The new UNEP report Limiting Overshoot treats climate overshoot as a process rather than a momentary breach of 1.5°C. That means preparing not only for exceedance but also for the long, uneven period of peak warming, decline and eventual stabilization that may follow.
Imagine two points in the future, separated by decades, at which the long-term global mean temperature is 1.5°C above the 1850–1900 pre-industrial baseline.
The first is imminent. Human-induced global warming is approaching 1.5°C compared to pre-industrial levels, with warming expected to continue beyond it. The second remains contingent: after peak warming, temperatures could return towards 1.5°C by the end of the century, but only if decisions taken now preserve that possibility.
Same temperature, but different worlds.
This is one of the most important implications of UNEP’s new Limiting Overshoot report. Overshoot is not simply a temporary crossing of a temperature threshold followed by a return to normal. It is a lived process unfolding through exceedance, peak warming, decline and eventual stabilization, with different risks and responses at each stage. And with deep uncertainties.
What we know, and what we do not
Some elements of the overshoot pathway are becoming clearer, but critical uncertainties remain. Exceedance of 1.5°C is now expected, but we do not know exactly when warming will peak, at what level, or how long the world will remain above 1.5°C. Nor do we know precisely how severe the impacts will be, or whether sustained global net-negative CO₂ emissions can be achieved at the scale required to bring temperatures back down.
Decisions taken now will strongly influence those outcomes. Near-term emissions reductions affect how high the warming peaks. The level of peak warming, in turn, affects how much net-negative CO₂ would be needed to drive temperatures back down. Delayed action increases both the magnitude and duration of overshoot and can reduce the chances of returning to 1.5°C within this century. Both matter: every additional fraction of a degree increases the severity of climate impacts, while every additional year above 1.5°C prolongs exposure to them and increases the likelihood of crossing irreversible thresholds and adaptation limits.
A world at 1.5°C on the way down will not resemble a world at 1.5°C on the way up. At first crossing (exceedance), some impacts may still be unfolding because of inertia in Earth systems. By the decline stage, however, the world will have passed through both exceedance and peak warming, accumulating impacts and biophysical shifts along the way. Ecosystems may have crossed tipping points, and ice sheets, permafrost and ecological functions may face changes that are effectively irreversible on human timescales.
Human systems will also have changed. For people, overshoot is not experienced as a temperature trajectory, but as a succession of changing risks to livelihoods, health, food and water security, shelter, mobility and access to essential services. As temperatures rise towards peak warming, repeated shocks may require successive rounds of crisis response and adaptation. By the time temperatures decline, households, communities and institutions may be adapting again from a very different starting point, with resources and capacities already altered or depleted by earlier impacts and responses.
These changes will not be evenly distributed. Existing inequalities may deepen as losses, adaptation costs, relocation and recovery burdens fall unevenly across places, groups and generations. Income, gender, age, disability, location and access to land, essential services and political power will shape both exposure and the capacity to respond.
What we must do
Adaptation must be anticipatory, iterative and transformational. The report frames the global response around three interconnected phases: immediate response, coping and containment, and long-term resilience. As temperatures approach and exceed 1.5°C, deep emissions reductions, including reductions of methane emissions and other short-lived climate pollutants, must proceed alongside urgent adaptation. Around peak warming, sustained decarbonization must continue while societies manage severe and compounding risks. During decline and stabilization, sustained net-negative CO₂ emissions would need to be accompanied by continued adaptation, including to impacts and losses that do not reverse even if global temperatures drop.
Sustaining climate-resilient development across all stages of the overshoot pathway will depend, in part, on how climate finance is structured. Finance needs to be anticipatory and flexible, responding to changing needs across the different stages of overshoot. It must build transformational capacity before peak warming and provide predictable, pre-arranged support where repeated shocks, rising borrowing costs and shrinking fiscal space risk trapping vulnerable countries in cycles of crisis response. The report highlights how debt service can crowd out adaptation and development spending, while insurance retreat can shift risks back onto governments and households.
Overshoot is also a test of multilateralism. International cooperation must mobilize and sustain finance, technology and capacity through peak warming and decline. Equity and justice must shape that cooperation so that countries and communities that have contributed least to warming are not left to bear its greatest burdens.
Finally, adaptation has limits. Beyond certain thresholds, impacts enter the domain of Loss and Damage, including economic and non-economic losses such as permanent displacement and the loss of ancestral land, culture, sovereignty or self-determination. This means anticipating where adaptation limits may be crossed, how irreversible losses will be addressed and what governance is needed when livelihoods must be reconfigured, landscapes are permanently altered or territories face increasing risks of uninhabitability.
Returning to 1.5°C would still be far safer than remaining above it. But the journey back will matter as much as the destination. Importantly, decisions taken before and during overshoot must be judged not only by whether they bend the temperature curve, but also by whether they preserve human security, agency and the ability to recover, especially for those who have contributed least to warming and face the greatest risks.
Across UNU-EHS and UNU-VIE, teams will advance knowledge on the human dimensions of climate overshoot – including implications for human security, ecosystems, migration and displacement, adaptation and transformation, climate finance and Loss and Damage.
