The Irreversible Impacts of Overshooting Climate Targets: A Call for Enhanced Protection Pathways

Table of Contents

Overall Summary

Overview

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.

Key Findings

Strengths

Areas for Improvement

Significant Elements

Figure 2

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.

Figure 3

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.

Conclusion

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.

Section Analysis

Introduction

Overview

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.

Key Aspects

Strengths

Suggestions for Improvement

Conceptual categories of peak and decline emission pathways

Overview

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.

Key Aspects

Strengths

Suggestions for Improvement

Non-Text Elements

table 1

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).

First Mention

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.

Critique
Visual Aspects
  • Consider using visual cues like color-coding or icons to differentiate the pathway categories more clearly.
  • Add a brief caption under the table title to explain the acronyms (PD, PD-OS, PD-EP) for improved readability.
Analytical Aspects
  • Provide a more concise summary of each pathway category within the table itself, perhaps using bullet points or a shorter description.
  • Include a column indicating the relative advantages and disadvantages of each pathway to facilitate comparison.
  • Consider adding a row summarizing the key differences between the three categories to aid understanding.
table Extended Data Table 1

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.

First Mention

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.

Critique
Visual Aspects
  • Use consistent formatting for temperature and emission characteristics across all categories to improve readability.
  • Consider adding a visual representation, such as a simplified flowchart, to illustrate the relationships between the different categories.
Analytical Aspects
  • Provide a more detailed explanation of the criteria used for categorizing pathways in both the authors' framework and the literature.
  • Clarify any discrepancies or overlaps between the different categories.
  • Discuss the implications of using different categorization schemes for policymaking and research.

Uncertain climate response and reversal

Overview

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.

Key Aspects

Strengths

Suggestions for Improvement

Non-Text Elements

figure 2

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.

First Mention

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.

Critique
Visual Aspects
  • In panel (a), use clearer labels to distinguish between the different percentile ranges. Perhaps use different colors or line styles.
  • In panel (b), add a brief explanation of the axes labels (T(2060), T(2100)-T(2060)) directly on the plot for improved clarity.
  • In panel (c), consider using a different color for the AR6 scenario uncertainty bars to distinguish them more clearly from the NNCE estimates.
Analytical Aspects
  • Provide a more detailed explanation in the caption about how the NNCE requirements were calculated, including the specific assumptions and limitations of the method.
  • Discuss the implications of the heavy-tailed climate response uncertainty distribution for CDR planning and policy.
  • Compare the estimated NNCE requirements with the potential and feasibility of different CDR technologies to provide a more concrete assessment of the challenges.
Numeric Data

Relying on CDR

Overview

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.

Key Aspects

Strengths

Suggestions for Improvement

Non-Text Elements

figure Extended Data Fig. 3

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.

First Mention

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.

Critique
Visual Aspects
  • Use distinct colors or line patterns for each CDR method to improve readability and differentiate them clearly.
  • Label the y-axis clearly with 'CDR (GtCO2 yr-1)' to explicitly indicate the units.
  • Add a legend explaining the different emission scenarios (C1-3) and their corresponding temperature targets.
Analytical Aspects
  • Provide more context on the assumptions behind the projected CDR ranges, including technological advancements, policy support, and economic feasibility.
  • Discuss the potential limitations and risks associated with each CDR method, such as land use change for AFOLU or energy requirements for BECCS and DACCS.
  • Consider showing the cumulative CDR amounts over the entire period (2020-2100) to provide a clearer picture of the total removal required under each scenario.
table Extended Data Table 2

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.

First Mention

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.

Critique
Visual Aspects
  • Use clear headings and subheadings to improve readability and organization.
  • Consider using visual cues like icons or color-coding to highlight key constraints or areas of overconfidence.
  • Provide a brief summary at the end of the table highlighting the most critical challenges and their implications.
Analytical Aspects
  • Provide more specific examples for each constraint, illustrating the potential consequences of overconfidence.
  • Discuss the potential trade-offs between different CDR methods and their associated constraints.
  • Consider adding a column that suggests potential solutions or research directions for addressing each challenge.

Regional climate change reversibility

Overview

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.

Key Aspects

Strengths

Suggestions for Improvement

Non-Text Elements

figure 3

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).

First Mention

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.

Critique
Visual Aspects
  • Use a consistent color scheme for overshoot and stabilization across all panels to avoid confusion.
  • Label the axes in panels (c) and (d) more clearly, explaining what 'scaling coefficients' represent.
  • In panels (e) and (f), consider using contour lines or different shading to highlight the areas with the largest temperature differences.
Analytical Aspects
  • Explain in the caption why the North Atlantic and Western/Northern Europe are chosen as focus regions.
  • Discuss the potential implications of the observed regional temperature differences for local climate impacts, such as changes in precipitation or extreme weather events.
  • Provide more context on the limitations of the two climate models used and how their results might differ from other models.
figure Extended Data Fig. 4

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.

