fitting of soil temperature and soil moisture explained 36% to 82% of the variation. ④ For the treatment of cultivated stropharia mushrooms, the ranked yield of each treatment was DSM (49.7 t·hm-2) > ASM (47.0 t·hm-2) > HSM (23.3 t·hm-2), and ASM had the highest soil CEE (1.14). Therefore, under the system of citrus tree/stropharia mushroom intercropping, straw return can increase soil CO2 emissions, with the highest emissions being obtained when a double dosage of straw was applied. However, the optimal amount of straw still needs to be determined in combination with changes in soil nutrients and crop yields.

Despite its potential future collapse and profound impacts, assessing the tipping dynamics of the Atlantic Meridional Overturning Circulation (AMOC) remains a significant challenge. Complex models such as Earth System Models (ESMs) and Earth System Models of Intermediate Complexity (EMICs) introduce substantial uncertainties in identifying tipping points. To address this, recent research has focused on developing conceptual models based on non-linear dynamics to capture the tipping behavior of the system. However, existing conceptual models typically simulate the AMOC response to a single temperature forcing, whereas it is well established that the AMOC is also influenced by freshwater flux.In this study, we develop and validate an AMOC Tipping Calibration module that incorporates two forcing parameters: global mean temperature and freshwater flux. This module is designed as an emulator for the AMOC response within cGenie, an EMIC. Following validation, the emulator is integrated into SURFER, a simplified climate model that enables rapid and efficient simulations of AMOC trajectories under various scenario-based pathways. Our results show that incorporating both forcing parameters improves the accuracy of AMOC trajectory predictions. The methodology used to develop the two-parameter emulator is generalizable and can be applied to other tipping elements. By facilitating a greater number of simulations than complex models while maintaining calibration to them, this tool represents a significant advancement in exploring and understanding the potential future behaviour of the AMOC and other tipping elements.

The transient climate response to cumulative CO2 emissions (TCRE) is a useful metric of climate warming that directly relates the cause of climate change (cumulative carbon emissions) to the most used index of climate change (global mean near-surface temperature change). In this paper, analytical reasoning is used to investigate why TCRE is near constant over a range of cumulative emissions up to 2000 Pg of carbon. In addition, a climate model of intermediate complexity, forced with a constant flux of CO2 emissions, is used to explore the effect of terrestrial carbon cycle feedback strength on TCRE. The analysis reveals that TCRE emerges from the diminishing radiative forcing from CO2 per unit mass being compensated for by the diminishing ability of the ocean to take up heat and carbon. The relationship is maintained as long as the ocean uptake of carbon, which is simulated to be a function of the CO2 emissions rate, dominates changes in the airborne fraction of carbon. Strong terrestrial carbon cycle feedbacks have a dependence on the rate of carbon emission and, when present, lead to TRCE becoming rate dependent. Despite these feedbacks, TCRE remains roughly constant over the range of the representative concentration pathways and therefore maintains its primary utility as a metric of climate change.

&amp;lt;p&amp;gt;The IPCC Special Report on 1.5&amp;amp;#176;C concluded that the maximum level of anthropogenic global warming is &amp;amp;#8220;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&amp;amp;#8221; 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.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;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&amp;amp;#176;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.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;We calculate remaining CO2 budgets for a 33, 50 or 66% chance of limiting peak warming to 1.5&amp;amp;#176;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 &amp;amp;#8220;lower 2&amp;amp;#176;C-compatible&amp;amp;#8221; are consistent with a best-estimate peak warming of 1.5&amp;amp;#176;C.&amp;lt;/p&amp;gt;

Abstract. The land surface models JULES (Joint UK Land Environment Simulator, two versions) and ORCHIDEE-MICT (Organizing Carbon and Hydrology in Dynamic Ecosystems), each with a revised representation of permafrost carbon, were coupled to the Integrated Model Of Global Effects of climatic aNomalies (IMOGEN) intermediate-complexity climate and ocean carbon uptake model. IMOGEN calculates atmospheric carbon dioxide (CO2) and local monthly surface climate for a given emission scenario with the land–atmosphere CO2 flux exchange from either JULES or ORCHIDEE-MICT. These simulations include feedbacks associated with permafrost carbon changes in a warming world. Both IMOGEN–JULES and IMOGEN–ORCHIDEE-MICT were forced by historical and three alternative future-CO2-emission scenarios. Those simulations were performed for different climate sensitivities and regional climate change patterns based on 22 different Earth system models (ESMs) used for CMIP3 (phase 3 of the Coupled Model Intercomparison Project), allowing us to explore climate uncertainties in the context of permafrost carbon–climate feedbacks. Three future emission scenarios consistent with three representative concentration pathways were used: RCP2.6, RCP4.5 and RCP8.5. Paired simulations with and without frozen carbon processes were required to quantify the impact of the permafrost carbon feedback on climate change. The additional warming from the permafrost carbon feedback is between 0.2 and 12 % of the change in the global mean temperature (ΔT) by the year 2100 and 0.5 and 17 % of ΔT by 2300, with these ranges reflecting differences in land surface models, climate models and emissions pathway. As a percentage of ΔT, the permafrost carbon feedback has a greater impact on the low-emissions scenario (RCP2.6) than on the higher-emissions scenarios, suggesting that permafrost carbon should be taken into account when evaluating scenarios of heavy mitigation and stabilization. Structural differences between the land surface models (particularly the representation of the soil carbon decomposition) are found to be a larger source of uncertainties than differences in the climate response. Inertia in the permafrost carbon system means that the permafrost carbon response depends on the temporal trajectory of warming as well as the absolute amount of warming. We propose a new policy-relevant metric – the frozen carbon residence time (FCRt) in years – that can be derived from these complex land surface models and used to quantify the permafrost carbon response given any pathway of global temperature change.

We use the HadGEM2-ES Earth System model to examine the degree of reversibility of a wide range of components of the Earth System under idealized climate change scenarios where the atmospheric CO2 concentration is gradually increased to four times the pre-industrial level and then reduced at a similar rate from several points along this trajectory. While some modelled quantities respond almost immediately to the atmospheric CO2 concentrations, others exhibit a time lag relative to the change in CO2. Most quantities also exhibit a lag relative to the global-mean surface temperature change, which can be described as a hysteresis behaviour. The most surprising responses are from low-level clouds and ocean stratification in the Southern Ocean, which both exhibit hysteresis on timescales longer than expected. We see no evidence of critical thresholds in these simulations, although some of the hysteresis phenomena become more apparent above 2 × CO2 or 3 × CO2. Our findings have implications for the parametrization of climate impacts in integrated assessment and simple climate models and for future climate studies of geoengineering scenarios.