Tropical rainfall is important for regional climate around the globe. In a warming climate forced by rising CO₂, the tropical rainfall will increase over the equatorial Pacific where sea surface warming is locally enhanced. Here, we analyze an idealized CO₂ removal experiment from the Carbon Dioxide Removal Model Intercomparison Project and show that the tropical rainfall change features a stronger pattern during CO₂ ramp-down than ramp-up, even under the same global mean temperature increase, such as the 2 °C goal of the Paris Agreement. The tropical rainfall during CO₂ ramp-down increases over the equatorial Pacific with a southward extension, and decreases over the Pacific intertropical convergence zone and South Pacific convergence zone. The asymmetric rainfall changes between CO₂ ramp-down and ramp-up result from time-varying contributions of the fast and slow oceanic responses to CO₂ forcing, defined as the responses to abrupt CO₂ forcing in the first 10 years and thereafter, respectively, in the abrupt-4xCO2 experiment. The fast response follows the CO₂ evolution, but the slow response does not peak until 60 years after the CO₂ peak. The slow response features a stronger El Niño-like pattern, as the ocean dynamical thermostat effect is suppressed under stronger subsurface warming. The delayed and stronger slow response leads to stronger tropical rainfall changes during CO₂ ramp-down. Our results indicate that returning the global mean temperature increase to below a certain goal, such as 2 °C, by removing CO₂, may fail to restore tropical convection distribution, with potentially devastating effects on climate worldwide.

How to cite: Zhou, S., Huang, P., Xie, S.-P., Gang, H., and Wang, L.: Varying contributions of fast and slow responses cause asymmetric tropical rainfall change between CO2 ramp-up and ramp-down, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4118, https://doi.org/10.5194/egusphere-egu23-4118, 2023.

Under current mitigation implementations, it is of increasing likelihood that global warming will exceed the target set by the Paris Agreement (PA) of “well below 2°C”. Correcting for this overshoot through intense carbon dioxide removal could reverse global warming back to safe levels, but the biophysical impacts associated with pathways exposing the planet to dangerous warming levels is essentially unknown. This is particularly the case for the ocean ecosystem, where peak warming could lead to ecosystem threshold exceedance and non-reversible changes. Here, we investigate spatial asymmetries in the response of surface ocean ecosystem stressors to temporarily overshooting the PA target using a novel model framework.

To advance the knowledge on temporary overshoots, we utilized the Adaptive Emission Reduction Approach to design first-order emission pathways to reach given stabilization and overshoot peak temperature targets. With the help of this framework, we performed simulations with the Earth System Model GFDL-ESM2M that overshoot by 0.5°C and 1.5°C, and thereafter returns to the quasi-stabilized warming level of a simulation that respects the PA target.  

Our preliminary analysis shows important differences in regional ocean characteristics between simulations that overshoot the PA target and a simulation that stabilizes at the PA target, despite ultimately reaching the same global surface temperature. For instance, regional sea surface temperatures can differ by over 0.5°C in the extreme overshoot of 1.5°C in comparison to the PA stabilization simulation, even following the overshoot. This spatial heterogeneity is illustrated through the divergent oceanic response of the polar oceans; In northern latitudes, cooler temperatures are simulated through the expansion of the North Atlantic cold spot during the overshoot, which arises through decrease of heat transport of the Gulf Stream owing to the weakening of the Atlantic Meridional Overturning Circulation (AMOC) of around 6 Sv or 30 %. In turn, the Southern Ocean is substantially warmed regionally in the overshoot simulations versus the stabilization, likely originating from increasing cross-ocean transport of heat during the overshoots, an implication that is ongoing even after the temporary overshoots return to the PA warming level. Similar spatial heterogeneities are also found for other ecosystem stressors such as O₂ and pH, hinting at potential disruption of regional ecosystems. Our analysis indicates that increased assessment of regional ocean responses to temporary warming exceedance levels and their impacts for regional ecosystems are urgently needed in the scope of fully evaluating trade-offs associated with delaying climate action and overshooting the PA.      

How to cite: Lacroix, F., Burger, F., Silvy, Y., and Froelicher, T.: Divergent Response of Ocean Regions to Temporarily Overshooting Paris Agreement Warming Levels, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6636, https://doi.org/10.5194/egusphere-egu23-6636, 2023.

Northern peatlands store 300~600 Pg carbon, with approximately half underlain by permafrost. Climate change is progressively threatening this large carbon stock. Future temperature rise is likely to trigger changes in this already vulnerable system and can cause irreversibility or strong hysteresis, increasing natural CO₂ and CH₄ emissions. However, the role of northern peatlands in carbon cycle under various future emission scenarios is still unclear. Elevated temperature and atmospheric CO₂ level may enhance the carbon sink in northern peatlands, while increased decomposition may lead to higher CO₂ emissions. At the same time, massive amount of CH₄ may be released due to permafrost thaw and other processes induced by future climate change.