First Mention

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.

Critique
Visual Aspects
  • Clearly label the units for the y-axes in panels (a) and (c) as 'CO2fe emissions (Pg C yr-1)'.
  • In panels (b) and (d), consider using different colors for the 'upward' and 'downward' phases to improve visual distinction.
  • Add a brief explanation in the caption about what CO2fe represents and why it's used instead of just CO2 emissions.
Analytical Aspects
  • Explain why different emission protocols were used for the two climate models (emission-driven for NorESM and AERA for GFDL-ESM2M).
  • Discuss the implications of the cumulative carbon budget differences for long-term climate change and the feasibility of achieving temperature reversal.
  • Compare the emission pathways used in these simulations with real-world emission scenarios to provide context and assess the realism of the model experiments.
figure Extended Data Fig. 5

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.

First Mention

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.

Critique
Visual Aspects
  • In panels (c) and (d), label the y-axis more clearly, perhaps with 'Precipitation Change Relative to GMST Change'.
  • Use a consistent color scheme for overshoot and stabilization across all panels. For example, always use dashed lines for overshoot and solid lines for stabilization.
  • In panels (e) and (f), consider using a different color or pattern for areas where precipitation is significantly lower under overshoot compared to areas where it's significantly higher.
Analytical Aspects
  • Explain in the caption what the regional scaling coefficients in panels (c) and (d) represent and why they are important.
  • Discuss the potential implications of the persistent precipitation differences shown in panels (e) and (f) for ecosystems and human societies.
  • Consider adding a panel showing the differences in precipitation during the overshoot period itself to provide a more complete picture of the regional impacts.
figure Extended Data Fig. 6

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.

First Mention

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.

Critique
Visual Aspects
  • Use a consistent color scheme for the temperature and precipitation maps to improve visual coherence.
  • Clearly label the color bars with units (degrees Celsius for temperature, percent change for precipitation).
  • Consider adding a small inset map showing the global regions being analyzed.
Analytical Aspects
  • Explain in the caption what 'transient changes' refer to and why they are important in the context of climate stabilization.
  • Discuss the potential causes of the regional differences observed in the maps, such as changes in ocean currents or atmospheric circulation patterns.
  • Consider adding a panel showing the changes in global mean temperature over the same period to provide context for the regional changes.
figure Extended Data Fig. 7

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.

First Mention

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.

Critique
Visual Aspects
  • The color scale could be improved for better contrast and easier interpretation. Consider using a diverging color scheme with a neutral midpoint for zero change.
  • Labeling the individual model panels with the model names would be helpful for clarity.
  • Provide a more detailed explanation in the caption about the meaning of stippling in the ensemble median panel.
Analytical Aspects
  • Discuss the reasons for the differences between the models, such as variations in climate sensitivity or ocean circulation patterns.
  • Quantify the magnitude of the temperature changes in key regions to provide a more concrete understanding of the impacts.
  • Relate the regional temperature changes to specific climate impacts, such as changes in precipitation, sea level, or extreme weather events.
figure Extended Data Fig. 8

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.

First Mention

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.

Critique
Visual Aspects
  • The color scale could be improved for better contrast and easier interpretation. Consider using a diverging color scheme with a neutral midpoint for zero change.
  • Labeling the individual model panels with the model names would be helpful for clarity.
  • Provide a more detailed explanation in the caption about the meaning of stippling in the ensemble median panel.
Analytical Aspects
  • Discuss the reasons for the differences between the models, such as variations in atmospheric circulation patterns or land-ocean interactions.
  • Quantify the magnitude of the precipitation changes in key regions to provide a more concrete understanding of the impacts.
  • Relate the regional precipitation changes to specific climate impacts, such as changes in water availability, agricultural yields, or ecosystem health.

Time-lagged and irreversible impacts

Overview

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.

Key Aspects

Strengths

Suggestions for Improvement

Non-Text Elements

figure 4

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).

First Mention

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.

Critique
Visual Aspects
  • In panel (a), label the two y-axes more clearly to indicate which data points correspond to each axis. Consider using different symbols for the temperature and sea-level rise data points.
  • In panel (b), add a brief explanation of what ΔT and ΔSLR represent directly on the plot.
  • Consider adding a third panel showing the cumulative emissions for the two scenarios in (b) to provide more context.
Analytical Aspects
  • Explain in the caption how the permafrost and peatland emissions were calculated and what models were used.
  • Discuss the potential uncertainties associated with the long-term projections of sea-level rise and permafrost thaw.
  • Provide more context on the implications of the additional temperature increase and sea-level rise in 2300 for human societies and ecosystems.
Numeric Data
  • Sea-level rise per 100 years of overshoot: 0.4 m
  • Temperature increase from permafrost and peatland emissions per 100 years of overshoot: 0.02 °C
figure Extended Data Fig. 9

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.

