This study examines the risks and implications of "overshoot pathways," where global temperatures temporarily exceed the Paris Agreement's warming limits before being brought back down. The authors argue that overshoot scenarios present significantly different and often irreversible climate risks compared to pathways that avoid exceeding the limits. Using various climate models and scenarios, they demonstrate that achieving declining temperatures after an overshoot is crucial for limiting long-term risks but also highlight the substantial uncertainties involved in temperature reversal, particularly concerning the effectiveness and feasibility of large-scale carbon dioxide removal (CDR). The study emphasizes the need for rapid near-term emission reductions to minimize reliance on CDR and mitigate climate risks effectively.
Description: This figure visually demonstrates the uncertainty in CDR requirements for achieving temperature reversal. It shows the range of potential warming outcomes for a given emission pathway and estimates the cumulative CDR needed to return to 1.5°C by 2100 under different scenarios. The figure highlights the potential for high warming outcomes and the corresponding need for large-scale CDR deployment, emphasizing the importance of considering uncertainty in CDR planning.
Relevance: Figure 2 effectively communicates the central message about the uncertainty and potential scale of CDR required for temperature reversal. It visually reinforces the key finding that relying on median warming outcomes can lead to underestimating the CDR needs and the risks associated with overshoot.
Description: This figure illustrates the regional differences in climate change between overshoot and stabilization scenarios. It compares regional temperature changes in two different climate models and shows that even after global temperatures stabilize, regional climates can differ significantly depending on whether an overshoot occurred. The figure emphasizes the complexity of regional climate change and the limitations of relying solely on global average temperature changes for predicting regional impacts.
Relevance: Figure 3 visually demonstrates the key finding that regional climate change is complex and cannot be accurately predicted based on global mean temperature changes alone, especially after an overshoot. It highlights the need for region-specific adaptation strategies and underscores the importance of considering regional impacts in climate risk assessments.
This study provides compelling evidence that overshoot pathways present distinct and often irreversible climate risks compared to scenarios that avoid exceeding temperature limits. The uncertainty surrounding temperature reversal, the potentially massive scale of CDR required, and the lasting regional and socioeconomic impacts underscore the need for a precautionary approach to climate policy. Future research should focus on quantifying irreversible impacts, exploring alternative emission pathways, and developing robust adaptation strategies that account for the long-term consequences of overshoot. Critically, the study highlights the ethical imperative to minimize climate risks and prioritize rapid, deep emission reductions to avoid exceeding temperature limits in the first place, rather than relying on uncertain and potentially infeasible technological solutions to reverse warming after an overshoot.
The introduction sets the stage by explaining the context of insufficient global emission reduction efforts to meet the Paris Agreement's temperature goals. It highlights the increasing importance of exploring "overshoot pathways," which involve temporarily exceeding the warming limit before bringing temperatures back down. The authors argue that climate change and associated risks after an overshoot differ significantly from a world that avoids it, emphasizing that achieving declining temperatures is crucial for limiting long-term risks. However, they caution against overconfidence in the feasibility and desirability of overshoot, highlighting uncertainties in Earth system feedbacks and the limitations of carbon dioxide removal technologies. The introduction concludes by stressing the need for rapid near-term emission reductions to mitigate climate risks effectively.
The introduction effectively establishes the context of climate overshoot and its relevance to the Paris Agreement. It clearly explains why exploring overshoot pathways has become necessary due to insufficient emission reductions.
The introduction provides a concise summary of the paper's key findings, including the differences between overshoot and non-overshoot scenarios and the importance of declining temperatures. This helps readers quickly grasp the main takeaways of the research.
The introduction acknowledges the uncertainties and limitations associated with overshoot pathways, including the potential for Earth system feedbacks and the scalability of CDR. This strengthens the paper's credibility by presenting a balanced perspective.
While the introduction mentions regional differences, it could briefly elaborate on specific regional risks associated with overshoot. This would provide a more concrete understanding of the potential consequences.
Rationale: Providing specific regional examples would make the risks of overshoot more tangible and relatable for readers.
Implementation: Include a brief sentence or two mentioning specific regions and the types of risks they might face, such as increased heatwaves in certain areas or changes in precipitation patterns in others.
The introduction could more explicitly define the intended audience for the paper. This would help tailor the language and level of detail to the specific readership.
Rationale: Clarifying the intended audience ensures that the paper effectively communicates its message to the relevant stakeholders.
Implementation: Include a sentence or phrase indicating whether the paper is primarily aimed at scientists, policymakers, or a broader audience.
While the introduction provides context, it could benefit from a stronger hook to immediately engage the reader. This could be achieved by starting with a more compelling statement or question.
Rationale: A stronger hook would capture the reader's attention and motivate them to continue reading.
Implementation: Consider starting with a striking statistic about the likelihood of overshoot or a provocative question about the long-term consequences of exceeding the temperature limit.
This section introduces a new framework for categorizing emission pathways based on their peak temperature and subsequent decline characteristics. It argues that the existing categorization, primarily based on peak temperature outcomes, is insufficient for understanding the nuances of pathways aligned with the Paris Agreement. The proposed framework distinguishes between three pathway categories: Peak and Decline (PD), Overshoot (PD-OS), and Enhanced Protection (PD-EP) pathways. Each category is defined by its temperature trajectory and corresponding emission reduction strategies, emphasizing the importance of considering both peak warming and the rate and duration of temperature decline.
The section presents a clear and well-defined framework for categorizing emission pathways based on peak and decline characteristics. This provides a more nuanced understanding of different pathway types and their implications.
The section explicitly links the proposed framework to the Paris Agreement, highlighting its importance for understanding and achieving the agreement's temperature goals.
While Table 1 is referenced, the section could benefit from a more detailed explanation of the table's content and how it illustrates the proposed framework. This would improve the clarity and accessibility of the information.
Rationale: A more thorough explanation of Table 1 would help readers better understand the specific characteristics of each pathway category and how they differ in terms of temperature and emission trajectories.
Implementation: Expand the discussion of Table 1 by providing more details on the specific temperature and emission characteristics of each pathway category, perhaps with illustrative examples or comparisons.
