In 2007, all the signatories agreed to accelerate the phasing out of hydrochlorofluorocarbons (HCFCs), the last remaining ozone-depleting substance which is still widely used in room air conditioners.

Yet, while the Montreal Protocol has achieved much of what it set out to do, it still has some weighty challenges ahead. The 2005 IPCC/TEAP Special Report on Ozone and Climate, of which I was a coordinating lead author, exposed some alarming trends.

* A threat from "banks" of ozone-depleting substances: Though the production of CFCs has been phased out, CFC produced in the past (before 2010) exists in various equipment that are still running, like old refrigerators. Such CFCs and other ozone-depleting substances that still exist in equipment all over the world are called "banks". About 21 gigatons of carbon dioxide equivalents contained in old equipment will inevitably seep into the atmosphere in the absence of any significant efforts to chemically destroy them by incineration.

* Market imperatives: The center of gravity for global air-conditioning with HCFCs is moving to China. The country faces multiple challenges. It has to supply the high global warming potential (GWP) - a measure of how much a given mass of greenhouse gas contributes to global warming - alternative air-conditioning systems to the developed countries where such systems are allowed, such as the United States, and low GWP alternatives in places where there are regulations that ban the high GWP systems, such as the European Union.

The world is also looking at China and India to develop low-GWP and energy-efficient air-conditioning systems that would be economically and environmentally beneficial. High ambient temperature in the developing countries would be the key barrier for energy-efficient technologies.

* High growth of HFCs: There is also the projected growth of HFCs, which were introduced to replace HCFCs. This increase in business-as-usual scenarios is alarming. HFCs are powerful greenhouse gases and their emissions have to be controlled under the Kyoto Protocol. Forecasts indicate that the share of HFCs in the global fluorocarbon market will jump from 35 percent in 2008 to 58 percent in 2018.

Today, the reputation of the Montreal Protocol is at stake. Without immediate action to address these challenges and strengthen it, the Montreal Protocol is in danger of becoming a liability to the global community.

Climate change and global warming are linked to the ozone. If we protect the ozone layer, we protect the planet. The agreement has shown how government and the public can work together, but they must continue to do so to overcome the remaining challenges.