UNEP Limiting Overshoot Report: Navigating a World Beyond 1.5°C

Context

The United Nations Environment Programme (UNEP) has released Limiting Overshoot: Navigating Exceedance of 1.5°C and Pathways Towards Return. The report examines how the world can respond if global warming temporarily crosses the 1.5°C threshold and identifies pathways to bring temperatures back down through deep decarbonisation and net-negative emissions.

Understanding Climate Overshoot

Climate overshoot describes a situation in which global temperatures rise above the 1.5°C threshold for a period before reaching a peak and subsequently declining. The pathway broadly involves four stages:

• Exceedance – Temperatures cross the 1.5°C level.
• Plateau/Peak – Warming reaches its maximum.
• Decline – Net-negative emissions gradually reduce atmospheric greenhouse gases.
• Stabilisation – Temperatures move towards a safer long-term level.

Major Findings

• 1.5°C is becoming harder to avoid: Human-induced warming is approaching 1.4°C, increasing at around 0.25°C per decade. The WMO estimates a 75% probability that the 2026–2030 average will exceed 1.5°C.
• Carbon budget is rapidly shrinking: Only around 130 GtCO₂ remains from 2026 for a 50% probability of staying within 1.5°C. At current annual emissions of nearly 40 GtCO₂, this could be consumed within roughly three years.
• Current policies remain insufficient: Existing policies could result in around 2.6°C warming by 2100. Even stronger implementation of NDCs and net-zero commitments may produce peak warming of approximately 1.8°C.
• Delay increases the challenge: Every additional five years of sustained high emissions could add about 0.1°C to peak warming. Reversing such an increase would require removing roughly 220 GtCO₂.
• Tipping-point risks: Prolonged overshoot could accelerate irreversible changes involving the Greenland and West Antarctic ice sheets, weaken the Atlantic Meridional Overturning Circulation (AMOC), and contribute to Amazon ecosystem decline.
• Sea-level and glacier impacts: Mountain glaciers could lose more than a quarter of their mass by 2100. Extended warming above 1.5°C could commit the planet to substantial additional sea-level rise over subsequent centuries.
• Food and livelihood risks: Without adequate adaptation, global food production could decline by as much as 14% by 2050, while extreme heat increasingly threatens labour productivity and human health.

Opportunities for Climate Action

Despite the risks, the report identifies significant opportunities for reducing warming:

• Large mitigation gains: Measures across energy, transport, industry and agriculture could potentially reduce emissions by up to 41 GtCO₂e annually by 2035, with many options costing less than $20 per tonne.
• Health benefits: Phasing down fossil fuels could substantially reduce sulphur dioxide and nitrogen oxide pollution, lowering respiratory illnesses and associated healthcare costs.
• Methane control: Rapid methane reduction can deliver relatively quick climate benefits because methane has a strong short-term warming effect.
• Economic value of adaptation: Climate-resilient infrastructure can prevent substantial future losses; in developing economies, every dollar invested could avoid around four dollars in damage.
• Stronger climate jurisprudence: International legal developments are increasingly reinforcing responsibilities relating to climate mitigation and adaptation.

Key Constraints

• Fiscal vulnerability: Climate disasters increase reconstruction costs and borrowing pressures in vulnerable economies. Around 70% of Small Island Developing States (SIDS) exceed the 40%-of-GDP debt threshold after repeated climate shocks.
• Carbon-removal limitations: Nature-based carbon removal can face ecological saturation and wildfire risks, while technologies such as DACCS and BECCS require considerable energy, water and land.
• Risk of maladaptation: Poorly designed interventions can create new environmental and social vulnerabilities. Commercial monoculture plantations, for instance, may deliver weaker ecological outcomes than restoration with native forests.
• Threat to island states: Sea-level rise and saltwater intrusion can undermine freshwater supplies, habitable land and territorial security of SIDS such as Tuvalu and Kiribati.
• Heat-related productivity losses: Increasing humid heat could significantly reduce outdoor labour capacity and deepen economic losses in vulnerable countries.

Way Forward

• Pursue mitigation and adaptation together: Emission reduction should not be postponed in favour of adaptation; both need to advance simultaneously.
• Speed up the clean-energy transition: Reduce dependence on fossil fuels, expand renewable energy and prioritise rapid methane reduction.
• Improve carbon-removal governance: Establish robust monitoring, reporting and verification systems so carbon removal supplements rather than substitutes for emission reductions.
• Build climate-resilient societies: Develop flexible infrastructure, strengthen coordination across water-energy-food systems and consider planned relocation where adaptation limits are reached.
• Expand loss-and-damage finance: Strengthen international financial support for climate-vulnerable nations facing unavoidable and irreversible impacts.

Conclusion

Crossing 1.5°C may increasingly become a temporary reality, but how high temperatures rise and how long they remain elevated are still matters of policy choice. Limiting every additional fraction of warming can reduce irreversible damage. The priority must therefore be rapid decarbonisation, resilient adaptation, responsible carbon removal and predictable climate finance to shorten the overshoot and enable a return towards a safer climate pathway.

Source : UNEP