The large carbon pool, as well as the often-delayed responses of northern peatlands, is an essential component in evaluating future climate responses, especially under overshoot scenarios. Ignoring the climate impact on this carbon pool may lead to misestimations of the carbon fluxes of terrestrial systems under future emission scenarios. Furthermore, it is critical to fully understand its feedback, which may make a significant contribution to the future carbon budget. Therefore, the role of peat carbon feedback in meeting the Paris Agreement goals must be investigated and quantified.

Significant progresses have been made in representing the northern peatlands with process-based complex land surface models. These results have laid a solid foundation to allow the development of our new peat carbon emulator. Compared to the extended run time required for complex land surface models, our emulator allows for running a large number of scenarios within a short time frame. The parameterizations of the peat carbon cycle, such as vegetation growth, soil carbon accumulation, organic matter decomposition, and CO₂ and CH₄ emissions are calibrated on five state-of-the-art complex land surface models that specifically represent high-latitude peatlands. By coupling the peat carbon emulator into the compact Earth System Model OSCAR, we incorporated the peatland feedback into global carbon-climate system and explored interactions between multiple feedbacks. Under a series of overshoot scenarios, we show that effective climate change mitigations are needed to prevent peat C loss and consequently positive climate feedback in the future.

How to cite: Zhu, B., Qiu, C., Gasser, T., Tanaka, K., and Ciais, P.: Future northern peatland responses and its climate feedback under overshoot scenarios, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7511, https://doi.org/10.5194/egusphere-egu23-7511, 2023.

Limited global gas emissions reductions would lead to exceeding 1.5˚C and even 2˚C of global warming compared to pre-industrial levels. Yet many emission scenarios included in the last IPCC report feature a decrease in Global Mean Temperature (GMT) after peak warming, even suggesting that levels compatible with the Long-Term Temperature Goal of the Paris Agreement could be reached again within the 21^st century. In contrast with the prominence of overshoot pathways in the literature, their implications for adaptation planning have not been discussed. Many factors are taken into account when it comes to decision-making about adaptation, however it could be argued that the perspective of seeing in the future Global Mean Temperature decrease after peak warming constitutes a disincentive to deploy adaptation measures dimensioned against impacts to be expected at peak warming.

In this study, we take the viewpoint of a decisionmaker who is trying to determine the extent to which they should adapt their assets to optimize costs. We assume that this decision is made in 2040, when global warming has just reached 1.5˚C and that the decisionmaker has full knowledge of the future evolution of GMT and resulting potential impacts from 1-in-100-year tropical cyclones on their assets. We consider three idealized pathways: two overshoot pathways in which GMT peaks in 2060 (at 1.65 or 1.8˚C) before coming back to 1.5 in 2080, and a third one in which it stabilizes at 1.65˚C as of 2060. We then compare the sum of the one-off costs of implementing adaptation measures and of the expected damages from tropical cyclones on the assets for two options: the decisionmaker decides to adapt against the level of damages expected at 1.5˚C, or at Peak Warming (1.65 or 1.8˚C). We also assess the sensitivity of the results to the evolving perceived value of the assets via a discount or growth (inflation) rate.

We find that adapting to impacts at peak warming is more cost-efficient for adaptation measures characterized by high efficiency (effectiveness divided by costs), while adapting to impacts at 1.5˚C is more cost-efficient for measures with low efficiency. In contrast, which option is more cost-efficient does not depend on the GMT pathway, although the differences in total costs between the two options become stronger in pathways that reach higher levels of global warming. Higher discount rates constitute incentives to adapt to lower levels of global warming, whereas this is the opposite for higher growth rates. Overall, these results suggest that the perspective of decreasing GMT in the future plays a limited role in adaptation decisionmaking, and that it should not be perceived as an incentive not to deploy adaptation measures.

How to cite: Lejeune, Q., Kropf, C. M., and Schleussner, C.: Does the perspective of Global Mean Temperature decreasing after peak warming in an overshoot pathway matter for adaptation?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8310, https://doi.org/10.5194/egusphere-egu23-8310, 2023.

With the current rate of climate change, exceeding the remaining global carbon budget for a warming target of 1.5 °C becomes increasingly likely. Hence, attention has been put towards carbon dioxide removal (CDR) strategies that, given the required socio-economic feasibility and sustainability, allow temporary emission overshoots. The balance between the potential viability of overshoot scenarios and the associated risks depends on their characteristics and how the global ecosystems respond during and after the overshoot. Here we investigate the global climate and regional ecosystem responses to emission-driven overshoot scenarios of different magnitude, duration and timing, imposing the potential for irreversible changes. Our analysis focuses on the behavior and reversibility of thermodynamic, hydrological, and biogeochemical processes and their impacts on a regional scale. Based on results from state-of-the-art Earth system models, physically driven mechanisms appear to be mostly reversible for moderate overshoot scenarios considering a response lag in the range of decades. However, feedbacks in high overshoot scenarios and biogeochemical processes show signs of irreversibility on a larger spatial scale – some of which have been brought into connection with the crossing of tipping points for certain elements of the Earth system. Our analysis informs about the reversibility of climate system processes, which allows refining thresholds for global climate change mitigation policies. Furthermore, the aspect of partial reversibility of temperature overshoot scenarios might not be the main concern, but rather the impacts and risks occurring during the periods of elevated temperatures during the overshoot.

