Teenage climate change activist Greta Thunberg warned critics not to "underestimate the force of angry kids" as she concluded a three-week yacht journey across the Atlantic.Greta, 16,set sail from the US on 13 November after Chile announced it would no longer be holding COP25 climate conference, with Madrid stepping up as a replacement - leaving her on the wrong continent.Arriving in the Portuguese capital Lisbon on Tuesday, Greta told her cheering supporters her journey had "energised" her in the fight against climate change.Image:'It feels good to be back,' said Greta as she arrived in LisbonImage:Greta's journey lasted three weeks "We've all been on quite an adventure it feels good to be back," she saidTalking about her critics, she added: "I think people are underestimating the force of angry kids."If they want us to stop being angry, then maybe they should stop making us angry."She has said she will now spend a few days in Lisbon, before heading to Madrid, where hundreds of people from around the world are gathering to discuss how to take on global climate change."We will continue the fight there to make sure that within those walls the voices of the people are being heard," she said.Explained: The rise of Greta ThunbergThe yacht Greta travelled on, La Vagabonde, also carried her father Svante, professional sailor Nikki Henderson and an Australian family.The boat has a carbon footprint of almost zero when its sails are up, and uses solar and hydro-electricity.Greta has faced criticism from some who say that despite being environmentally friendly, her modes of transport are often unsustainable for modern life.Responding, she said: "I am not travelling like this because I want everyone to do so.:: A New Climate is a series of special podcasts from the Sky News Daily.

Listen on Apple Podcasts, Google Podcasts, Spotify, Spreaker"I'm doing this to sort of send the message that it is impossible to live sustainable today, and that needs to change.

It needs to become much easier."On the opening day of climate conference, UN Secretary-General Antonio Guterres warned that if the world's greenhouse gas emissions are not reduced by 7.6% each year the impact on the planet will be "catastrophic", and called for the world's biggest polluters to take responsibility.