The biggest emitters of greenhouse gases, China and the United States, are finally on board.
There is real money on the table to help poor countries cut their emissions and cope with warming.
They have even adopted a target of holding the warming to only +1.5degC, instead of the limit of +2degC, the goal when the conference opened.
So the several thousand delegates who spent two weeks dickering over the details of the deal in a draughty exhibition hall north of Paris felt fully justified in cheering and congratulating one another on a job well done.
Given all that, it's a pity the deal won't actually stop the warming.
The +2degC limit was always too high.
It began as a scientific estimate of when natural feedbacks, triggered by the warming human beings had caused, take over and started driving the temperature much, much higher.
It was actually quite a fuzzy number: at somewhere between +1.75degC and +2.25degC, the feedbacks will kick in and it will be Game Over.
So +2degC, for political purposes, became the limit.
Beyond that, governments told us, we would have "dangerous warming''.
Nonsense.
We are having dangerous warming now - bigger storms, worse floods, longer droughts - and we are only at +1degC.
At +2degC, or thereabouts, what we get is catastrophe: runaway warming that can no longer be halted just by stopping human emissions of carbon dioxide.
Nature will take over, and we will be trapped on a one-way escalator taking us up to +3, +4, +5, even +6degC.
Hundreds of millions - or even billions - of people would die as large parts of the planet ceased to be habitable by human beings.
If you don't want to risk unleashing that, then you don't want to go anywhere near +2degC, so the official adoption by the world's governments of +1.5degC as the never-exceed limit is a major step forward.
But note that they have only pledged "to pursue efforts to limit the temperature increase to 1.5C'', not to succeed.
The hard-and-fast promise is still not to go past +2degC - and there is not even any guarantee that will be achieved.
In order to avoid a collapse like the one at the last climate summit in Copenhagen six years ago, nobody even tried to put enforceable limits on national carbon dioxide emissions this time.
Each country was just invited to submit the emission cuts it was willing to make.
The sum of all those promised cuts (if the promises are kept) is what we will get by way of global emission cuts in the next five years.
United Nations experts did the math and concluded these emission cuts fall far short of what is needed.
If this is all that is done, then we are headed for at least +2.7degC - or rather, for a lot more, because of the feedbacks.
None of the negotiations at the Paris conference changed those numbers, or even tried to.
So are we doomed to runaway warming?
Not necessarily.
Most of the negotiators know the cuts which are politically impossible now may become quite possible in five or 10 years if the cost of renewable energy goes on dropping, if techniques like carbon capture and sequestration (CCS) become economically viable - and if people are sufficiently frightened by a climate that is getting wilder and less predictable by the year.
So there is a review process built into the treaty.
Every five years, starting in 2018, there will be a "stock-taking'' exercise in which everybody's progress in cutting their emissions will be reviewed, and everybody will be encouraged to increase their commitments and speed up their cuts.
Whether they will actually do that depends on political, economic and technological factors that cannot yet be calculated, but fear is a great incentive, and there is no government on the planet that is not frightened by the prospect of major climate change.
In fact, most of them would have gone a lot further in Paris if they were not nervous about getting too far ahead of public opinion at home.
Public opinion will eventually change, because there is going to be a very large amount of damage and suffering in the world as we move past +1degC and head up towards +1.5degC.
Will it change fast enough to allow governments to act decisively and in time?
Nobody knows.
Will new green technologies simply sweep the field, making fossil fuels uneconomic and government intervention unnecessary?
Nobody knows that either, although many people pin their hopes on it.
We are not out of the woods yet, but we are probably heading in the right direction - and it would be right at this point to put in a good word for that much maligned organisation, the United Nations.
It is the only arena in which global negotiations like this can be conducted, and its skills, traditions and people were indispensable in leading them to a more or less successful conclusion.
● Gwynne Dyer is an independent London journalist.
