Executive Summary

A new analysis of climate change projections after the cessation of CO₂ emissions indicates a more significant cooling effect than previously estimated. This report details findings from a study utilizing a novel method, Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (CEEMDAN), to filter for climate variability across multiple timescales. The results provide critical data for policymakers working towards the United Nations’ Sustainable Development Goals, particularly SDG 13 (Climate Action). The multi-model median temperature change is estimated to be -0.19 °C fifty years after an abrupt halt in emissions, with a range of -0.44 °C to 0.04 °C. These revised, cooler estimates underscore the importance of robust analytical methods in shaping effective long-term climate strategies.

Introduction: Aligning Climate Projections with Sustainable Development Goal 13

Global efforts to achieve net-zero emissions are central to fulfilling the objectives of the Paris Agreement and Sustainable Development Goal 13 (Climate Action), which aims to limit global warming to well below 2 °C, preferably to 1.5 °C. With global warming approaching 1.3 °C, understanding the climate’s response after reaching net-zero is crucial for effective long-term policymaking and assessing the feasibility of these targets. The climatic behavior following the cessation of emissions, known as the Zero Emissions Commitment (ZEC), has significant implications for whether global temperature targets can be met and sustained. This report presents a revised estimate of the ZEC, employing an advanced methodology to provide a more accurate foundation for strategies aligned with SDG 13.

Methodology for Enhanced Climate Modeling

Zero Emissions Commitment (ZEC) Framework

The standard framework for analyzing post-net-zero climate response involves simulating an abrupt cessation of CO₂ emissions. This study utilizes data from the Zero Emissions Commitment Model Intercomparison Project (ZECMIP), which has been run by nine Earth System Models (ESMs). The primary experiment involves a 1% annual increase in atmospheric CO₂, followed by an immediate halt to emissions.

The conventional method for calculating ZEC relies on comparing 20-year average temperatures. However, this approach has limitations:

• Inadequate Timescale: A 20-year period is often too short to serve as a robust climatology and fails to account for natural multi-decadal variability.
• Base Period Misalignment: The base period used for comparison can be misaligned with the simulation’s starting point, leading to inaccurate estimates.
• Variability Noise: Natural climate variations can be of a similar magnitude to the ZEC signal, potentially leading to over- or underestimation of the true post-net-zero response.

Advanced Time-Series Analysis: CEEMDAN

To overcome the limitations of previous methods, this study employs the Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (CEEMDAN) technique. This data-driven method decomposes a time series into its constituent parts:

1. Intrinsic Mode Functions (IMFs): These represent oscillations at various timescales, such as inter-annual and multi-decadal variability.
2. Residual Trend: This captures the long-term, underlying trend in the data.

By filtering out IMFs with wavelengths shorter than 50 years, the CEEMDAN method isolates the long-term climate response from the noise of natural variability. This approach provides a more robust estimate of temperature changes without relying on a potentially misaligned base period, thereby offering a clearer signal for informing climate action policies under SDG 13.

Results: Revised Projections for Post-Net-Zero Climate Change

Comparison of Methodologies

The application of the CEEMDAN method yields substantially different results compared to the standard 20-year average method. The revised estimates indicate a stronger cooling trend after emissions cease.

• Median Temperature Change: The median Global Mean Surface Temperature (GMST) change 50 years post-cessation is -0.19 °C using CEEMDAN, compared to -0.11 °C with the 20-year average method.
• Temperature Range: The projected range shifts to more negative values, from -0.44 °C to +0.04 °C with CEEMDAN, whereas the previous method gave a range of -0.33 °C to +0.29 °C.

With the CEEMDAN method, eight of the nine ESMs analyzed show a cooling trend 50 and 90 years after emissions cease. This contrasts with previous findings where the range was centered closer to zero, often interpreted as a negligible change.

Implications for Global and Regional Temperature Changes

The revised methodology significantly alters the projections for certain models. For instance, the UKESM1-0-LL model, which previously showed a warming of +0.29 °C, is now estimated to warm by only +0.04 °C. This large discrepancy is attributed to the correction of a misaligned base period and the removal of multi-decadal variability by CEEMDAN. Similarly, the ACCESS-ESM1-5 model now shows a cooling of -0.1 °C, reversing a previously projected warming of +0.01 °C.

Local changes are also substantially different. In the CESM2 model, widespread warming previously projected for the Southern Hemisphere is now confined primarily to the Southern Ocean, presenting a more nuanced picture of regional climate responses. These refined local projections are vital for developing targeted adaptation strategies as part of SDG 13.