While the proposed framework is valuable, it's important to acknowledge its limitations. This would enhance the section's objectivity and provide a more balanced perspective.
Rationale: Acknowledging limitations strengthens the analysis and provides opportunities for future research.
Implementation: Add a brief discussion of the potential limitations of the framework, such as the challenges of accurately projecting long-term temperature decline or the uncertainties associated with achieving net-negative emissions.
Table 1 categorizes peak and decline emission pathways based on temperature and emission characteristics. It distinguishes between three pathway categories: PD (peak and decline), PD-OS (overshoot), and PD-EP (enhanced protection). Each category is described in terms of its temperature trajectory (peak, decline, stabilization) and the corresponding emission reduction strategies (net-zero CO2, net-negative CO2, timing and stringency of reductions). Think of it like different routes to reach a destination: some routes might be faster but require sharper turns (rapid emission reductions), while others are slower but smoother (gradual reductions).
Text: "categorize pathways in terms of their peak and decline characteristics (Table 1)."
Context: Global GHG emission pathways have a central role in informing the development of policy benchmarks in line with the Paris Agreement and are a core part of climate change assessments by the Intergovernmental Panel on Climate Change (IPCC)2,15. These assessments categorize pathways principally based on their peak temperature outcome2,15. Because a peak and gradual reversal of global warming turns out to be a fundamental feature of Paris-compatible pathways16, we propose to henceforth categorize pathways in terms of their peak and decline characteristics (Table 1).
Relevance: This table is crucial for understanding the framework the authors propose for classifying emission pathways. It lays the groundwork for the subsequent analysis of overshoot pathways and their implications.
Extended Data Table 1 compares the authors' proposed pathway categorization (PD, PD-OS, PD-EP) with existing categories from scientific literature, particularly those used in the IPCC AR6 report. It clarifies how the peak and decline framework relates to existing classifications based on temperature limits and overshoot levels. Imagine it as a translation guide between different languages used to describe emission pathways.
Text: "Several categories of peak and decline pathways have been proposed in the scientific literature2,17 (Extended Data Table 1)."
Context: of peak warming for median climate outcomes6,16 (Fig. 1a). The latter determines the pace of potential temperature reversal16. Both aspects are further dependent on the temporal evolution of non-CO2 emissions. Several categories of peak and decline pathways have been proposed in the scientific literature2,17 (Extended Data Table 1). A prominent example is the latest contribution of Working Group III (WGIII) to the Sixth Assessment Report (AR6) of the IPCC, which includes two pathway categories explicitly referring to the term overshoot (Extended Data Table 1).
Relevance: This table provides context and strengthens the authors' argument by demonstrating how their framework builds upon and clarifies existing pathway classifications. It helps readers familiar with IPCC categories understand the nuances of the peak and decline approach.
This section explores the uncertainties associated with predicting how the climate will respond to emission changes, particularly in overshoot scenarios. It emphasizes that simply focusing on the median, or most likely, warming outcome can lead to underestimating the potential risks and the amount of carbon dioxide removal (CDR) needed to bring temperatures back down. The section uses a model to illustrate how different levels of climate sensitivity could result in significantly different warming outcomes, even with the same emission pathway. It also highlights the uncertainty in how much CDR would be required to return to 1.5°C warming by 2100, showing that hundreds of gigatonnes of CO2 removal might be necessary in some scenarios. Essentially, the section argues that we need to consider the full range of possible warming outcomes, not just the most likely one, to adequately plan for and mitigate the risks of overshoot.
The section effectively explains the concept of climate response uncertainty and its implications for overshoot scenarios. It uses clear language and avoids technical jargon, making the information accessible to a broader audience.
The use of the FaIR model provides a concrete example of how climate response uncertainty can affect warming outcomes and NNCE requirements. This helps readers visualize the potential range of outcomes and understand the importance of considering uncertainty.
The section explicitly addresses high-risk futures where warming exceeds 1.5°C even after achieving net-zero CO2 emissions. This highlights the potential for severe consequences and the need for proactive measures to mitigate these risks.
While the section focuses on global warming outcomes, it could briefly discuss how climate response uncertainty might vary across different regions. This would provide a more complete picture of the potential risks and challenges.
Rationale: Regional variations in climate response uncertainty can have significant implications for adaptation and mitigation strategies.
Implementation: Add a paragraph or a few sentences discussing how climate sensitivity and other factors might differ across regions and how this could affect warming outcomes and CDR needs.
The analysis focuses on a single illustrative emission pathway. Exploring alternative pathways with different emission reduction timelines and levels could provide a more comprehensive understanding of NNCE requirements and the feasibility of temperature reversal.
Rationale: Considering different emission pathways would strengthen the analysis and provide more robust insights into the challenges of achieving the Paris Agreement goals.
Implementation: Extend the analysis to include a range of emission pathways, perhaps using existing scenarios from the IPCC or other sources, and compare the NNCE requirements and warming outcomes across these pathways.
While the FaIR model is useful for illustrating the concept of uncertainty, it's important to acknowledge its limitations. This would enhance the section's credibility and provide a more balanced perspective.
Rationale: All models have limitations, and acknowledging these limitations is crucial for interpreting the results accurately.
Implementation: Add a brief discussion of the limitations of the FaIR model, such as its simplified representation of Earth system processes or its reliance on certain assumptions about climate sensitivity.
Figure 2 estimates the cumulative net-negative CO2 emissions (NNCE) needed to achieve specific temperature targets, considering uncertainties in climate response. Panel (a) shows the net CO2 emissions pathway and the range of possible warming outcomes. Think of climate response uncertainty like predicting the temperature of a pot of water on a stove: you know the heat setting (emissions), but the actual temperature depends on factors like the pot's material and the amount of water. Panel (b) compares warming at the time of net-zero CO2 (around 2060) with the change in temperature between 2060 and 2100. It shows how much additional warming might occur even after CO2 emissions reach zero. Panel (c) estimates the NNCE required to bring warming back to 1.5°C by 2100 under different scenarios, including stabilizing warming at its peak and achieving a decline after stabilization. It highlights the potentially large amounts of CDR needed to counteract high warming outcomes.
