Overshoot scenarios

Overshoot scenarios are modelled emissions pathways in which global mean surface temperature rises above a stated policy threshold — most commonly the 1.5°C limit referenced in the Paris Agreement — peaks, and then declines back toward or below that threshold later in the century. The "return" phase depends on achieving net-negative CO₂ emissions, meaning carbon dioxide removal (CDR) must exceed residual gross emissions for a sustained period.

The IPCC's Sixth Assessment Report (AR6, 2021–2022) formalised the terminology used today. Working Group III's scenario database classifies pathways by the degree and duration of overshoot:

• No or limited overshoot: peak warming stays within roughly 0.1°C of the target.
• High overshoot: temperature exceeds the target by a larger margin, often 0.2–0.4°C, before drawdown.

AR6 found that nearly all modelled pathways that hold warming to 1.5°C with at least 50% probability involve some overshoot, and that limiting overshoot magnitude sharply reduces the risk of triggering irreversible impacts.

Key policy and scientific concerns include:

1. Reliance on CDR at scale — bioenergy with carbon capture and storage (BECCS), afforestation, and direct air capture are assumed in large volumes, raising feasibility and land-use questions.
2. Irreversibility risks — coral reef loss, ice-sheet destabilisation, and permafrost thaw may not reverse even if temperatures later decline.
3. Equity and intergenerational burden — overshoot effectively borrows from future generations who must deliver the removals.
4. Tipping points — higher peak temperatures increase the probability of crossing thresholds in the Amazon, the Atlantic Meridional Overturning Circulation, and polar ice systems.

For negotiators, the distinction between overshoot and no-overshoot pathways matters in debates over NDC ambition, long-term strategies submitted under Article 4 of the Paris Agreement, and the operationalisation of the Global Stocktake.