Discussion: Policy Implications for SDG 13 and Future Climate Action

The findings presented in this report have direct and significant implications for global climate policy and the pursuit of SDG 13. By demonstrating that post-net-zero temperature changes are likely more negative than previously understood, this research provides policymakers with a more robust scientific basis for long-term planning.

Strengthening Climate Policy and Targets

The cooler ZEC estimates suggest that once net-zero is achieved, there may be a more pronounced natural cooling effect, which could provide a slightly wider margin for meeting the Paris Agreement’s 1.5 °C target. This information is critical for Target 13.2 of the SDGs, which calls for integrating climate change measures into national policies and planning. Accurate projections are essential to avoid both over- and under-investment in mitigation and adaptation efforts.

Enhancing Scientific Robustness

This study highlights the critical importance of the methodology used to analyze climate projections, especially when the signal-to-noise ratio is low. The CEEMDAN method offers a more reliable tool for filtering out natural variability, ensuring that policy is based on the most accurate possible understanding of the underlying climate trajectory. This contributes to SDG Target 13.3 by improving the scientific capacity for climate change mitigation and early warning.

Future Outlook

As the international community develops the next generation of climate models (e.g., CMIP7), these findings underscore the need to refine analytical methodologies. Ensuring that future climate projections are robust and relevant is paramount for effective global governance and achieving the ambitious goals set forth in the 2030 Agenda for Sustainable Development.

1. Which SDGs are addressed or connected to the issues highlighted in the article?

SDG 13: Climate Action

• The article is fundamentally about climate change, focusing on the scientific understanding of global temperature changes after achieving net-zero CO₂ emissions. This directly relates to SDG 13, which calls for urgent action to combat climate change and its impacts. The entire study is framed within the context of global climate policy, referencing the Paris Agreement and the goal of limiting global warming. The text states, “Policies are being developed in recognition that reaching net zero sooner will limit damaging climate change impacts. This motivated the formation of the Paris Agreement, which aims to limit global warming to well below 2 °C, with a preference for staying below 1.5 °C above pre-industrial levels.”

2. What specific targets under those SDGs can be identified based on the article’s content?

Target 13.2: Integrate climate change measures into national policies, strategies and planning.

• The article directly references the implementation of this target by stating, “Many governments worldwide have implemented targets to achieve net zero emissions in the coming decades.” The research presented aims to provide more accurate scientific data to “inform policymaking,” which is essential for the effective planning and revision of these national climate strategies. The study’s goal is to refine the understanding of post-net zero climate dynamics to better achieve the goals set out in these policies.

Target 13.3: Improve education, awareness-raising and human and institutional capacity on climate change mitigation, adaptation, impact reduction and early warning.

• This research contributes to strengthening institutional capacity by improving the scientific methods used for climate projections. The article highlights the limitations of current methods for calculating the Zero Emissions Commitment (ZEC) and proposes a more robust methodology (CEEMDAN). By doing so, it enhances the tools available to the scientific community and policymakers, as stated in the conclusion: “these findings underscore the importance of refining methodologies for assessing post-net zero temperature changes to ensure that future climate projections remain robust and relevant for both policymaking and climate science.”

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

1. Global Temperature Increase

   The article uses the global temperature increase as a primary indicator for climate change, directly linking it to the Paris Agreement’s goals. It mentions, “…the Paris Agreement, which aims to limit global warming to well below 2 °C, with a preference for staying below 1.5 °C above pre-industrial levels.” It also notes the current status: “With global warming already reaching around 1.3 °C.” The entire study revolves around projecting this indicator after emissions cease.

2. Net-Zero Emissions

   The concept of “net zero emissions” is a central indicator of climate action discussed throughout the article. It is presented as a key policy goal: “Many governments worldwide have implemented targets to achieve net zero emissions in the coming decades.” The study analyzes the climate response to achieving this state, referring to it as “abrupt cessation of CO₂ emissions.”

3. Zero Emissions Commitment (ZEC)

   The ZEC is a specific, technical indicator used to measure the committed warming or cooling after emissions stop. The article is dedicated to calculating this indicator more accurately. It provides specific quantitative values, stating, “Our findings indicate that 50 years after abrupt emission cessation, the multi-model median temperature change is −0.19 °C, with a range spanning from −0.44 °C to 0.04 °C, which is cooler than previous estimates.” This provides a measurable metric for assessing the long-term impacts of achieving net-zero emissions.

4. Table of SDGs, Targets, and Indicators

Source: nature.com