How to cite: Steinert, N. J., Schwinger, J., and Lee, H.: Regional surface climate irreversibility under temperature overshoot scenarios, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8383, https://doi.org/10.5194/egusphere-egu23-8383, 2023.

How to cite: Goris, N., Schwinger, J., Fransner, F., Fröb, F., and Lauvset, S.: Potential changes of finfish thermal habitat under negative emissions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11505, https://doi.org/10.5194/egusphere-egu23-11505, 2023.

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.

How to cite: Ganti, G., Lejeune, Q., Gidden, M., Smith, C., Nauels, A., and Schleussner, C.-F.: A precautionary approach to carbon dioxide removal needs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14436, https://doi.org/10.5194/egusphere-egu23-14436, 2023.

With ongoing greenhouse gas emissions it becomes increasingly unlikely that the global mean temperature (GMT) can be stabilized at 1.5°C without considerable negative emissions. As a result, most emission scenarios that would allow to reach 1.5°C GMT at the end of the century are overshoot scenarios: In these scenarios GMT warms until net-zero emissions are reached and slowly starts to cool afterwards. Here we want to have a closer look at the local climate responses after peak warming to get a first idea of potential consequences of overshoots.

The analysis is mainly based on the overshoot scenarios SSP119 and the SSP534-over from the “Coupled Model Intercomparison Project (Phase 6)”. We identify regions in which precipitation or temperature has an asymmetric response to GMT changes around peak warming. In some regions, and especially for temperature related variables, the asymmetries could result from lagged responses in the climate system. However, there are also a number of dynamic mechanisms that could influence local climate signals after peak warming and there are only few regions where analyzed earth system models (ESM) agree on the sign of change.

In many regions, the projected trends in precipitation or temperature after peak warming are in the range of trends that can be found in control runs without anthropogenic forcings. Here, single model initial-condition large ensembles (SMILEs) are necessary to estimate the forced response in overshoot scenarios. For a comprehensive understanding of the mechanisms explaining these non-linear responses to GMT changes around peak warming, more large ensemble simulations of idealized overshoot scenarios for different ESMs would be required.

How to cite: Sillmann, J., Pfleiderer, P., and Schleussner, C.-F.: Physical climate impacts in overshoot scenarios, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15179, https://doi.org/10.5194/egusphere-egu23-15179, 2023.

Glaciers have a delayed response to climate change. As a result, glacier mass loss is expected to continue long after greenhouse gas emissions have stopped as they seek to reach a new equilibrium, with consequences for sea level rise, infrastructure and freshwater resources. However, most glacier projections stop in 2100 and use a small number of greenhouse gas emission scenarios that do not allow linking global temperature targets to glacier change beyond 2100. Here, we compute mountain glacier volume and runoff changes until 2300 under a large suite of synthetic greenhouse gas emission scenarios leading to various levels of overshoots and temperature decline after peak. We show that early temperature stabilization leads to less glacier loss than the overshoot “peak-and-decline” scenarios. We also discuss the potential relevance of global temperature overshoots on water availability from glaciers, before and after peak global temperature.

How to cite: Maussion, F., Schmitt, P., and Schuster, L.: Global glacier response to temperature goals overshoot, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15477, https://doi.org/10.5194/egusphere-egu23-15477, 2023.

With 1.2°C current global warming, it becomes increasingly important to think about overshoot and what this would imply for adaptation. In the face of increasing impacts, more and more thresholds and limits to adaptation will be reached – but if global warming levels are brought down again through the deployment of negative emission technologies, what does this imply for adaptation?

Here, we present a methodology which aims to provide concrete entry points for integrating overshoot risks into adaptation planning, with the objective of strengthening resilience, reducing vulnerability and avoiding maladaptation. We explore concepts such as impact (un)avoidability and (ir)reversibility, key elements of long-term adaptation planning. Ultimately, we aim to develop a step-based approach allowing adaptation planners to formulate and review adaptation policies adequately integrating the concept of overshoot and its implications.

How to cite: Yesil, B., Theokritoff, E., Pringle, P., Menke, I., and Schleussner, C.-F.: Overshoot proofing adaptation policies and plans, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15724, https://doi.org/10.5194/egusphere-egu23-15724, 2023.