Even in the December cold, you could sense their red-hot fury as you walked by their placards each morning.
But I felt that, for all of the news coverage of their desperate marches through the streets of the French capital, the urgency of the crisis still hadn't hit home.
The next year, as the amount of carbon dioxide in the planet's atmosphere passed the grim milestone of 400 parts per million, it became clear the world needed a hero.
Now we have her.
A courageous teen named Greta Thunberg who means it when she says she'd rather be in school than having to call out lawmakers for categorically failing her generation and each one after it.
A 16-year-old luminary who took all of the complexities of climate science, and all of the anxiety of her peers, and this week packed it into an incendiary, 479-word indictment of inaction and apathy.
If you go and Google the scenarios that will play out under something called RCP 8.5 – that's the representative concentration pathway used by the UN's Intergovernmental Panel on Climate Change, assuming on-going emissions rise this century – then you come some way to understanding Thunberg's sadness and rage.
Scientists tell us that the best case scenario, or RCP 2.6, is that the world succeeds in ramping down man-made emissions of carbon dioxide, within this century, to net zero.
Emissions might even have to be forced down to negative toward the end of the century, something that would likely require CO2 to be actively sucked back out of the atmosphere.
Mean global warming would likely flatten off a little under 2C above pre-industrial temperatures – the target number of the Paris Agreement.
But even under this optimistic, and perhaps unlikely, trajectory, our world would still be transformed, with a big change in extreme events like floods and droughts.
Across the globe, rainfall levels could rise or drop by 10 per cent, and the occurrence of heatwaves and the risk of forest fires would soar.
Warming could reach up to 4C near the Arctic, while global sea level rise could climb anywhere between 0.28m and 0.61m by the close of the century, with more to come.
Meanwhile, experts like Victoria University's Professor James Renwick tell us that we're still on that business-as-usual RCP8.5 trajectory, headed toward the scarier end of the scale.
So what does that mean?
Think several metres of sea level rise by the end of the century. Think billions of dollars of homes and buildings being swamped by storm surge and risen tides.
Think extreme weather and extreme fire danger.
Think widespread, damaging drought and unbearable heatwaves that have become a summer norm.
If global temperatures climb just one, two or three degrees above current levels, researchers tell us that annual heat-related death rates in Auckland and Christchurch rise to 28, 51 and 88 respectively, by the end of the century.
Think a new wave of foreign pests and disease. Think mass migration, food insecurity and global conflict over resources.
Think ecological catastrophe.
Think a global climate unlike anything the world has seen for several hundred thousand years.
"We will not let you get away with this," Thunberg furiously told the UN Climate Action summit in New York this week.
"Right here, right now is where we draw the line. The world is waking up."
That she's now drawing enough attention to have become a target of some commentators who, even in 2019, argue that the threat of climate change is over-hyped, proves her point.
Even the world's most powerful climate change denier, US President Donald Trump, childishly had a crack at her on Twitter.
Will the newfound awareness she's helped raise translate to nations cutting enough emissions to avoid that RCP 8.5 future?
For Greta, and the rest of our children, we can only hope.
• Jamie Morton is the Herald's science reporter.
