Limiting Overshoot: UNEP on Crossing and Returning from 1.5°C

Understanding the 1.5°C threshold

The Paris Agreement seeks to hold warming well below 2°C. It also pursues efforts to limit warming to 1.5°C. These figures compare global average temperature with the 1850–1900 baseline. They do not describe the temperature on any single day or in one country.

A particularly hot year can exceed 1.5°C without proving long-term exceedance. Climate targets concern sustained warming measured across a longer period. This distinction matters when interpreting records and forecasts. Temporary annual observations and long-term climate states answer different questions.

UNEP says exceedance is now widely assessed as unavoidable under current policies. This finding reflects accumulated emissions and slow near-term action. It does not make every future pathway identical. The height and duration of warming still depend greatly on present choices.

What climate overshoot means

Overshoot describes a pathway with three connected stages. Global warming first exceeds the selected limit. It then reaches a peak before declining. A complete analysis must examine the magnitude, duration and eventual return.

The report’s most optimistic examined scenario peaks near 1.8°C. Other scenarios produce peaks above 2°C. Every additional fraction of warming increases physical and social risks. Each added year can also deepen exposure and make adaptation harder.

Returning towards 1.5°C does not restore every earlier condition. Lost species, displaced communities and damaged ice systems may not recover. Some changes can continue after global temperature falls. Overshoot must therefore never be treated as harmless temporary borrowing.

How temperatures could decline

Rapid greenhouse-gas cuts are needed to limit the peak. Carbon dioxide emissions must reach net zero, while methane reductions can slow near-term warming. A later decline requires sustained net-negative carbon dioxide emissions. Humanity would then remove more carbon dioxide than it releases.

Carbon dioxide removal, or CDR, can include restoring forests and engineered approaches. Removal must supplement deep emissions cuts, not replace them. Its land, energy, cost and permanence constraints require strong governance. Unrealistic future removal assumptions can excuse avoidable emissions today.

UNEP says credible return becomes much harder if warming peaks near 2°C. Residual emissions must also be reduced before removals create a net-negative balance. The lower the peak, the smaller the removal burden. Immediate mitigation therefore keeps more future options open.

Adaptation during an overshoot

Even strong mitigation cannot prevent all near-term harm. Governments must prepare health systems, cities, farms and water management for changing risks. Adaptation should respond to compound and unexpected events. Measures designed for historical conditions may become inadequate.

There are limits to what adaptation can achieve. Extreme heat can exceed safe conditions for outdoor work. Sea-level rise can force relocation from low-lying coasts. Ecosystem loss may remove natural protection and livelihoods together.

Vulnerability is distributed unequally between and within countries. Many exposed communities contributed little to historical emissions. Fair finance and participation are therefore central to effective adaptation. Decisions about relocation or changed livelihoods also require legitimacy and consent.

Policy implications

Governments should not replace the 1.5°C goal with a looser destination. Targets need faster implementation across energy, transport, buildings, industry and land use. Methane control can complement the transition away from fossil fuels. Protection of natural sinks must accompany, not delay, emissions reductions.

Plans should also separate proven reductions from uncertain future removals. Transparent accounting can prevent double counting and temporary storage claims. Regular assessment should track both peak risk and adaptation readiness. This makes overshoot policy an immediate governance task.

Conclusion

The report presents overshoot as a dangerous pathway, not a new comfort zone. Lower peaks and shorter exceedance periods still save lives and ecosystems. Rapid mitigation determines whether temperature decline remains achievable. Adaptation must advance alongside those cuts. The remaining choice concerns the scale and permanence of harm, not whether action still matters.