Exceeding the 1.5°C threshold: the UNEP report on peak and decline pathways

On 2 September 2026 the United Nations Environment Programme (UNEP) published the report Limiting Overshoot, on the exceedance of the 1.5°C threshold set out in the Paris Agreement. The document examines what entering a period of exceedance involves and which choices remain available to bring temperatures back below that threshold.

The report and the notion of overshoot

The full title of the document is Limiting Overshoot: Navigating exceedance of 1.5°C and pathways towards return. It is a UNEP spotlight report, presented in Nairobi on 2 September 2026. The starting point of the analysis is that, under current policies and under the near-term trajectories that can be envisaged today, exceedance of 1.5°C above pre-industrial levels is widely assessed as unavoidable and is expected within a few years.

The report describes overshoot not as a state but as a dynamic pathway made up of three moments: exceedance of the threshold, peak warming and the subsequent decline in temperatures. On that basis UNEP identifies the overshoot, peak and decline pathway as the best remaining option to limit the magnitude and duration of exceedance and to return below 1.5°C as soon as possible. The document states that overshoot is not a new destination, and that the 1.5°C goal remains the reference point of the Paris Agreement.

Peak projections and expected risks

In the most favourable scenario considered by the report, temperature rise peaks at 1.8°C above pre-industrial levels. Most other scenarios place the peak at higher values, in several cases above 2°C.

Above 1.5°C, risks and impacts intensify with every additional fraction of a degree and with the length of the exceedance. Among the expected effects the document lists intensified extreme weather, ecosystem loss and the submersion of small island developing States and low-lying coastal cities, with significant damage to health, food security, water supplies, nature, cities, infrastructure and economies.

The report also points to the co-benefits of climate interventions. A rapid transition away from fossil fuels towards renewable energy is associated with improved air quality and public health, lower energy costs and increased access to electricity.

Tipping points and the limits of adaptation

According to the report, the likelihood of crossing irreversible tipping points increases with the magnitude and the duration of overshoot. The processes identified include the destabilization of major ice sheets, the degradation of the Amazon rainforest and the disruption of the Atlantic Meridional Overturning Circulation (AMOC), the system of currents that moves warm surface water north and cold deep water south and that plays a role in regulating the climate. Some effects may emerge abruptly, while others accumulate over time.

The document notes that adaptation set up now forms the foundation for later efforts, but that limits to adaptation and irreversible impacts exist, particularly where peak warming is not minimized. Hence the call for transformational adaptation, responsive to non-linear and abruptly arising risks, including those that may harm mitigation efforts. The report gives priority to interventions that deliver both mitigation and adaptation outcomes, such as nature-based cooling, and stresses that the two dimensions are mutually reinforcing and must advance together.

Mitigation, carbon dioxide removal and enabling conditions

The pathway described by UNEP requires immediate and sustained cuts in greenhouse gas emissions, including methane, on the road to net zero, followed by long-term net-negative emissions. Within this framework Carbon Dioxide Removal (CDR), that is the removal of carbon from the air and its storage through afforestation or other means, is identified as a necessary component.

The report sets out the terms of this approach: net zero is a milestone towards net negative, and carbon dioxide removal is additional to emission reductions, not a substitute for them. Removal can contribute credibly to a return below 1.5°C only if peak warming remains well below 2°C and all residual emissions are reduced. The document also stresses the need for strong governance frameworks for removal, to allow responsible deployment and to manage the implications for environmental integrity, equity and scale.

Among the conditions that enable action, the report lists political will, multilateralism, effective policies, inclusive governance and finance, with equity and justice as cross-cutting criteria; countries with greater responsibility for climate change should act fastest and with the greatest ambition. The document finally notes that years of inaction make certain irreversible losses and permanently changed conditions certain: many communities may be forced to relocate or to change livelihoods, raising issues of justice, power and political legitimacy that will require new, inclusive governance arrangements.

Outlook

The report is released during the preparatory phase of the thirty-first Conference of the Parties to the United Nations Framework Convention on Climate Change (COP31), convened in Antalya, Türkiye, from 9 to 20 November 2026, under a presidency exercised in partnership between Türkiye and Australia. The level of ambition of emission reduction measures, the governance arrangements for carbon dioxide removal and the financing of adaptation remain to be defined there and in related processes. UNEP places the overshoot, peak and decline pathway among the matters requiring immediate decisions, so that this trajectory remains available.

Sources

• UNEP, press release of 2 September 2026 on crossing 1.5°C
• UNEP, publication page for Limiting Overshoot
• UNEP, key messages of the report
• UNFCCC, official page for COP31 in Antalya