Text: "We explore NNCE requirements for an illustrative pathway with the following characteristics (Fig. 2a)"
Context: Peak warming depends on the cumulative CO2 emissions until global net-zero CO2 and the stringency of reductions in non-CO2 GHGs. Achieving net-negative CO2 emissions (NNCE) after peak warming can result in a long-term decline in warming6. Most estimates of NNCE consistent with a long-term reversal of warming in peak and decline pathways have focused on median warming outcomes15. However, to comprehensively assess overshoot risks and NNCE requirements for warming reversal, uncertainties in the climate response must also be considered. These include uncertainties during the warming phase (for example, high warming outcomes due to amplifying warming feedbacks)18 and in the long-term state (potential for continued warming post-net-zero CO2 and the response of the climate system to NNCE)7. We explore NNCE requirements for an illustrative pathway with the following characteristics (Fig. 2a): (1) it achieves net-zero CO2 around mid-century; (2) limits median peak warming close to 1.5 °C above pre-industrial levels; and (3) requires no NNCE to do so (for the median warming outcome).
Relevance: This figure is central to the paper's argument about the uncertainties and potential scale of CDR required for temperature reversal. It demonstrates that relying on median warming outcomes can lead to underestimating the NNCE needed to achieve the Paris Agreement's temperature goals.
This section discusses the challenges and potential overconfidence in relying on Carbon Dioxide Removal (CDR) to achieve temperature declines after an overshoot. It highlights the uncertainties in CDR deployment, including technical, economic, and sustainability constraints. The section emphasizes the need for rapid emission reductions to minimize reliance on CDR and suggests a preventive CDR capacity to address potential high warming outcomes.
The section provides a comprehensive overview of the challenges associated with relying on CDR, including technical, economic, and sustainability aspects. This comprehensive approach ensures that readers understand the multifaceted nature of CDR deployment.
The introduction of the harm prevention principle provides a clear ethical framework for guiding CDR deployment decisions. This principle emphasizes the importance of minimizing potential harm and prioritizing proactive measures to reduce risks.
While the section mentions a preventive CDR capacity of "several hundred gigatonnes," it could benefit from more specific quantification. This would provide a clearer understanding of the scale of CDR deployment needed to address high warming outcomes.
Rationale: More precise quantification would allow for a better assessment of the feasibility and implications of establishing a preventive CDR capacity.
Implementation: Provide a range or a best estimate for the preventive CDR capacity, perhaps based on specific scenarios or model simulations.
The section could elaborate on the role of policy and governance in facilitating the development and deployment of CDR. This would provide a more practical perspective on how to overcome the challenges associated with CDR.
Rationale: Addressing the policy and governance aspects of CDR is crucial for ensuring its effective and responsible implementation.
Implementation: Add a paragraph or a few sentences discussing specific policy instruments and governance mechanisms that could incentivize CDR development and deployment, such as carbon pricing, research and development funding, or international agreements.
This figure shows the median (middle value) and range (5th to 95th percentiles) of projected carbon dioxide removal (CDR) needed under different emission reduction scenarios (C1-3) from 2020 to 2100. It breaks down CDR into different methods like BECCS (bioenergy with carbon capture and storage), DACCS (direct air capture with carbon storage), enhanced weathering, and AFOLU (agriculture, forestry, and other land use). Think of it like a recipe for reducing CO2 in the atmosphere, with different ingredients (CDR methods) and amounts depending on how ambitious the overall recipe (emission scenario) is. The shaded areas around the median lines show the range of possible CDR amounts, acknowledging that there's uncertainty in how much each method can realistically achieve.
Text: "C1, Extended Data Fig. 3"
Context: Scale-up of CDR is most rapid in pathways with the lowest peak warming (low or no overshoot 1.5 °C pathways, C1, Extended Data Fig. 3).
Relevance: This figure illustrates the reliance on CDR in various emission reduction scenarios, highlighting the scale and uncertainty of CDR deployment required to achieve different temperature targets. It's important for understanding the feasibility and potential limitations of relying on CDR to achieve climate goals.
This table lists the challenges and potential overconfidence associated with large-scale CDR. It's organized into categories like 'Readiness' (how developed the technologies are), 'Permanence & Resilience' (how long the captured CO2 will stay stored), 'System Feedbacks' (how CDR might affect other parts of the Earth system), 'Policy & Governance' (the rules and regulations needed), and 'Sustainability & Acceptability' (the environmental and social impacts). Think of it as a checklist of things to consider before relying heavily on CDR, pointing out where our current understanding or planning might be overly optimistic.
Text: "Extended Data Table 2"
Context: Here we highlight that there are multiple areas in which current pathways might be overconfident in their assumed use of CDR (Extended Data Table 2).
Relevance: This table is crucial for understanding the limitations and potential downsides of relying heavily on CDR. It provides a comprehensive overview of the challenges that need to be addressed to ensure that CDR can play a safe and effective role in climate mitigation.
This section explores the regional impacts of climate change after an overshoot period, comparing them to scenarios where global warming is stabilized without overshoot. Even if global temperatures are reversed, regional climates might not simply return to their previous states. The authors use model simulations to show that regional climate change can differ significantly between overshoot and stabilization scenarios, particularly in the North Atlantic and adjacent European land regions. These differences are attributed to the time-lagged response of the Atlantic Meridional Overturning Circulation (AMOC). The key takeaway is that regional climate change is complex and cannot be accurately predicted solely based on global average temperature changes, especially after an overshoot.
The section effectively focuses on the regional implications of overshoot, which is a crucial aspect often overlooked in global climate change discussions. This regional focus provides valuable insights for adaptation planning and risk assessment.
The use of model simulations from NorESM2-LM and GFDL-ESM2M provides concrete evidence for the differences in regional climate responses between overshoot and stabilization scenarios. This strengthens the section's arguments and provides a basis for further investigation.
While the section mentions the importance of understanding regional impacts for adaptation, it could expand on the specific implications for adaptation planning. This would provide more practical guidance for policymakers and stakeholders.
