What happens to the Paris Agreement once the world breaches 1.5 degrees of global warming?
This question, long debated in climate circles, has now become unavoidable. In 2024, the hottest year ever recorded, global temperatures averaged 1.6 degrees above pre-industrial levels.
One year doesn’t define a trend, and the Paris Agreement focuses on long-term averages. But, at this stage, it’s virtually certain that the 1.5 degrees threshold will be exceeded this decade, likely before the end of 2027.
Is the goal of limiting warming to 1.5 degrees now a lost cause?
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Some argue it always was. For these pragmatists, 1.5 degrees was always aspirational and unrealistic. With current policies steering us towards 3 degrees of warming this century, they suggest aiming for an interpretation of the Paris Agreement temperature goal that is more feasible, and less likely to induce paralysis and disillusionment.
But others disagree. The consequence of accepting a higher temperature goal would lock in such profound damages and irreversible tipping points in Earth’s climate systems, that 1.5 degrees must remain our North Star. Letting go of this goal accepts locking in greater irreversible changes to Earth’s climate systems: the collapse of coral reefs, melting glaciers and ice sheets, disruption of ocean currents (including the one that keeps Ireland’s climate mild) and the dieback of the Amazon rainforest.
The Paris Agreement’s 1.5 degree target was grounded in the understanding of this harm, and is more urgent than ever: the scientific evidence linking catastrophic tipping points to overshoot scenarios has grown even stronger, but policymakers did not anticipate time was so short, and the target would be breached so soon.
Keeping 1.5 alive now means facing a tough truth – we are already in climate debt. For many high-emitting countries such as Ireland, this means that net-zero is not sufficient. Any excess carbon we put into the atmosphere after overshooting 1.5 degrees will have to be removed by today’s young people and future generations. That will require not just cutting greenhouse gases rapidly, but also removing carbon dioxide from the atmosphere at gigantic scales.
Like cleaning up a sticky mess, carbon dioxide removal is far more difficult than avoiding it in the first place. Its concentration in the atmosphere is only 0.04 per cent, and there is no “low hanging fruit”.
Nature-based solutions such as reforestation and soil carbon require vast land areas and are vulnerable to fire, drought and disease, which are exacerbated by climate change itself. Technical approaches like direct air capture, are promising, but still in their infancy and will demand huge energy inputs and infrastructure.
There is no guarantee these measures will be technically feasible at the scales necessary to return temperatures to 1.5 degrees. Societal and political feasibility will be an even greater challenge. Look at the current dire situation: climate change is not just getting worse, but getting worse faster, and climate action is being unravelled in the United States. How is it conceivable that humanity will come together to draw down carbon dioxide at vast scales, while running the global economy on carbon-free energy and food?
Nevertheless, a recent article in Science made the case that the 1.5 degree goal must endure as both a legal and ethical imperative, arguing that “exceedance of 1.5 degrees, rather than rendering the goal irrelevant, is a wake-up call alerting parties [Paris agreement signatory countries] to redouble their efforts to bring emissions down and thereby halt or even reverse the increase in global warming”.
Regardless of whether climate leaders reaffirm the 1.5 degree goal or accept another interpretation of the Paris Agreement, stabilising global temperatures requires that emissions of carbon dioxide fall rapidly and reach net-zero. Only then will warming stop.
It is also incumbent on wealthier countries – which bear more responsibility and have greater capacity – to support decarbonisation efforts and adaptation in developing countries.
Addressing methane is another key element. Responsible for about one-third of today’s warming, methane’s short-term life in the atmosphere means that reducing its emissions reverses its historical warming impact. This very powerful short-term lever to reverse warming is critical for buying time and reducing the peak temperature.
For high methane-emitting countries such as Ireland, there is a responsibility to lead. Instead, this government has committed to advocating for reclassifying how methane is accounted for in the EU and international forums, to downplay its warming role rather than committing to strong emissions reductions.
Whether 1.5 degrees is feasible or not is a matter for scientific debate, but may be a distraction from political action. At this stage, the more urgent and practical question is how far we overshoot, for how long, and what we do to bring temperatures back down.
Focusing solely on the feasibility of 1.5 risks paralysis; instead, debate should centre on how to minimise overshoot, reduce peak warming and prepare for large-scale carbon drawdown. These are the levers we still have, and the outcomes we can still shape.
Prof Hannah Daly is professor of sustainable Energy at University College Cork
