Greta Thunberg arrives in Lisbon on her way to U.N. climate talks

Following a three-week voyage across the Atlantic, climate activist Greta Thunberg has arrived in Lisbon via catamaran.

Portugal is just a pitstop though: She's on her way to Madrid for COP25, the U.N. Climate Change Conference.

The conference, which was originally scheduled to take place in Chile, had to change locations on short notice due to civil unrest in Chile's capital, Santiago.

This posed some problems for Thunberg, who famously eschews air travel due to its high CO2 emissions. When the change of venue was announced, Thunberg was still in the U.S., where she spent almost three months participating in climate strikes and meeting with local leaders.

Luckily, she was able to hitch a ride with an Australian family as they sailed across the Atlantic on a 48-foot catamaran.

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Now, Thunberg will rest and meet with Portuguese climate activists before finally making her way to Madrid, according to Reuters.

Thunberg's travels are often met with fanfare, and her arrival in Lisbon was no different. Reuters reports that more than 150 supporters greeted Thunberg when she arrived.

Meanwhile, COP25, which convenes the body that oversees the implementation of the Paris Agreement, is underway.

During opening remarks on Sunday, United Nations Secretary General Antonio Guterres urged leaders to avoid sleepwalking "past the point of no return." Currently, the U.S. is the only country to leave the Paris Agreement. While the Trump administration's decision to leave will become official in November 2020, House Speaker Nancy Pelosi made an appearance at the climate summit to affirm an American commitment to reducing greenhouse gas emissions, saying that "Congress's commitment to action on the climate crisis is iron-clad."

"By coming here we want to say to everyone we are still in, the United States is still in," Pelosi added.

The summit in Madrid marks the last time the COP group will meet before 2020, the year that nations signed onto the Paris Agreement agreed that global greenhouse gas emissions must be capped, and then finally decrease.

Avoiding the "point of no return" would mean curbing emission 7.6% every year of the next decade, according to CNN.

Priorities at the conference include the establishment of a shared time frame for countries in implementing their climate commitment plans, and coming to an agreement about international carbon markets.

Guterres told journalists at a COP25 press conference that his message is one of "hope not despair."

"The signals of hope are multiplying. Public opinion is waking up everywhere. Young people are showing remarkable leadership and mobilization," Guterres said.

What's missing, according to Guterres, is the political will to put a price on carbon, stop fossil fuel subsidies, and tax pollution instead of people.