Rationale: Connecting the findings to adaptation strategies would enhance the section's relevance and usefulness for practical applications.
Implementation: Add a paragraph or a few sentences discussing how the regional differences between overshoot and stabilization scenarios could inform adaptation measures, such as infrastructure development, water resource management, or coastal protection.
The section could benefit from a more explicit discussion of the uncertainties associated with regional climate projections. This would provide a more balanced perspective and acknowledge the limitations of current modeling capabilities.
Rationale: Acknowledging uncertainties is crucial for responsible scientific communication and informed decision-making.
Implementation: Add a paragraph or a few sentences discussing the sources of uncertainty in regional climate projections, such as model limitations, emission pathway uncertainties, or the representation of feedback mechanisms.
The section mentions differences in model dynamics between NorESM2-LM and GFDL-ESM2M. Clarifying these differences and their potential implications for the results would strengthen the analysis.
Rationale: Understanding the model-specific differences is important for interpreting the results and assessing the robustness of the findings.
Implementation: Provide a more detailed explanation of the key differences between the two models, such as their representation of AMOC or other relevant processes, and discuss how these differences might affect the regional climate projections.
Figure 3 compares regional temperature changes after a period of overshoot (temporarily exceeding a target temperature) with regional temperature changes under a stabilization scenario (where the target temperature is not exceeded). It uses two different climate models (NorESM and GFDL-ESM2M) and shows how temperatures in specific regions, like the North Atlantic and Western/Northern Europe, respond differently to overshoot compared to stabilization. Panels (a) and (b) show the global mean surface air temperature (GMST) over time for both scenarios. Panels (c) and (d) show how the relationship between regional temperatures and global temperature changes over time. Panels (e) and (f) show the difference in regional temperatures between the overshoot and stabilization scenarios after 100 years of stable global temperatures. Imagine the Earth as a giant bathtub: even if the overall water level (global temperature) stabilizes, the water temperature in different parts of the tub (regions) might not be the same, especially if there was a period of higher water levels (overshoot).
Text: "regional climate changes (Fig. 3"
Context: Even if global warming is stabilized at a certain level without overshoot, the climate system continues to change as its components keep adjusting and equilibrate30, with implications for regional climate patterns. The question then becomes what additional imprints on regional climate may originate directly from the overshoot. Here we explore a unique set of dedicated modelling simulations comparing overshoot and long-term stabilization in two ESMs and find substantial differences in regional climate impact drivers on multi-century timescales (Fig. 3
Relevance: This figure highlights that even if global temperatures stabilize after an overshoot, regional temperatures can behave differently compared to a scenario without overshoot. This is important because regional changes are what directly impact people and ecosystems.
Extended Data Figure 4 shows the CO2 equivalent emissions (CO2fe) used in the overshoot and stabilization simulations for the two climate models (GFDL-ESM2M and NorESM). CO2fe combines the warming effects of all greenhouse gases, not just CO2. Panels (a) and (c) show the emissions over time for both scenarios. Panels (b) and (d) show the difference in the total amount of CO2fe emitted between the overshoot and stabilization scenarios, separated into the period when temperatures are increasing ('upward phase') and the period when temperatures are decreasing ('downward phase'). Think of it like comparing the fuel consumption of two cars driving the same distance: one car accelerates quickly and then brakes hard (overshoot), while the other maintains a steady speed (stabilization). The total fuel consumed might be different, and the difference will be most pronounced during acceleration and braking.
Text: "Extended Data Fig. 4"
Context: We use the results of the NorESM2-LM model following an emission-driven protocol conceptualizing an overshoot of the carbon budget, as well as GFDL-ESM2M simulations following the Adaptive Emission Reduction Approach (AERA) to match a predefined global mean temperature trajectory (Methods and Extended Data Fig. 4).
Relevance: This figure provides details on the emission pathways used in the simulations presented in Figure 3. It helps understand how the different emission profiles in the overshoot and stabilization scenarios contribute to the observed temperature changes.
Extended Data Fig. 5 compares regional precipitation patterns before and after a period of temperature overshoot with patterns seen under global temperature stabilization. It uses two Earth System Models (ESMs), NorESM and GFDL-ESM2M. Panels (a) and (b) show how global temperature changes over time in each model under both overshoot and stabilization scenarios. Panels (c) and (d) show how precipitation in specific regions (global land, global ocean, Amazon, Mediterranean) changes relative to global temperature change. Think of it like comparing how rainfall in your garden changes with the overall temperature in your city versus in a nearby town with a different climate. Panels (e) and (f) are maps showing the differences in precipitation between overshoot and stabilization scenarios after the global temperature has stabilized. Areas with significant differences are hatched. Imagine comparing two maps of average rainfall after a heatwave: one where the heatwave was followed by a return to normal temperatures, and one where temperatures stayed high.
Text: "Extended Data Fig. 5"
Context: Here we explore a unique set of dedicated modelling simulations comparing overshoot and long-term stabilization in two ESMs and find substantial differences in regional climate impact drivers on multi-century timescales (Fig. 3 and Extended Data Fig. 5).
Relevance: This figure supports the argument that regional climate impacts, specifically precipitation, don't simply reverse when global temperatures decline after an overshoot. It highlights the complex regional variations in precipitation patterns that can persist even after global temperature stabilization.
Extended Data Fig. 6 shows how regional temperatures and precipitation change over time within a single scenario where global mean temperature (GMT) is stabilized. It uses the same two models as Extended Data Fig. 5: NorESM and GFDL-ESM2M. Panels (a) and (c) compare temperatures during the first 50 years of stabilization to the last 50 years. Think of it like comparing the temperature in your house during the first and second half of winter after the thermostat is set. Panels (b) and (d) do the same for precipitation. Hatched areas show where the differences are statistically significant. Imagine comparing two maps of your region's average winter temperature: one from the beginning of the stabilization period and one from the end.
Text: "Extended Data Fig. 6"
Context: We find substantial long-term imprints of overshoot on regional climate (Fig. 3c,d) that are distinct from transient changes in stabilization scenarios (Extended Data Fig. 6).
