MADRID — A year ago, a little known 15-year-old Swedish girl in a checked shirt and pigtails stood before world leaders in a snowy Polish coal town and told them they were blowing their chance to aggressively combat climate change, and their inaction would no longer be tolerated.

“Change is coming, whether you like it or not,” she vowed in a blistering three-minute speech that lit a fire under the otherwise sleepy proceedings.

On Wednesday, Greta Thunberg — now 16, now a global celebrity and the most recognizable face of the climate movement — returned to the annual United Nations climate summit, this time on the outskirts of Spain’s capital.

Only this time, she was the main attraction.

Thunberg’s globe-trotting, headline-making, movement-forming journey to push for urgent climate action has transformed her from a solitary protester into an international icon. On Wednesday, not long after she spoke, she was named Time magazine’s Person of the Year, its youngest ever, for becoming “the biggest voice on the biggest issue facing the planet.”

So much has changed since that cold day in Poland when she emerged on the world stage. People flocked to the vast room where she spoke. A swarm of media documented her every move. A sea of young activists filled the conference halls.

And yet, so much has not changed. The world has hardly budged when it comes to taking more collective action on climate change. The trajectory of global emissions is still headed the wrong direction. And Thunberg has not lost her bluntness or indignation.

“The changes required are still nowhere in sight. The politics needed does not exist today, despite what you hear from world leaders,” she said, criticizing the lofty rhetoric and distant goals from countries here, which have yet to result in concrete promises. “I still believe the biggest danger is not inaction. The real danger is when politicians and CEOs make it look like real action is happening, when in fact, almost nothing is being done, apart from clever accounting and creative PR.”

In the late summer of 2018, Thunberg sat alone each Friday outside the Swedish parliament, quietly protesting with a handmade sign that read: Skolstrejk for Klimatet. School strike for climate.

The sign and its owner have had an astonishing journey since those lonely early days.

Over the past year, Thunberg, who has forgone air travel due to carbon emissions, has sailed across the Atlantic Ocean twice, delivering fiery speeches from New York to North Dakota, from Berlin to Brussels. She inspired millions of people across hundreds of countries to march in the streets — again and again and again — demanding that leaders move more quickly and more forcefully to wean the world off fossil fuels.

She endured waves of online vitriol, tangled with President Trump and Russian President Vladimir Putin, testified before Congress, found herself on magazine covers and television shows and became a contender for the Nobel Peace Prize. A 60-foot-tall mural of her face now stares down upon the streets of San Francisco, and her likeness has been plastered around the streets of Washington.

She has amassed millions of followers on social media, published a book of her speeches and delivered the same plea about climate change in countless interviews: “Listen to the scientists.”

Katrin Uba, a political science professor at Sweden’s Uppsala University, who with a colleague has been gathering data on Thunberg’s impact for months, said several factors have played into her rapid rise to fame.

She had good timing, undertaking her strikes at a time when the United States had made clear it would withdraw from the Paris accord and climate impacts were becoming more clear. She also had a simple message — “listen to the science” — that people can grasp. In addition, media coverage and the ability of social media to amplify her message helped spur the Fridays for Future movement around the globe.

The “Greta effect,” Uba said, helped encourage young people worried about climate change, and in particular young women, to speak out for the first time. “A large proportion of these activists had never been active in protests or in the environmental movement before,” she said.

Despite her skyrocketing fame, Thunberg has repeatedly tried to turn the spotlight away from herself and onto other young activists, as well as on climate scientists.

In October, she declined a major environmental award, writing that “the climate movement does not need any more awards. What we need is for our politicians and the people in power start to listen to the current, best available science.”

On Monday in Madrid, Thunberg and another high-profile German activist, Luisa Neubauer, yielded the stage at a climate panel composed of other young people who had received a fraction of the coverage.

“We are privileged, and our stories have been told over and over again,” she told the overflowing crowd, saying she felt a “moral duty” to use her global reach to spotlight others wrestling firsthand with the impacts of climate change, some of whom had worked for years to gain attention. “It is not our stories that need to be told and listened to. It is the others, especially the people from the global south and indigenous communities.”

She then sat silent as young people from the Philippines, the Marshall Islands, Minnesota and Uganda shared their experiences.

On Tuesday, Thunberg and Neubauer headed another panel, this time asking the overflow crowd and the flock of reporters and television cameras to focus on a group of scientists who had come to speak about the grave risks climate change poses.

She spoke of having a “big platform that must be used in a wise way. So that’s why we wanted to create this event — to create a platform for scientists to be able to make their voices heard.”

But on Wednesday morning, the spotlight was on Greta once again.

By the time she stepped to one more podium in front of one more crowd, the question was not what the diminutive teenager might say, but rather what words she might choose to once again excoriate world leaders. She had told them in the past to act as if “the house is on fire,” and in September at the United Nations she had repeatedly said “how dare you” when listing their failures to act more quickly.

On this day, she steered clear of pithy reprimands, instead asking leaders once again to consider the science. She recited figures for how swiftly the world must halt its greenhouse gas emissions to head off worsening of climate impacts that have already devastated parts of the globe.

“This is what I want you to focus on,” Thunberg said. “How do you react to these numbers without feeling at least some level of panic? How do you respond to the fact that basically nothing is being done about this without feeling the slightest bit of anger? And how do you communicate this without sounding alarmist? I would really like to know.”

As she spoke, cameras clicked. Twitter sent her words around the globe at lightning speed. Applause interrupted her speech again and again.

But another question hung over the proceedings: Would the change that she insisted was coming actually materialize? Would the millions of activists she helped rally actually spur leaders to act with more urgency? And when?

The answer is one tied up in economic realities, national and global politics and bureaucratic delays that have left nations far off course from the promises they made in Paris in 2015.

The drumbeat of grim reports on the world’s pace of emissions, coupled with raging wildfires, devastating storms, rising seas and other unfolding catastrophes, suggest there is no time to waste. But as this and other climate gatherings have shown, change often comes at a glacial pace.

Still, there is little doubt that Thunberg has made it more uncomfortable for world leaders to ignore the growing pressure to act.

“A year and a half ago, I didn’t speak to anyone unless I had to,” Thunberg said Wednesday at the beginning of her speech. “But then I found a reason to speak.”

She also was reminded millions of times along the way that she is hardly alone. As if to drive home that point, dozens of young activists took to the stage as Thunberg’s session wound to a close.

“We are unstoppable,” they chanted in unison. “Another world is possible.”