That's a lot of people! The conference organisation must have been daunting; and just imagine arranging the hotel accommodation and restaurant facilities and dealing with the additional human-generated waste.
Imagine the carbon and nitrogen emissions from the associated air travel!
The 40 or more decisions made were announced as an historic success.
Supposing this proves to be so, will it be sufficient to secure an acceptable quality of life for the generations to come?
What about the myriad other planetary-scale human impacts - for example on land cover, the water cycle, the health of ecosystems, and biodiversity?
What about our release of other chemicals into the environment?
What about our massive transport and mixing of biological material worldwide, and our unsustainable consumption of resources?
All of these effects interconnect and add up to the collective "footprint" of humankind on our planet's life support systems.
The consequences extend to the ends of the Earth (recall the hole in the ozone layer over the Antarctic) and each is as difficult to predict and as challenging to deal with as the link between carbon emissions and climate.
It would surely be impractical and almost certainly ineffective to assemble 10,000 delegates to address each one of these issues, and especially to do so in the necessary "joined up" way?
And in particular, what about the net 76 million annual rise in the world's population, which currently stands at about 6.5 billion - more than twice what it was in 1960 - and which is heading towards eight billion or so by mid-century)?
That's an annual increase 7,500 times the number of delegates in Montreal.
Imagine organising the accommodation, feeding arrangements, schooling, employment, medical care, cultural activities and general infrastructure - transport, power, water, communications, waste disposal - for a number of people slightly larger than the population of the UK, and doing it each year, year on year for the foreseeable future.
Combined with ongoing economic growth, what will be the effect on our collective human "footprint"? Will the planet cope?
Steps to Utopia
Although reducing human emissions to the atmosphere is undoubtedly of critical importance, as are any and all measures to reduce the human environmental "footprint", the truth is that the contribution of each individual cannot be reduced to zero.
Only the lack of the individual can bring it down to nothing.
So if we believe that the size of the human "footprint" is a serious problem (and there is much evidence for this) then a rational view would be that along with a raft of measures to reduce the footprint per person, the issue of population management must be addressed.
Let us assume (reasonably) that an optimum human population level exists, which would provide the physical and intellectual capacity to ensure a rich and fulfilling life for all, but would represent a call upon the services of the planet which would be benign and hence sustainable over the long term.
A scientific analysis can tell us what that optimum number is (perhaps 2-3 billion?).
With that number and a timescale as targets, a path to reach "Utopia" from where we are now is, in principle, a straightforward matter of identifying options, choosing the approach and then planning and navigating the route from source to destination.
In practice, of course, it is a bombshell of a topic, with profound and emotive issues of ethics, morality, equity and practicability.
Only the lack of the individual can bring the footprint down to nothing
As found in China, practicability and acceptability can be particularly elusive.
So controversial is the subject that it has become the "Cinderella" of the great sustainability debate - rarely visible in public, or even in private.
In interdisciplinary meetings addressing how the planet functions as an integrated whole, demographers and population specialists are usually notable by their absence.
Rare indeed are the opportunities for religious leaders, philosophers, moralists, policymakers, politicians and indeed the "global public" to debate the trajectory of the world's human population in the context of its stress on the Earth system, and to decide what might be done.
Unless and until this changes, summits such as that in Montreal which address only part of the problem will be limited to at best very modest success, with the welfare and quality of life of future generations the ineluctable casualty.
Professor Chris Rapley is Director of the British Antarctic Survey, based in Cambridge, UK.
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