First Mention

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.

Critique
Visual Aspects
  • Use different colors or symbols for the data points in each panel to clearly distinguish between CO2 and CH4 emissions.
  • Label the y-axes more clearly with the units (GtCO2 for CO2 emissions, TgCH4 for CH4 emissions).
  • Consider adding error bars or confidence intervals to the data points to represent the uncertainty in the projections.
Analytical Aspects
  • Explain in the caption how the emissions were calculated and what models or data sources were used.
  • Discuss the potential uncertainties associated with the projections of permafrost and peatland emissions.
  • Provide more context on the relative importance of CO2 and CH4 emissions from these sources in terms of their contribution to global warming.
Numeric Data
  • Permafrost cumulative CO2 emissions per 100 years of overshoot: 36.7 GtCO2
  • Permafrost cumulative CH4 emissions per 100 years of overshoot: 314.3 TgCH4
  • Peatland cumulative CO2 emissions per 100 years of overshoot: -8.6 GtCO2
  • Peatland cumulative CH4 emissions per 100 years of overshoot: 465.6 TgCH4
figure Extended Data Fig. 10

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).

First Mention

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.

Critique
Visual Aspects
  • In panel (a), clearly label the two y-axes with units and indicate which data points correspond to which axis. Perhaps use different marker shapes or colors.
  • In panel (b), consider using different colors for the box plots representing temperature change and sea-level rise to improve visual distinction.
  • Add a brief explanation in the caption about what the 95th percentile represents and why it's important to consider high-end risks.
Analytical Aspects
  • Explain the mechanisms behind the increased permafrost and peatland emissions and sea-level rise with longer overshoot durations.
  • Discuss the potential implications of these high-end impacts for human societies and ecosystems.
  • Compare the magnitude of these impacts with other potential climate change consequences to provide context and perspective.
Numeric Data
  • Permafrost and peatland feedback on 2300 global mean temperature increase per 100 years of overshoot: 0.04 °C
  • 2300 global mean sea-level rise per 100 years of overshoot: 1.5 m

Socioeconomic impacts

Overview

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.

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Adaptation decision-making and overshoot

Overview

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.

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figure 5

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.

First Mention

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.

Critique
Visual Aspects
  • In panel (a), label the y-axis more clearly, perhaps with 'Climate Impact Driver (stylized)', and provide examples of what this driver could represent (e.g., sea level, drought intensity).
  • In panel (b), use different colors or patterns for the horizontal bars representing different adaptation measures to improve visual distinction. Add a legend explaining the different discount rates and their corresponding values.
Analytical Aspects
  • Explain more clearly in the caption how the stylized impact driver in panel (a) relates to global mean temperature changes.
  • Discuss the implications of the different discount rates for adaptation decision-making and intergenerational equity.
  • Provide examples of specific adaptation measures that might be affected by the timescales of overshoot and reversal.

Reframing the overshoot discussion

Overview

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.

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Methods

Overview

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.

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figure Extended Data Fig. 1

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.

First Mention

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.

Critique
Visual Aspects
  • In panel (a), label the lines representing different NNCE levels more clearly, perhaps with a legend or direct labels.
  • In panel (b), explain the significance of the purple line and what it represents in terms of temperature change.
  • In panel (c), consider adding a trend line or regression equation to illustrate the relationship between eTCREup and eTCREdown more clearly.
  • In panel (d), use different colors or markers to distinguish between the data points for different scenarios.
Analytical Aspects
  • Provide a more detailed explanation of how eTCREup and eTCREdown are calculated and what they represent physically.
  • Discuss the limitations of using a single emission scenario (PROVIDE REN_NZCO2) and the potential implications of using different scenarios.
  • Explain how the results of this analysis inform the overall discussion of overshoot and CDR in the paper.
figure Extended Data Fig. 2

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.

First Mention

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.

Critique
Visual Aspects
  • In panels (a) and (b), clearly label the x-axes with units (degrees Celsius) and indicate the median and 5-95% ranges.
  • In panel (c), consider adding a trend line or correlation coefficient to quantify the relationship between ECS and ZEC.
  • In panel (d), add a legend to clearly identify the observed warming data and the model simulations.
Analytical Aspects
  • Provide a more detailed explanation of how ECS and ZEC are calculated and their physical significance.
  • Discuss the limitations of using idealized model diagnostics like ECS and ZEC for assessing real-world climate change.
  • Explain how the model's performance in simulating historical warming affects the confidence in its projections of future temperature changes.

Extended Data Figures and Tables

Overview

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.

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