Why 1.5C matters
Breaching the Agreement heightens our obligations to mitigate faster to reduce warming and protect the most vulnerable.
By James Fletcher and Bill Hare.
Almost a decade ago, in the second week of the Paris climate talks, St Lucia was asked to co-facilitate a pivotal process – one that proved key to securing a high ambition outcome for the Paris Agreement.
We – James, then St Lucia’s Environment Minister, and Bill, his science advisor – were laser-focussed on securing a warming limit of 1.5˚C. This was a crucial guardrail for the Alliance of Small Island States (AOSIS) which had first proposed "I.5 to survive" at the climate talks in Poznan. By Paris, over 100 countries supported it.
The negotiations were fierce, but we had strong support from the French Presidency. With the backing of AOSIS, the Least Developed Countries (LDCs) and aligned nations, we secured inclusion of the 1.5˚C warming limit in the Paris Agreement, alongside a commitment to net zero greenhouse gas emissions in the second half of the century, which, together would operationalise the 1.5°C limit.
The Paris Agreement was a momentous occasion for the small island states, the most vulnerable to climate change. While the Agreement was not perfect, it represented long-overdue recognition that limiting warming to 1.5˚C offers these countries a chance of survival in the long-term.
This target was never about feasibility; it was about safety. Even at just over 1°C of warming, impacts on vulnerable nations were already severe. 1.5°C was a survival threshold, not a cost-free limit for small island states.
At 2°C, the pre-Paris global goal, the impacts on small islands would be devastating. A 1.5°C warming limit meant that at least some of the AOSIS 40 member states had a hope of surviving with the majority of their land intact.
Scientific assessments at the time confirmed our position. The UNFCCC’s own Structured Expert Dialogue declared the 2°C warming limit “ inadequate,” identifying limiting warming to below 1.5°C as the safer path.
A decade later, the scientific case has only grown stronger. The 1.5°C limit is increasingly seen as a physical limit to avoid major Earth system tipping points — thresholds that, if crossed, would cause catastrophic damages to ecosystems, people and infrastructure.
Today, we know that warming beyond 1.5°C carries disproportionate risk. Heatwaves, water scarcity, and biodiversity loss all intensify. Recent massive coral bleaching events, driven by extreme marine heat waves, raise fears that one such tipping point may already be behind us.
Equity and climate justice are the core the 1.5°C limit. It aims to limit harm to those least responsible for climate change—small island states and least developed countries—and provides a critical ceiling to the threats they face.
Overshooting this limit would constitute an immense moral failure and a betrayal of these nations and younger generations globally. Let’s be frank: big emitting countries tried to relegate the 1.5°C limit to a non-binding, aspirational goal. But AOSIS and the LDCs fought successfully for its inclusion as an operational, legally binding directive.
This, combined with the net-zero goal in the Agreement, has shaped nearly all climate goals since. From halving emissions by 2030 to tripling renewables, to the EU’s 90% emissions reduction by 2040 — these are all driven by the 1.5°C limit.
Anyone concerned about planetary habitability must fiercely defend the 1.5°C pathway.
Extreme heat already threatens lives, especially in regions already struggling with food and water insecurity. Ecosystems, too, stand on the brink: biodiversity loss escalates dramatically past 1.5°C, and protected areas won’t be enough to prevent mass species extinction.
Carbon emissions and warming are driving dire risk for our oceans, from extreme heatwaves to acidification and deoxygenation, risks that can only be limited if we get emissions onto a 1.5˚C pathway. Coral reefs might cling on at 1.5°C, but anything above spells their demise, further endangering marine biodiversity and livelihoods.
The economic cost of overshooting is stark. Developing countries face disproportionate harm that could set back development gains. Small island states, though least responsible, are among the hardest hit, accounting for two-thirds of countries suffering major climate disaster damages.
More than three billion people are at increased risk of losses and damages if 1.5°C is breached. Even modest warming could lock in multiple metres of sea-level rise, with devastating impacts on low-lying islands and coastal cities.
Is 1.5°C still in reach? Recent projections from the WMO indicated that 2024 was the hottest year ever recorded at ~1.5°C above pre-industrial levels. That spike includes both human-caused warming and natural variability. The Paris Agreement limit also refers to a long-term average over about 20 years, so the answer is: not quite yet.
However, without urgent action, we are on track to blow past the 1.5°C limit by the early 2030s. There’s a 1 in 100 chance of seeing 2°C warming before this decade is out.
Yet, hope remains. Renewable energy technology and electric vehicles have become more accessible and affordable. The global shift away from fossil fuels is not only possible but underway.
Concurrently, the imperative to protect and expand natural carbon sinks, especially forests, has intensified. Forests must be preserved not as carbon offsets, but as essential allies in stabilising the climate. Continued fossil fuel reliance cannot be masked by reforestation alone.
Today, we stand at a critical juncture. Temporary overshoot does not invalidate the enduring, legally binding goals of the Paris Agreement. But it does increase our responsibility to act faster and with more ambition.
Countries must set 1.5 °C-aligned targets for 2030 and 2035 in new NDCs before the Belém COP this November. The 1.5°C limit is not symbolic—it’s a vital boundary for planetary stability.
Getting on the 1.5°C pathway is not just achievable—it is essential. Our collective challenge is to translate this imperative into immediate, equitable and effective policies.
Limiting warming to 1.5°C is more than a matter of hope. It is a necessity grounded in science and driven by equity and the relentless pursuit of a safer world.