Relevance: This figure highlights that even in a stabilization scenario without overshoot, regional climate patterns continue to evolve over time. This reinforces the message that regional climate change is complex and doesn't always directly mirror changes in global mean temperature.
Extended Data Fig. 7 displays the differences in regional annual temperatures before and after an overshoot scenario, using a CMIP6 model ensemble. Each of the first 12 panels represents a different climate model's projection of temperature change. The colors on the maps show how much warmer or cooler a region is after the overshoot compared to before the overshoot, when global mean temperatures are 0.2°C below the peak warming. Hatching indicates areas where the change is statistically significant, meaning it's likely not just random variation. The 13th panel shows the ensemble median, which is like the average of all the models. Stippling in this panel shows where at least two-thirds of the models agree on the direction of the temperature change (warmer or cooler). Imagine comparing temperatures before and after a fever: this figure shows which parts of the body (regions of the world) experience the most significant and consistent temperature changes.
Text: "Multi-model transient overshoot simulations further corroborate the finding that AMOC dynamics and related changes in regional climate are a dominant feature of overshoot pathways5,32 (Methods and Extended Data Figs. 7 and 8)."
Context: They also indicate a continuous warming of the Southern Ocean relative to the rest of the globe as a result of fast and slow response patterns, and changes in regional climate following reduced aerosol loadings (in particular in South and East Asia)18. Taken together, our results suggest that regional climate changes cannot be approximated well by GMST after peak warming.
Relevance: This figure provides evidence that regional temperature changes following an overshoot are complex and can differ significantly from the global average temperature change. This is important because it highlights the potential for uneven distribution of warming impacts across the globe.
Extended Data Fig. 8 is similar to Fig. 7, but it focuses on changes in regional annual *precipitation* before and after an overshoot scenario. Again, the first 12 panels show projections from individual CMIP6 models, with hatching indicating statistically significant changes. The 13th panel shows the ensemble median, with stippling indicating agreement among at least two-thirds of the models on the direction of change (wetter or drier). Think of it like comparing rainfall patterns before and after a major weather event: this figure shows which regions experience the most consistent and significant changes in rainfall.
Text: "Multi-model transient overshoot simulations further corroborate the finding that AMOC dynamics and related changes in regional climate are a dominant feature of overshoot pathways5,32 (Methods and Extended Data Figs. 7 and 8)."
Context: They also indicate a continuous warming of the Southern Ocean relative to the rest of the globe as a result of fast and slow response patterns, and changes in regional climate following reduced aerosol loadings (in particular in South and East Asia)18. Taken together, our results suggest that regional climate changes cannot be approximated well by GMST after peak warming.
Relevance: This figure complements Fig. 7 by showing that regional precipitation changes are also complex and can differ significantly from global average changes. This is important because precipitation is a key driver of many climate impacts, such as droughts, floods, and agricultural productivity.
This section emphasizes that certain climate impacts, even after a temperature overshoot and subsequent decline, will not immediately reverse. These impacts include changes in the deep ocean, marine ecosystems, land-based ecosystems, carbon stocks, crop yields, and biodiversity. Sea levels, for example, will continue to rise for centuries even if temperatures decrease. The section also highlights the importance of considering not only the irreversible consequences of an overshoot but also the benefits of long-term temperature decline compared to stabilizing at a higher temperature.
The section clearly identifies a range of climate impacts that are not expected to reverse immediately after an overshoot. This provides a valuable overview of the long-term consequences of exceeding temperature limits.
The section highlights the long-term commitment to sea-level rise, even with temperature decline. This emphasizes a critical and widely understood impact of climate change with significant consequences for coastal regions.
While the section identifies irreversible impacts, it could benefit from quantifying these impacts whenever possible. This would provide a more concrete understanding of the magnitude of the long-term consequences.
Rationale: Quantifying the impacts would make the risks more tangible and facilitate comparisons between different impacts and scenarios.
Implementation: Where possible, provide estimates or ranges for the magnitude of irreversible impacts, such as the extent of sea-level rise commitment or the potential loss of biodiversity under different overshoot scenarios.
The section mentions that some impacts are irreversible, but it could benefit from discussing the timescales of irreversibility. This would provide a clearer understanding of the long-term implications of overshoot.
Rationale: Understanding the timescales of irreversibility is crucial for long-term planning and adaptation strategies.
Implementation: For each irreversible impact, provide an estimate or a range for the time it would take for the system to recover, if at all, after temperatures decline. This could be expressed in years, decades, centuries, or millennia, depending on the specific impact.
The section mentions the increased probability of triggering tipping elements. Connecting this to specific tipping elements, such as the collapse of ice sheets or the dieback of the Amazon rainforest, would make the risks more concrete and relatable.
Rationale: Discussing specific tipping elements would provide a more compelling and understandable illustration of the potential consequences of overshoot.
Implementation: Provide examples of specific tipping elements that are particularly vulnerable to temperature overshoot and briefly explain the potential consequences of their activation.
Figure 4 illustrates the long-term irreversible impacts of overshooting the 1.5°C warming limit on permafrost, peatlands, and sea-level rise. Panel (a) shows how the length of time we spend above 1.5°C affects both the global temperature increase caused by permafrost and peatland emissions (blue) and the sea level rise (purple) by the year 2300. The longer the overshoot, the greater these impacts. Think of it like leaving ice cream out in the sun: the longer it sits, the more it melts. Panel (b) compares two scenarios: one where we only achieve net-zero CO2 emissions (stabilizing temperatures at their peak), and one where we achieve net-zero greenhouse gas emissions (leading to a long-term temperature decline). It shows that achieving net-zero GHG emissions significantly reduces the additional temperature increase and sea-level rise in 2300 compared to just stabilizing CO2 emissions. It's like comparing the final mess after spilling two different liquids: one that evaporates quickly (GHG scenario) and one that leaves a sticky residue (CO2 only scenario).
Text: "Sea levels will continue to rise for centuries to millennia even if long-term temperatures decline37."
