New paper proposes next generation Earth system modeling frameworks to facilitate impactful policy decisions

Every few years, the IPCC assesses a multitude of climate scenarios, which are bucketed into pathways to provide a consistent foundation for comparison. A new paper makes a case for a new generation of “framing pathways” to best support policy decisions in the critical years to come.

The selection and criteria of pathways significantly influence:

• how climate futures are perceived,
• risk assessments,
• mitigation strategies, and
• adaptation decisions.

They inform and are used in regard to

• climate policy,
• private sector investments,
• civil society’s attitudes,
• climate litigation,
• financial risk analysis, and
• regional adaptation planning.

Meinshausen et al, take these critical considerations into account when creating their suggested framing pathways they term “Representation Emission Pathways” (REPs). Taking into account the importance of meeting the Paris Agreement objectives, risks associated with carbon removal strategies, the consequences of delay and overshoot, adaptation needs, loss and damage, and mitigation in the context of societal development goals, they propose six framing pathways for future Earth System Models (ESMs).

Six Suggested Framing Pathways

1. TEWA (The Emission World Avoided): Pathways with high or very high emissions beyond what is expected from current commitments. These are not “business-as-usual” scenarios and should not be seen as such and are thus lower priority. However, they are useful in exploring low-probability, high-warming outcomes and exploring breaking points and resiliency of earth systems. Expected warming by 2100 is 3˚C and above.
2. NFA (No Further Action): These reflect current emission trends in the absence of additional climate action. It is recommended to accompany these pathways with a perturbed physics ensemble for assessing worst-case high-end warming outcomes under current policies. Important in both assessing progress with respect to the TEWA scenarios and providing a baseline and rationale for future, more ambitious climate policies. Expected warming by 2100 is 2.5˚C-3˚C.
3. DASMT (Delayed Action and Stabilization but Missing Target): These scenarios explore emissions approximately in line with current Nationally Determined Contributions (NDCs) and long-term targets proposed around the time of COP26. These pathways miss the Paris Agreement's long-term temperature goal, resulting in global warming of around 2°C in 2100.
4. DAPD (Delayed Action Peak and Decline): This category encompasses high overshoot scenarios that stabilize temperatures through strong net negative GHG emissions in the second half of the century. These scenarios are important in providing a comparison for low overshoot 1.5˚C scenarios with similar cumulative CO₂ emissions. Essentially, they will demonstrate the effects of delayed but aggressive climate action. Expected warming peaks below 2˚C and declines to just above 1.5˚C by 2100.
5. IAPD (Immediate Action Peak and Decline): These pathways involve the immediate onset of decisive emission reductions by 2025, achieving net zero CO₂ emissions by mid-century in alignment with the Paris Agreement goals. These scenarios are important in describing the ‘best possible’ mitigation future and ‘adaptation minimum.’ They exhibit strong and fast methane reductions and sustainable land use. Expected warming peaks only slightly above 1.5˚C and declines to ~1.5˚C by 2100.
6. IA2015 (Immediate Action starting in 2015): A category for a pathway resembling a world that could have been with low emissions, assuming emission reductions toward net zero had started in 2015. This is a lower priority category since it is beyond currently possible futures, but it can inform assessments of loss and damage and serve as a reference pathway. Would achieve 1.5˚C of by 2100 without overshoot.

These categories allow for a structured approach to examining climate outcomes under various emissions and policy scenarios. The authors note that every increment of global warming matters, emphasizing the need for highly granular resolutions in framing pathways, particularly concerning mid-century decision-making. They intentionally propose scenarios that do not dictate specific socio-economic futures but are adaptable, making them flexible enough to accommodate different perspectives on burden-sharing, equity, and fairness. Ultimately, they hope these ESMs provide a strategic approach to modeling policy-relevant futures so they can best inform climate policy and action.