UNEP says 1.5°C warming overshoot is now ‘unavoidable’. What does it mean for climate action?

Notably, UN Climate Change head Simon Stiell had only last year insisted that the 1.5°C target remained “within reach”.

First, why is the 1.5°C target significant?

The 1.5°C target was a key outcome of the Paris Agreement, signed in 2015 by 193 countries and the European Union. Under Article 2 of the Paris Agreement, nations must commit to keep global average temperatures well below 2°C above pre-industrial levels, and ideally capped at 1.5°C.

The lower threshold matters because climate risks increase with every additional fraction of a degree of warming. The 1.5°C limit was never considered a completely “safe” level of warming, but staying closer to it reduces the severity of risks to people and ecosystems.

For years, 2°C was the threshold widely used to assess dangerous climate change. Yale economist William Nordhaus first suggested that if global warming were to exceed 2°C, climate change would push living conditions outside the range experienced by human civilisation. Over time, scientists and policymakers increasingly used 2°C as an organising principle for climate action.

It was only with the Cancun Agreements, signed at the UN Climate Change Conference (COP 16), that both 2°C and 1.5°C were formally written into an international climate accord. The 2015 Paris Agreement then established 2°C as the upper limit while identifying 1.5°C as the more desirable goal, because the risks of extreme heat, drought, water stress and other climate impacts are substantially lower at 1.5°C than at 2°C.

This does not mean that limiting warming to 1.5°C is “safe”. The report notes that climate impacts intensify as warming rises, with growing risks of irreversible changes such as Amazon dieback, permafrost thaw and ice-sheet collapse. The further temperatures rise above 1.5°C, the harder and more expensive it also becomes to bring them back down, increasing reliance on carbon dioxide removal in addition to emissions cuts.

At the same time, exceeding 1.5°C in a few months, or even for a year or two, does not automatically mean the Paris Agreement target has been breached. The 1.5°C goal refers to long-term warming, measured over decades, rather than the average temperature in any calendar year.

So what does the latest report say?

UNEP acknowledges that even in an optimistic scenario where governments fully implement current climate plans and net-zero pledges, warming is expected to peak at about 1.8°C.

In this light, the report offers five main takeaways.

SHIFT STRATEGY, DO NOT ABANDON 1.5°C: 

UNEP says the world will exceed the 1.5°C limit, but that does not mean abandoning the target. Countries will now have to shift strategy and approach 1.5°C “from above rather than below” this limit, meaning exceed this limit, keep the peak as low as possible for as short a time as possible, and then work on bringing temperatures down.

“Decisions, science and, most importantly, mindsets need to shift to seeing 1.5°C as a goal we’ll be approaching from above rather than below,” said Mirey Atallah, Chief of UNEP’s Adaptation and Resilience Branch and coordinator of the report. “This is the starting point for realising just how imperative it is to massively reduce emissions and minimise the severity of the overshoot pathway we’re on.”

SOME DAMAGE CANNOT BE UNDONE:

Second, overshooting 1.5°C will still cause harmful, long-lasting damage, even if temperatures are eventually brought back down. The higher temperatures rise, and the longer they stay above 1.5°C, the greater the risk of extreme heat, wildfires, glacier loss and damage to ecosystems such as coral reefs.

Some changes may be difficult or even impossible to reverse. The report notes risks involving the Greenland and West Antarctic ice sheets, the Amazon rainforest, and the Atlantic Meridional Overturning Circulation, a major system of ocean currents that helps regulate climate.

Bringing temperatures back down is itself difficult. UNEP says carbon dioxide removal will be necessary, but even optimistic deployment is unlikely to reverse more than a few tenths of a degree of warming this century. At current rates, the report estimates that afforestation and forest management alone would take more than 100 years, assuming no residual CO₂ emissions, to reduce warming by just 0.1°C.

And returning below 1.5°C would not undo all the damage caused during the overshoot. Sea-level rise, extinctions and ecosystem shifts may continue even as temperatures decline.

EXTENT OF OVERSHOOT:

The extent of overshoot matters. A smaller, shorter overshoot would mean less damage and improve the chances of bringing warming back below 1.5°C. Cutting methane and other short-lived climate pollutants in the immediate future can also help slow warming relatively quickly.

MITIGATION:

The report lays out a three-step response to overshoot. In the immediate future, countries need to slash emissions and protect vulnerable people and ecosystems. At peak warming, the world would need to reach at least net zero while coping with severe impacts. After that, bringing temperatures down would require sustained net-negative CO₂ emissions, or removing more cabron dioxide from the atmosphere than is emitted.

The report stresses that mitigation and adaptation have to happen together. Weak emissions cuts can make adaptation harder and more expensive, while poor adaptation can leave people and infrastructure exposed to climate risks.

UNEVEN BURDEN:

The consequences of overshoot will not be shared equally. Countries that have contributed least to global warming are often among the most exposed to its effects. This was also a major concern at COP30 in Belém, Brazil last year, where countries agreed on the need to sharply scale up adaptation finance for developing countries. UNEP had estimated ahead of the summit that their adaptation needs could exceed $310 billion a year by 2035, around 12 times current international public finance flows.