Context: Comprehensively assessing future climate risks under peak and decline pathways requires a focus not only on the (irreversible) consequences of a temporary overshoot but also on the benefits of long-term temperature reversal, compared with stabilization at higher levels. Here we explore the consequences of overshoot in an ensemble of peak and decline pathways (Methods) that achieve net-zero GHGs and thereby long-term temperature decline compared with stabilization at peak warming (by maintaining net-zero CO2). For global sea-level rise, we find that every 100 years of overshoot above 1.5 °C leads to an additional sea-level rise commitment of around 40 cm by 2300 (central estimate) apart from a baseline of about 80 cm without overshoot (Fig. 4a).
Relevance: This figure emphasizes the long-term consequences of overshooting the 1.5°C warming limit, even if temperatures eventually decline. It highlights the irreversible nature of some climate impacts, like sea-level rise and permafrost thaw, and the importance of achieving net-zero greenhouse gas emissions rather than just net-zero CO2 emissions.
Extended Data Figure 9 shows how CO2 and methane (CH4) emissions from permafrost and peatlands increase with the duration of a temperature overshoot above 1.5°C. Each panel is a scatter plot showing the relationship between the time spent above 1.5°C and the cumulative emissions. The longer the overshoot, the more emissions are released. Think of it like squeezing a sponge: the longer you squeeze, the more water comes out. Panel (a) shows CO2 emissions from permafrost, (b) shows CH4 emissions from permafrost, (c) shows CO2 emissions from peatlands, and (d) shows CH4 emissions from peatlands. The diagonal lines indicate the average rate of increase in emissions for each 100 years of overshoot.
Text: "Extended Data Fig. 9"
Context: A similar pattern emerges for 2300 permafrost thaw and northern peatland warming leading to increased soil carbon decomposition and CO2 and CH4 release (Fig. 4 and Extended Data Fig. 9).
Relevance: This figure provides detailed information on the emissions from permafrost and peatlands, which contribute to the long-term temperature increase shown in Figure 4. It helps understand the specific mechanisms by which overshoot duration affects future warming.
Extended Data Figure 10 illustrates the high-end (95th percentile) long-term irreversible impacts of temperature overshoot on permafrost, peatlands, and sea-level rise. Panel (a) shows the relationship between the duration of overshoot above 1.5°C and the resulting feedback on global mean temperature from permafrost and peatland emissions (blue markers) and the 95th percentile sea-level rise (purple markers) in 2300. The longer the overshoot, the greater the temperature increase from permafrost and peatland emissions and the higher the sea level rise. Panel (b) compares the additional temperature increase and sea-level rise in 2300 caused by stabilizing temperatures at peak warming (net-zero CO2) versus achieving a long-term temperature decline (net-zero GHG). It shows that stabilizing at peak warming leads to more warming and sea-level rise than achieving net-zero GHG and a temperature decline. Imagine two scenarios: one where a bathtub overflows (overshoots) and the water level is then held constant, and another where the water is drained back down after the overflow. This figure shows the long-term consequences of these two scenarios for the bathroom (Earth system).
Text: "Extended Data Fig. 10 | High-end long-term irreversible permafrost, peatland and sea-level rise impacts of overshoot."
Context: For a range of climate impacts, there is no expectation of immediate reversibility after an overshoot. This includes changes in the deep ocean, marine biogeochemistry and species abundance34, land-based biomes, carbon stocks and crop yields35, but also biodiversity on land36. An overshoot will also increase the probability of triggering potential Earth system tipping elements33. Sea levels will continue to rise for centuries to millennia even if long-term temperatures decline37. Comprehensively assessing future climate risks under peak and decline pathways requires a focus not only on the (irreversible) consequences of a temporary overshoot but also on the benefits of long-term
Relevance: This figure highlights the long-term irreversible consequences of even a temporary temperature overshoot, emphasizing the importance of not just stabilizing temperatures but actively reducing them after an overshoot to minimize these risks. It focuses on high-end risk outcomes, providing a cautionary perspective on the potential severity of impacts.
This section discusses the societal and economic consequences of a temperature overshoot. It emphasizes that the severity of climate risks during an overshoot depends heavily on the adaptive capacity of human systems, particularly in the first half of the 21st century when adaptive capacity is projected to be relatively low, even under optimistic development scenarios. The coincidence of overshoot with low adaptive capacity can amplify climate risks, particularly for vulnerable populations. The section also highlights the potential for lasting negative impacts on human well-being, including climate-related deaths, and the long-term economic consequences, especially for less developed countries. It raises ethical concerns about the extent of loss and damage that vulnerable populations might face due to overshoot.
The section effectively focuses on the human dimensions of overshoot, highlighting the socioeconomic consequences and the differential impacts on vulnerable populations. This focus is crucial for understanding the broader implications of overshoot beyond biophysical changes.
The section correctly emphasizes the role of adaptive capacity in shaping the impacts of overshoot. This highlights the importance of investing in adaptation measures and building resilience, particularly in vulnerable regions.
While the section discusses socioeconomic impacts in general terms, it could benefit from more specific examples of how overshoot might affect different sectors or communities. This would make the impacts more tangible and relatable for readers.
Rationale: Concrete examples would enhance the section's impact and provide a clearer understanding of the potential consequences of overshoot.
Implementation: Include specific examples of how overshoot could affect sectors like agriculture, health, or infrastructure, or how it might disproportionately impact specific communities, such as coastal populations or those reliant on climate-sensitive resources.
The section could be strengthened by quantifying the socioeconomic impacts of overshoot whenever possible. This would provide a more precise understanding of the potential costs and benefits of different scenarios.
Rationale: Quantifying the impacts would facilitate comparisons between different scenarios and inform policy decisions.
Implementation: Where possible, provide estimates or ranges for the economic costs of overshoot, such as the potential losses in GDP or the costs of adaptation measures. This could be based on existing research or model simulations.
While the section focuses on the negative impacts of overshoot, it could also briefly discuss potential solutions and strategies for mitigating these impacts. This would provide a more balanced perspective and offer some hope for addressing the challenges.
