Warning on Warming: On the UNEP’s Limiting Overshoot Report

The world is on course to cross the 1.5°C global warming threshold in the coming years, a development that marks a sobering moment for international climate action. But a new report by the United Nations Environment Programme (UNEP) says crossing the threshold should not be treated as a reason to abandon the fight against climate change.

UNEP’s “Limiting Overshoot” report, released on September 2, says exceeding 1.5°C above pre-industrial levels is now widely considered unavoidable under current policies and near-term trajectories. The focus, therefore, must shift towards limiting how high temperatures rise, reducing the length of time the world remains above the threshold and eventually bringing temperatures back down.

The report describes this as an “overshoot, peak and decline” pathway. In simple terms, the world may temporarily exceed 1.5°C, but the ultimate objective should remain to limit the peak and return global temperatures to below the threshold as quickly as possible.

Why crossing 1.5°C matters

The 1.5°C threshold became a central objective of the 2015 Paris Agreement, which seeks to keep global warming well below 2°C while pursuing efforts to limit it to 1.5°C.

UNEP now warns that every fraction of a degree above 1.5°C brings additional risks. Higher temperatures are associated with more intense extreme weather, ecosystem damage, glacier loss and increasing pressure on food and water supplies. The likelihood of crossing potentially irreversible climate tipping points also rises as warming becomes greater and lasts longer.

Among the risks highlighted by the report are destabilisation of major ice sheets, degradation of the Amazon rainforest and disruption of the Atlantic Meridional Overturning Circulation, a major system of ocean currents that influences global climate.

The consequences will not be evenly distributed. Vulnerable communities, low-lying coastal areas and small island nations face some of the most severe consequences despite often contributing relatively little to historical greenhouse gas emissions.

The world cannot afford climate resignation

One of the report’s most important messages is that exceeding 1.5°C must not become an excuse for political inaction.

UNEP argues that the lower the eventual temperature peak and the shorter the period of overshoot, the lower the risks to people, economies and ecosystems. Every fraction of a degree avoided can reduce future damage.

The report therefore calls for immediate and sustained reductions in greenhouse gas emissions. Cutting carbon dioxide is crucial, but reducing methane emissions is also important because methane is a highly potent greenhouse gas and contributes significantly to near-term warming.

Net zero will not be enough to reverse warming

The report also highlights an important distinction between stopping additional warming and actually reducing global temperatures.

Reaching net-zero emissions can help stabilise temperatures, but bringing temperatures back below 1.5°C would require the world to move towards net-negative emissions. That means removing more carbon dioxide from the atmosphere than humanity emits.

Carbon dioxide removal could involve measures such as restoring forests and other approaches capable of capturing and storing atmospheric carbon. However, UNEP stresses that carbon removal must be carefully governed and cannot be treated as a substitute for rapidly cutting fossil fuel emissions.

Adaptation must move alongside mitigation

Another central message of the report is that mitigation and adaptation can no longer be treated as separate climate strategies.

Even with aggressive emissions cuts, some impacts of climate change are already unavoidable. Countries therefore need to strengthen infrastructure, protect communities, improve disaster preparedness and make food, water and urban systems more resilient.

At the same time, insufficient mitigation could eventually push climate impacts beyond the limits of adaptation. Some ecosystems and communities may reach a point where adapting to further warming is no longer possible.

This makes the choices being made today particularly important. The amount of warming the world ultimately experiences will determine not only the scale of future damage but also how much room societies have to adapt.

A warning, but not a surrender

The UNEP report represents a significant change in the way the climate challenge is being framed. The question is no longer simply whether the world can prevent 1.5°C from ever being exceeded. It is increasingly about how high temperatures rise, how long they remain elevated and whether humanity can eventually reverse the increase.

The report’s best-case scenario puts peak warming at around 1.8°C, while less ambitious pathways could result in warming above 2°C.

That is a warning, but it is not a declaration of defeat.

The message from UNEP is that climate action remains consequential even after the 1.5°C threshold is crossed. Faster emissions reductions can lower the eventual peak. Stronger adaptation can protect vulnerable communities. Responsible carbon removal could help bring temperatures down over the longer term.

The challenge, therefore, is not to accept a hotter world as inevitable. It is to ensure that the overshoot is as small and as short as possible.

As the UNEP report makes clear, the trajectory remains partly within human control. The decisions governments, businesses and societies make now will determine how high the planet’s temperature climbs — and how difficult the journey back will be.