Forty years ago, humanity faced its first truly planetary threat: the specter of an unraveling ozone layer. Heeding the unequivocal warnings of science, every nation on Earth came together under the Vienna Convention. This unprecedented unity was followed by the Montreal Protocol, which brilliantly targeted and began phasing out the culprit chemicals—CFCs—setting the ozone layer on a path to recovery.

This triumph is rightly celebrated. It stands as a landmark of multilateralism, a testament to what is possible when science is our guide and collective action is our response. The ozone layer is healing.

But on this World Ozone Day, as we commemorate this success, we must confront its complex and urgent legacy. The story of the ozone layer is not a closed chapter of past glory; it is a living lesson in the perpetual cycle of global stewardship. We succeeded in solving one problem, only to inadvertently unleash another.

The very alternatives that saved the ozone layer—hydrofluorocarbons (HFCs)—are potent greenhouse gases, thousands of times more effective at warming the planet than CO₂. Our global action, brilliant in its conception, created a new global threat. Science sounded the alarm once again, leading to the essential Kigali Amendment to phase down HFCs.

This is the critical, often-forgotten lesson: global action is not a one-time event. It is a continuous process where today’s solution can become tomorrow’s problem, demanding ever more vigilant and predictive science.

The Montreal Protocol’s journey epitomizes this cycle: from science to global action, to emerging science, to further global action.

We now stand at the next inflection point. The success of the ozone regime is threatened by new challenges that demand our immediate focus:

1. Accelerating the Kigali Implementation
The 2016 Kigali Amendment to the Montreal Protocol, which commits countries to phase down HFCs, must be urgently accelerated. Governments need to ratify and implement the amendment without delay. More importantly, HFC phase-down commitments should be fully integrated into enhanced Nationally Determined Contributions (NDCs). This single action could avoid up to 0.5°C of warming by 2100—a monumental contribution in the race to stay below 1.5°C.

2. Tackling the Dumping Crisis
Developed countries continue to offload obsolete refrigerators and air conditioners onto poorer nations, particularly in Africa. While this may seem like a phase-out of old equipment, the reality is grim: these appliances leak stored CFCs and HFCs, operate with extremely low energy efficiency, and create toxic waste problems. This is not just an environmental hazard but an ethical failure—shifting the burden of emissions to those least responsible and least equipped to handle it.

3. Addressing the PFAS Peril
A new challenge is emerging from the decomposition of substitutes for ozone-depleting substances. These can form persistent “forever chemicals” (PFAS), which are highly toxic and accumulate in the environment, risking human health, animal life, and even marine ecosystems. Food security itself may be at risk.

4. Avoiding the Complacency Trap
Perhaps the greatest threat of all is the narrative of “mission accomplished.” Complacency can poison the momentum needed to address the unintended consequences of our own success.

The parallels to other technological leaps are striking. Nuclear energy, the internet, and now artificial intelligence all offer profound benefits, but their risks often reveal themselves only after widespread adoption. The ozone story is no different. Our scientific models must become more predictive, actively identifying potential downsides before solutions are deployed at planetary scale.

On this World Ozone Day, we must do more than celebrate. We must recommit—with sharper vision and renewed urgency.

Let us honor the Vienna Convention not just by remembering its past success, but by embracing its enduring spirit: the humility to listen to science, the courage to act collectively, and the wisdom to expect the unintended.

Every fraction of a degree of warming matters. Every ton of HFC phased out counts. And every lesson from our past victories must be learned—not drowned out by applause.

We are well on our way to sealing the ozone hole. But let us ensure that in doing so, we do not ignore the new fissures opening at our feet. The work of healing our planet is never complete—it simply evolves.Let our legacy be that we were wise enough to see the whole board, not just the move we just made. The ozone hole is being sealed, but the deal is not done. The lesson is not over. The action must continue—even in celebration.

(The author is a noted environmentalist, former Director of UNEP, Coordinating Lead Author, IPCC 2007 (Nobel Peace Prize laureate), IIT Alumnus, and Founder, Green TERRE Foundation, Pune, India. Views are personal. He can be reached at shende.rajendra@gmail.com.)