Rationale: Discussing potential solutions would enhance the section's practical relevance and provide readers with a sense of agency.
Implementation: Add a paragraph or a few sentences discussing potential strategies for building adaptive capacity, reducing vulnerability, and addressing the ethical concerns associated with overshoot. This could include examples of successful adaptation measures, international cooperation initiatives, or policy interventions.
This section examines the relevance of potential long-term impact reversals after a temperature overshoot for current adaptation planning. It argues that the long timescales involved in such reversals (50 years or more) often exceed the typical planning horizons of adaptation plans and infrastructure lifecycles. The section also discusses the influence of discount rates on adaptation decisions, noting that higher rates prioritize near-term costs and benefits, making adaptation to peak warming seemingly more attractive than adapting to a potentially lower long-term outcome. The section concludes that long-term reversibility may only be relevant in specific adaptation decisions, particularly those involving irreversible impacts like sea-level rise. However, given the uncertainty of achieving temperature decline after overshoot, limiting peak warming remains crucial for effective adaptation.
The section effectively addresses the practical relevance of long-term impact reversals for adaptation planning. By comparing the timescales of reversal with typical planning horizons, it highlights the potential disconnect between long-term climate projections and near-term adaptation needs.
The discussion of discount rates and intergenerational equity adds an important economic and ethical dimension to the analysis. It highlights how different discount rates can influence adaptation decisions and prioritize short-term versus long-term considerations.
While the section discusses adaptation planning in general terms, it could benefit from providing more concrete examples of specific adaptation measures and how they are affected by the timescales of impact reversal. This would make the discussion more practical and relatable for readers.
Rationale: Concrete examples would enhance the section's practical relevance and provide a clearer understanding of the challenges and opportunities for adaptation planning.
Implementation: Include examples of specific adaptation measures, such as coastal protection, water resource management, or agricultural practices, and discuss how the timescales of impact reversal might influence the design and implementation of these measures.
The section mentions the uncertainty of achieving temperature decline after overshoot. However, it could further explore how this uncertainty should be incorporated into adaptation planning processes. This would provide more practical guidance for decision-makers facing uncertain future climate conditions.
Rationale: Addressing uncertainty explicitly in adaptation planning is crucial for developing robust and flexible strategies that can cope with a range of possible future outcomes.
Implementation: Discuss specific approaches for incorporating uncertainty into adaptation planning, such as scenario planning, robust decision-making, or adaptive management. Provide examples of how these approaches have been used in practice.
Figure 5 illustrates the timescales relevant for adaptation planning in the context of a potential temperature overshoot. Panel (a) shows two stylized examples of how a climate impact driver (like sea level or drought intensity) might change over time under a peak and decline temperature scenario. The green shaded areas represent the period of overshoot, with the dashed lines showing different possible overshoot magnitudes and durations. Panel (b) visualizes the time horizons typically considered in adaptation planning (decadal, multi-decadal) and the lifetimes of various adaptation measures, ranging from annual adjustments (like crop choices) to long-term infrastructure projects (like dams). It also shows how different discount rates, which reflect how we value future costs and benefits, can influence the effective time horizon for adaptation decisions. The higher the discount rate, the less we value future impacts, and the shorter the effective planning horizon becomes.
Text: "prospects of a long-term decline would start to affect adaptation decisions today or in the immediate future (Fig. 5a)."
Context: Even under the optimistic assumption of nearly full reversibility of a climate impact driver under overshoot, a planning horizon of 50 years or more might be required before prospects of a long-term decline would start to affect adaptation decisions today or in the immediate future (Fig. 5a). Few adaptation plans and policies operate on these timescales: for example, the EU Adaptation Strategy spans three decades, whereas other national adaptation plans have similar or shorter time horizons44.
Relevance: This figure is crucial for understanding the challenge of incorporating long-term temperature decline into near-term adaptation planning. It highlights the mismatch between the timescales of overshoot and reversal and the typical time horizons considered in adaptation decisions. This mismatch suggests that the potential for future temperature decline might not be a primary factor in current adaptation planning.
This section argues against framing overshoot as a viable alternative for achieving desired climate outcomes. It highlights that climate impacts differ significantly between a world with overshoot and one without, emphasizing that impact reversibility is not guaranteed. Even when reversible, the timescales often exceed typical adaptation planning horizons. The section underscores the ethical implications of overshoot, particularly the irreversible socioeconomic impacts and disproportionate burden on vulnerable populations. It also cautions against relying on solar geoengineering as a solution, citing concerns about its effectiveness and governance. The section concludes by advocating for a reframing of the overshoot discussion to prioritize minimizing climate risks, accelerating emission reductions, and developing a preventive carbon dioxide removal (CDR) capacity to hedge against high-risk outcomes.
The section presents a clear and concise argument against framing overshoot as a viable alternative. It effectively summarizes the key findings of the paper and synthesizes them into a compelling narrative.
The section explicitly addresses the ethical dimensions of overshoot, highlighting the disproportionate impacts on vulnerable populations. This ethical focus adds an important dimension to the discussion and underscores the social justice implications of climate change.
While the section advocates for a preventive CDR capacity, it could provide more details on what this capacity would entail in terms of specific technologies, deployment scales, and governance mechanisms. This would make the recommendation more concrete and actionable.
Rationale: Providing more details on the preventive CDR capacity would strengthen the recommendation and provide a clearer roadmap for implementation.
Implementation: Add a paragraph or a few sentences discussing specific CDR technologies, their potential deployment scales, and the governance mechanisms needed to ensure their effective and responsible use. This could include examples of existing CDR projects or policy proposals.
The section could benefit from discussing the role of public communication in reframing the overshoot discussion. Public understanding and acceptance of the risks and limitations of overshoot are crucial for mobilizing support for ambitious climate action.
Rationale: Addressing public communication is essential for fostering informed public discourse and building consensus on climate policy.
Implementation: Add a paragraph or a few sentences discussing the importance of communicating the risks of overshoot to the public and engaging stakeholders in discussions about alternative pathways. This could include recommendations for effective communication strategies or examples of successful public engagement initiatives.
