One thing I've learned in the many global environmental summits I've attended over the years, is that these gatherings tend to mark progress rather than make it.
The collective will of leaders is not determined at the meetings themselves, but by events in between. This basic fact is highly pertinent to the United Nations climate change talks taking place in Paris later this year. Unlike the political disaster that unfolded in Copenhagen six years before - where talks broke down in diplomatic acrimony driven by mistrust and failed communications - careful groundwork and real-world progress mean that Paris is set to put the world onto a bright new path.
Critical to the progress made during the last year has been the ever-clearer accord between the world's two biggest greenhouse gas emitters - China and the United States. On top of this, by signing off last year's Intergovernmental Panel on Climate Change report, all nations have agreed that emissions from fossil fuels need to taper over a period of decades to zero, marking another sea change in political conditions. Rapid growth in renewable energy deployment is beginning to show in a measureable decoupling between development and emissions, demonstrating how "green growth" works in practice.
Another key factor for Paris is that the deal likely to emerge will be based on national commitments to cut greenhouse gas emissions rather than the near impossible job of everyone trying to negotiate everyone else's targets. And the Sustainable Development Goals to be adopted this month will create a new context for joined-up global action, linking ecological goals with economic ones in more meaningful ways than seen before.
However, the sum of national commitments on display before and during Paris will almost certainly not add up to keeping the world below two degrees of warming, the globally agreed upper limit in restricting temperature increase.
While unfortunate, we need to keep this in perspective. No single meeting or event can 'solve' climate change; Paris is part of a long process. And there is a major danger here. Should the Paris summit be judged a failure because the sum of the commitments made does not reach two degrees, this will strengthen the narrative of climate change deniers and detractors who are already primed, gleefully, to proclaim failure. They'll use that storyline to block national implementation, foster mistrust and to prop-up the high-carbon status quo.
They'll also try to exploit divisions between 'rich' and 'poor' nations. Many of us grew up with the familiar division of countries within the UN process into these two simple blocs. But the world is very different now, and the alliance of constructive states encompasses countries in every stage of development. So while it's right to press for justice in the Paris outcome, we should recognise that the formal negotiating blocs no longer mirror an increasingly complex and nuanced reality.
With all this in mind, it's vital that people looking for a positive outcome at Paris and beyond adopt the right metric for judging 'success' or 'failure'. The fallout from Copenhagen was so bad as to threaten the very future of climate change negotiations; the fact that we are in this far more optimistic state just six years later is already a victory of sorts. A broadly good agreement in Paris will increase confidence among governments and businesses for taking the more rapid steps we need in the coming years. That in turn will address the very real concerns of some countries about potential negative impacts of low-carbon plans on development. Through this cycle, combined with a "ratchet up" mechanism, to increase the rate of emissions reductions, in the Paris agreement, we can still achieve the two-degree trajectory that we need.
However the Paris meeting concludes, it will not be the end of the story. To quote Winston Churchill after the Allied victory at El Alamein: 'Now this is not the end; it's not even the beginning of the end. But it is, perhaps, the end of the beginning'. With the original climate change treaty agreed in 1992, it has for many of us been a very long beginning, but now, finally, we might be getting somewhere.
Even without the UN process, technology, pubic opinion, patterns of investment, commercial planning and political consensus are turning more and more toward meaningful action. A good Paris outcome will accelerate the transition already in train; and that is ultimately how Paris should and will be judged.
There is little doubt that opponents of the low-carbon transition know they are on the wrong side of history. But they have fought long and hard to delay the progress we need, and will continue doing so in the road through Paris. It is incumbent on all of us in the environment movement to make sure that we do not inadvertently hand them any ammunition.
As Churchill also remarked: "Before Alamein we never had a victory. After Alamein, we never had a defeat". Paris could be our pivot point, and despite not being a total win, history might see it as the moment the tables turned.
Follow Tony Juniper on Twitter: www.twitter.com/tonyjuniper