Sober exit from the ozone party
In Montreal and Nairobi - the headquarters of the UN Environment Programme, Unep - celebration permeates the air.
The occasion: that on the 22nd birthday of the Montreal Protocol on Substances that Deplete the Ozone Layer, every UN member country has ratified the treaty - the first time that's happened with any international environmental agreement.
The latest adherent is East Timor, whose Prime Minister Xanana Gusmao declared his country's pride in "joining the rest of the world in the fight against the depletion of the ozone layer and the effort towards its recovery".
Having played a leading role in East Timor's push for independence, Mr Gusmao knows a thing or two about fighting long fights; and that's just as well, as it is far from clear how long the journey to ozone "recovery" is going to take.
This graph from the World Meteorological Organization (WMO), which collects and collates data on the Antarctic ozone "hole", makes clear that although the exact size of the hole varies from year to year, it's stubbornly resistant to going away.
This is despite the fact that within months, the Montreal Protocol's list of banned chemicals will rise to 100 - all the variants of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrobromofluorocarbons (HBFCs) and so on whose stratospheric chemical offspring gnawed away the ozone layer as their usage soared in the determinedly white-coated years following World War II.
Why the ozone hole has not started to recover, as scientists initially believed it would, is a complex and only partially understood issue.
Firstly, not all ozone-depleting chemicals were phased out at the same time, with developing countries given longer to change over; a few substances, such as the fumigant methyl bromide, are still in use.
A second factor was the development of a thriving international black market.
Some companies in developing countries that were permitted to continue manufacturing and using CFCs found it more profitable to export them to western nations where they were officially banned.
At one point, the Environmental Investigation Agency estimated that CFCs ranked second only to drugs in their value to the black market stallholders of Miami.
Those days are almost over. The big industrial consumers such as refrigeration plants have largely switched to other chemicals, a process encouraged by chemical manufacturers such as Dupont.
The original bad boys, CFCs, are out of circulation (apart from possibly a few "essential uses") next year.
But many of these substances persist in the atmosphere for decades. And it appears that man-made global warming is increasing their impact.
More heat trapped in the lower atmosphere means that less permeates up to the stratosphere - which is cooling as a result.
That encourages the chemical reactions that take ozone molecules apart.
Climate change is increasing water vapour concentrations in the stratosphere above the poles. That means more polar stratospheric clouds, which again speeds ozone destruction.
The current best estimates are that the hole will be back to its pre-CFCs size by about 2050.
Paradoxically, though, the Montreal Protocol has helped to curb climate change, as some of the heavy ozone-depleting molecules are also among the most potent greenhouse gases that humanity has invented.
A recent study concluded that the treaty has had more impact on greenhouse gas emissions than the Kyoto Protocol would do even if all its targets are implemented - which currently looks unlikely.
However, a companion study calculated that use of popular ozone-friendly replacements, HFCs, is likely to increase so fast (for air-conditioning and other applications) that they could become major contributors to global warming in the coming decades - perhaps contributing 20% of the man-made greenhouse by 2050.
That analysis prompted a number of countries, led by Mauritius and Micronesia, to suggest using the mechanisms of Montreal to regulate HFCs - an initiative that has just received backing from the three nations of North America.
As with all of the protocol's phase-outs, it will need industry support to be successful.
The Montreal Protocol is widely hailed as a treaty that has worked - and by some measures, it has, with an internationally-endorsed political process, backed up by technical and financial help for countries that needed it, leading to a phase-out of the chemicals of concern.
Unep reckons that by curbing damage to the ozone layer, it has prevented 1.5 million cases of melanoma, about 20 million other cancers, and 130 million eye cataracts.
But it's worth reflecting that if 2050 turns out to be about right, it will have taken humanity more than a century to create, understand, discuss, regulate and solve a relatively small-scale and tractable environmental problem.
What does that timescale imply for our capacity to solve biodiversity decline, ocean acidification, climatic change, the spread of deserts, and the other symptoms of our swelling human population?
Raise a glass, if you will, in Montreal and Nairobi; but a swift sobriety ought perhaps to follow.