This section details the methodologies employed to analyze the climate impacts of overshoot pathways. It describes the use of the FaIR (Finite Amplitude Impulse Response) model to estimate net-negative CO2 emission (NNCE) needs under climate uncertainty, and the use of two Earth System Models (ESMs), GFDL-ESM2M and NorESM2-LM, to investigate regional climate change reversibility under overshoot and stabilization scenarios. The section also outlines the methods used to project long-term impacts on sea-level rise, permafrost, and peatlands, and the analysis of CMIP6 model projections.
The section provides detailed descriptions of the models and methods used, including the FaIR model, the two ESMs, and the CMIP6 ensemble. This level of detail allows for transparency and reproducibility of the results.
The section clearly explains the calculations performed, including the equations used to estimate NNCE needs, climate response metrics, and long-term impacts. This clarity ensures that readers understand the methodology and can follow the analysis.
While the section describes the models used, it could benefit from a more explicit discussion of the limitations of these models. This would enhance the section's objectivity and provide a more balanced perspective on the results.
Rationale: Acknowledging model limitations is crucial for interpreting the results accurately and understanding the potential uncertainties associated with the projections.
Implementation: Add a paragraph or a few sentences discussing the limitations of the models used, such as their simplified representation of certain processes, their sensitivity to input parameters, or their ability to capture regional climate variations.
The section could provide more context on the specific scenarios used in the analysis, such as the PROVIDE REN_NZCO2 scenario and the SSP5-34-OS and SSP1-19 pathways. This would help readers understand the rationale behind the scenario choices and their implications for the results.
Rationale: Understanding the scenario choices is important for interpreting the results and assessing their relevance to different policy contexts.
Implementation: Add a paragraph or a few sentences explaining the characteristics of the scenarios used, their underlying assumptions, and their relevance to the research questions addressed in the paper.
Extended Data Figure 1 illustrates the method used to derive the necessary amount of net-negative CO2 emissions (NNCE) under conditions of climate uncertainty, specifically for the PROVIDE REN_NZCO2 scenario. This scenario initially assumes net-zero CO2 emissions around 2060. Panel (a) shows the original scenario (in black) alongside modified versions where varying levels of NNCE are introduced after 2060. These modifications help explore how different amounts of CO2 removal affect temperature change. Panel (b) shows the difference in warming between 2100 and 2060 for each scenario. The red highlight indicates the scenario where 20 Gt of CO2 is removed annually. Values to the right of the purple line represent continued warming, even with CO2 removal. Panel (c) shows the relationship between two metrics: eTCREup (how much warming occurs for a given amount of CO2 emissions *before* net-zero) and eTCREdown (how much cooling occurs for a given amount of CO2 removal *after* net-zero). Panel (d) compares the cooling between 2100 and 2060 with the warming in 2060. This helps visualize how much CO2 removal is needed to offset the warming that has already occurred.
Text: "Extended Data Fig. 1 | Method to derive net-negative CO2 emissions under climate uncertainty for PROVIDE REN_NZCO2."
Context: Elements to analyze
Relevance: This figure is important because it explains the methodology used to estimate the amount of CO2 removal needed to achieve specific temperature targets, accounting for the uncertainties in how the climate system responds to emissions changes. This is crucial for understanding the feasibility and potential challenges of achieving temperature reversal after an overshoot.
Extended Data Figure 2 validates the FaIR v1.6.2 model used in the study by comparing its outputs to established climate assessments (AR6 WG1). Panel (a) shows the distribution of Equilibrium Climate Sensitivity (ECS) values from the model, which is a measure of how much the Earth's temperature would eventually increase if atmospheric CO2 doubled. Panel (b) shows the distribution of Zero Emissions Commitment (ZEC) values, which is the amount of warming that would still occur even if all CO2 emissions stopped immediately. Panel (c) shows the relationship between ECS and ZEC, indicating that higher ECS values tend to be associated with higher ZEC values. Panel (d) compares the model's simulated historical warming with actual observed warming, showing good agreement. This validation is important for building confidence in the model's ability to project future warming and cooling.
Text: "Extended Data Fig. 2 | FaIR v1.6.2 ensemble diagnostics consistent with AR6 WG1 assessment"
Context: Elements to analyze
Relevance: This figure is essential for establishing the credibility of the FaIR model used to estimate NNCE requirements. By demonstrating consistency with established climate assessments, it provides confidence in the model's ability to project future warming and cooling under different scenarios.
This appendix provides supplementary figures and tables that support the findings and analyses presented in the main paper. It includes detailed information on the methods used to estimate net-negative CO2 emissions, regional climate change reversibility, long-term impacts on sea-level rise and permafrost, and carbon dioxide removal (CDR) deployment. The appendix spans from Extended Data Figure 1 to Extended Data Table 2, offering valuable data and context for a deeper understanding of the main paper's arguments.
The appendix provides a comprehensive collection of supplementary figures and tables that cover a wide range of topics relevant to the main paper. This comprehensive approach ensures that readers have access to detailed information supporting the key findings and analyses.
The descriptions of the figures and tables are clear and concise, providing sufficient information for readers to understand their purpose and content without being overly technical or verbose.
While the appendix provides valuable information, it could benefit from more explicit cross-referencing with the main text. This would help readers connect the supplementary materials to the specific sections where they are relevant.
Rationale: Clear cross-referencing would improve the integration between the main text and the appendix, making it easier for readers to find and utilize the supplementary information.
Implementation: Add specific references to the relevant sections of the main text within the descriptions of each figure and table. For example, indicate which figure or table corresponds to which analysis or discussion in the main paper.
While the descriptions are generally clear, some technical terms or concepts might not be readily accessible to a non-expert audience. Consider providing brief explanations or definitions of these terms to enhance the appendix's accessibility.
Rationale: Improving accessibility would broaden the reach of the paper and ensure that a wider audience can understand and appreciate the findings.
Implementation: Identify any technical terms or concepts that might be unfamiliar to a non-expert audience and provide brief explanations or definitions within the descriptions of the figures and tables. Consider using analogies or examples to illustrate complex concepts in a more accessible way.