The 1.5°C climate threshold is expected to be exceeded persistently over the course of a few years.
The United Nations Environment Programme now estimates that this exceedance is widely considered inevitable under current policies and short-term trajectories. The threshold corresponds to a 1.5°C rise in the global average temperature compared with the pre-industrial period.
Pixabay illustrative image
A single year above this value does not mean that the climate target has already been definitively exceeded. In 2024, for example, the average temperature exceeded 1.5°C. To track climate change, scientists examine trends over several years, since temperatures naturally vary from one year to the next.
The new report therefore focuses on what happens after the threshold is crossed. The scenario studied involves three stages, provided that action is taken quickly: the temperature exceeds 1.5°C (now inevitable), reaches a peak, and then falls again (if drastic action is taken).
The United Nations Environment Programme calls this trajectory a temporary exceedance, or “overshoot.”
The height of this peak matters greatly. In the most favorable scenario studied, warming would peak at around 1.8°C. Other trajectories exceed 2°C. Every additional fraction of a degree increases the risks associated with extreme heat, ecosystems, water, food, infrastructure, and low-lying coastal areas.
The time spent above 1.5°C also matters. The longer this period lasts, the greater the likelihood of irreversible changes. The report specifically cites the major ice sheets, the Amazon rainforest, and Atlantic Ocean circulation (AMOC). Some damage may also exceed the adaptive capacities of populations or ecosystems.
In practical terms, limiting the peak requires rapid and sustained reductions in greenhouse gas emissions, particularly methane, until net-zero emissions are reached. To subsequently lower the temperature, more carbon dioxide would need to be permanently removed from the atmosphere than is added to it.
These removals could notably involve forests or technical processes capable of capturing and storing carbon dioxide. The more warming rises before falling again, the greater the volumes that must be removed and the more climate risks increase.
The report also emphasizes adaptation, since some effects are already present and others will occur before any potential decline in temperatures. The next step is therefore to act simultaneously on two fronts: reduce the peak in warming and prepare societies for the climatic conditions encountered during this period of exceedance.