In 2009, world leaders met in Copenhagen but got nowhere.
I was among the last to leave that summit and delegates were in tears. The glass roof of the conference centre was dark with snow.
But since then, a remarkable set of changes has allowed the Paris Agreement to take shape.
Paris climate deal enters force as focus shifts to action
UN review says carbon plans fall well short of climate goals
For a start, the world's two biggest emitters of greenhouse gases, China and US, jointly decided to make a serious push.
For President Obama, this has always been a priority. And for China the new middle class is demanding an end to the toxic pollution smothering the biggest cities.
And that's spurred another shift: renewable energy is falling in price.
Out in the deserts of western China, along the ancient Silk Road, I've seen for myself new forests of wind turbines. Recently, China has planted two of them every hour.
The Chinese push for renewable energy is the largest the world has ever seen. And look at the global figure for solar panels installed last year - it was half a million every day.
A third big change is the involvement of leading companies.
A decade ago, it was rare for me to get a corporate press release making a green boast. Now my inbox is bursting with them.
On Friday, 10 oil companies announced a one-billion-dollar fund for low-carbon technology - something inconceivable not long ago but, let's be clear, the sum is dwarfed by the money spent developing new sources of oil.
So will the Paris Agreement get anywhere? On the one hand, it doesn't oblige any government to cut their greenhouse gases.
But, on the other, each country's efforts will be scrutinised every five years.
And although the agreement has no teeth, it does represent something unique: the first time that every nation on the planet has signed up to try to head off the worst effects of global warming.

The alleged attack happened at the Glenbourne unit, part of the Derriford Hospital site in Plymouth.
Bester Mashayamombe, 50, from Plymouth, who is also accused of sexual assault by touching, is due to appear before Plymouth magistrates on 7 October.
Plymouth Community Healthcare, which runs the unit, said it was "actively contributing" to the investigation.
The unit cares for up to 46 patients with serious mental health disorders.
It has previously been criticised for standards of care but in its latest inspection report in September 2013, the Care Quality Commission found the unit was doing well.

Toulon's Halfpenny, 27, has started at full-back in the autumn internationals against Australia and Argentina, with Wales to host Japan on Saturday.
Williams says Scarlets' Liam Williams should move over to full-back instead.
"I used to enjoy playing on the opposite wing to Leigh and he used to score a lot of tries and create a lot of opportunities," Williams said.
"For that reason I'd love to see Leigh back on the wing.
Media playback is not supported on this device
"Liam Williams is playing with such confidence at the moment you could put him in any position and he would still perform."
Liam Williams missed Wales' defeat to Australia through injury but started on the wing and scored his side's first try in the win over Argentina last Saturday.
Wales host Japan in their third game of the autumn series at Cardiff's Principality Stadium, with interim coach Rob Howley considering resting players.
Howley has hinted Ospreys wing Keelan Giles could win his first senior cap against Japan and Shane Williams believes the 18-year-old should be given his chance.
"I think a game against a team like Japan would suit Keelan," said Williams, who has been inducted into World Rugby's Hall of Fame.
"I think it will be an open game and probably more chances and room for him to create than there would have been against Australia and Argentina.
"Perhaps it's time Wales take the shackles off him and let us see what he can do.
"He's a great players that's only going to get better."
Only seven years ago, the very idea that an international climate agreement would come into force today looked hopeless.
[NEXT_CONCEPT]
A care worker at an NHS psychiatric unit has been charged with raping a young female patient.
[NEXT_CONCEPT]
Wales' record try-scorer Shane Williams wants to see Leigh Halfpenny starting back on the wing.