The 1.8°C Peak: How Global Climate Models Shifted to Overshoot and Carbon Removal

The original architecture of the 2015 Paris Agreement treated 1.5 degrees Celsius of warming as an absolute atmospheric ceiling—a hard limit that global emissions had to halt before crossing. The current scientific consensus, formalized this week in the United Nations Environment Programme's "Limiting Overshoot" report, treats that same temperature not as a ceiling, but as a baseline to return to. The difference is the assumption of a temporary breach.

Because the remaining carbon budget is too small to prevent temperatures from crossing 1.5°C, the global climate strategy has officially shifted to an "overshoot, peak, and decline" pathway. This framework accepts that the planet will heat beyond the target, forces warming to the lowest possible peak, and then relies on removing carbon dioxide from the atmosphere to cool the Earth back down.[1]

The shift acknowledges a mathematical reality that climate models have projected for years. According to the UNEP, global action to cut greenhouse gas emissions has been too slow to avoid crossing the 1.5°C threshold within the next few years.

Even if governments fully implement their existing national climate plans and meet additional net-zero commitments, the most optimistic scenario now sees warming peak at around 1.8°C above pre-industrial levels. "Let's be clear that the 1.5 target is still the goal," UNEP Executive Director Inger Andersen stated. "But now we need to approach it differently — from above."[1]

The overshoot pathway divides the climate response into three interconnected phases. The first is an immediate response focused on slowing warming through rapid emissions reductions, particularly by cutting methane and other short-lived pollutants. The second phase, coping and containment, begins as temperatures approach their peak. During this period, economies must reach at least net-zero emissions while simultaneously adapting to increasingly severe climate impacts. The final stage, long-term resilience, involves bringing temperatures back down through sustained net-negative carbon dioxide emissions.[2]

This final stage elevates carbon dioxide removal from a backup plan to a mandatory climate tool. Returning to 1.5°C requires taking CO2 out of the atmosphere and storing it permanently, utilizing both nature-based systems like afforestation and engineered carbon removal technologies.

However, the UNEP report emphasizes that carbon removal cannot replace immediate emissions reductions. Companies and governments cannot continue emitting at current levels and rely on removal technologies to solve the problem later; emissions must fall sharply first, with the removal infrastructure addressing residual emissions and driving the net-negative cooling phase.[1][2]

The difference between 1.5°C and a 1.8°C peak carries severe consequences. Every additional fraction of a degree increases pressure on ecosystems, food systems, and human health. The report warns that global food production could decline by up to 14 percent by 2050 in the absence of effective adaptation. Furthermore, higher peak temperatures and longer overshoot durations increase the likelihood of crossing irreversible tipping points, such as the destabilization of major ice sheets, the degradation of the Amazon rainforest, and the disruption of the Atlantic Meridional Overturning Circulation.[1]

Because impacts are already accumulating, mitigation and adaptation must advance together. The UNEP stresses that adaptation will require greater investment, stronger institutions, and new, inclusive governance arrangements. Even if a return to below 1.5°C is secured, many nations and communities will face irreversible losses, raising profound issues of climate justice. "We need to consider equity and justice, because that has to be at the core of all actions," Andersen noted, adding that countries with greater historical responsibility for climate change must lead the effort.[1][4]

In terms of the planetary carbon budget, only about 130 gigatonnes of additional CO2 can be emitted from 2026 onwards to maintain a 50 percent chance of limiting global warming to 1.5°C without overshoot. With global greenhouse gas emissions reaching a record 54 billion tonnes of CO2 equivalent in 2025, that budget is effectively exhausted. To stay within it, emissions would need to decline steadily to zero in just over six years—a trajectory the UNEP report concedes is no longer feasible, making the overshoot pathway the best remaining option.[3][8]

The formal adoption of the overshoot framework marks a new phase in international climate policy. Reaching net-zero emissions is no longer the final destination, but rather an essential milestone on the way to net-negative emissions. The severity of the coming decades will depend entirely on how quickly the global economy can force temperatures to peak. As UN Secretary-General António Guterres summarized, "We must make the overshoot above 1.5 degrees as small and short as possible. That demands an overshoot of ambition."[1][6]