Introduction

The long-term objective of limiting global warming to 1.5°C remains the foundation of international climate policy. However, scientific assessments indicate that a temporary exceedance of 1.5°C is now inevitable under current emissions trajectories and policy settings. Consequently, the policy challenge has shifted from avoiding overshoot to navigating it. Every fraction of a degree avoided reduces risks, and every year by which overshoot duration is shortened reduces exposure.

Entering an era of 1.5°C overshoot

Human-induced global warming is nearing 1.4°C above pre-industrial levels, with observed temperatures increasing at approximately 0.25°C per decade. There is a 3-in-4 chance that the 2026–2030 five-year mean in global temperatures will exceed 1.5°C. To maintain a 50 per cent chance of limiting warming to 1.5°C, only approximately 130 billion tons of CO2 can be emitted from 2026 onwards. At current emission rates, this remaining carbon budget would be exhausted in roughly three years. Every five years of sustained high emissions adds approximately 0.1°C to peak warming. The transient climate response to cumulative emissions indicates that the best estimate for a 0.1°C increase is 220 GtCO2. Recent studies show that even stringent mitigation pathways starting around 2025 result in best-estimate peak warming exceeding 1.5°C by about 0.2°C.

Overshoot risks and impacts: Implications on human and environmental systems

Climate risks evolve across the different stages of overshoot pathways, including exceedance, peak, decline, and stabilisation. Multi-sector risk exposure is projected to double for global warming levels between 1.5°C and 2°C, and double again for warming between 2°C and 3°C. In the cryosphere, glaciers could lose more than a quarter of their mass by 2100, raising sea levels by 9 to 12.5 cm and permanently altering water availability. Every 100 years of exceedance above 1.5°C could lead to an additional median sea-level rise commitment of around 40 cm by 2300. Regarding food security, projections indicate declines of up to 14 per cent in global food production by 2050 in the absence of effective adaptation. Low-resilience ecosystems, such as warm water coral reefs, face a warming threshold of 1.2°C and are already experiencing mass bleaching events on a near-annual basis. Furthermore, permafrost thaw may trigger persistent, cascading changes that continue even after peak warming.

Limiting overshoot – mitigation pathways

Rapid and sustained mitigation is both necessary and feasible to hold warming as close to 1.5°C as possible. This requires reaching global net-zero CO2 emissions in the 2050s. Methane contributes around 0.5°C of current warming, and early reductions can significantly lower peak temperatures. Pathways limiting warming to 1.5°C with limited exceedance typically require global greenhouse gas emissions to decline by 55 per cent from 2019 levels by 2035. On the supply side, scenarios suggest scaling renewables to 60–70 per cent of global electricity generation by 2030, which is consistent with trilateral global targets of 11,000 GW. Demand-side measures, such as dietary shifts, could reduce food system emissions by 0.7–8.0 GtCO2-eq per year by 2050. Carbon dioxide removal (CDR) will be essential to achieve net-negative emissions, but its sustainable scale is constrained by land use and risks to biodiversity. Even optimistic CDR scaling is unlikely to deliver temperature reversal beyond a few tenths of a degree this century.

Adapting to overshoot realities in the context of development

Adaptation under overshoot is no longer optional; it is urgent and must be proactive. Adaptation is not only a complement to mitigation but also a precondition for it. Preventive transformational adaptation investment is highly cost-effective; in low- and middle-income countries, each dollar invested in climate-resilient infrastructure yields about four dollars in avoided losses. Infrastructure and buildings have long lifespans, meaning decisions taken today will lock in either vulnerability or resilience through the entire overshoot trajectory. Current adaptation implementation efforts remain insufficient in scale and depth, particularly for the most vulnerable who contributed least to warming. Effective adaptation depends on inclusive, systems-based planning that addresses underlying drivers of vulnerability across practical, institutional, and political spheres.

Conclusion: Through overshoot and beyond – Risks, choices and frontiers

The destination remains a safer climate, but the conditions of the journey are still being shaped. Decisions taken today influence how high warming rises, how long exceedance lasts, and which losses can be avoided. A return to 1.5°C is not a return to previous climatic conditions, but a transition into a future shaped by the legacies of overshoot. Navigating this future requires observation, institutional memory, and governance anchored in equity and sustainable development.