Pope Francis on Thursday urged young people to combine fighting to save the planet with tackling poverty during the second day of his visit to Portugal.
The 86-year-old Francis has made the protection of the environment a cornerstone of his pontificate.
He returned to the theme during an open-air address to students at Lisbon's Catholic University on the second day of his visit to Portugal.
"We must recognise the dramatic and urgent need to care for our common home," he said, speaking in his native Spanish. "Yet this cannot be done without a real change of heart.
"We cannot be satisfied with mere palliative measures or timid and ambiguous compromises," he added.
Around 6,500 people, including Portuguese President Marcelo Rebelo de Sousa and bishops, were gathered under a bright sun in a main square of the campus of the institution, one of Portugal's top universities, to listen to the pope.
The crowd cheered and applauded when the leader of the world's 1.3 billion Roman Catholics pontiff took to the stage.
"Keep in mind that we need an integral ecology, attentive to the sufferings both of the planet and the poor," he told them.
"We need to align the tragedy of desertification with that of refugees, the issue of increased migration with that of a declining birth rate.
"Instead of polarised approaches, we need a unified vision, a vision capable of embracing the whole."
- Paints mural -
After his speech, the pope headed to Cascais, a seaside town some 30 kilometres west of Lisbon, to visit the local branch of his Scholas Occurrentes foundation, a movement he founded in 2013 to bring young people from different backgrounds and nationalities together.
In keeping with his unpretentious style, the pontiff was driven to the foundation in a white Toyota car. Well-wishers lined the route, cheering, waving and taking pictures with their mobile phones.
At the foundation he answered questions from youths of different nationalities before putting the final brushstroke on a mural that the community has been working on.
"This is your Sistine Chapel," the pope said, sparking laughter from the assembled youths.
Before leaving the pope watered an olive tree, the symbol of peace, in the patio of the foundation.
Francis began his day meeting 15 youth from Ukraine at the Holy See's diplomatic mission in Lisbon where he is staying, the Vatican said in a statement.
The meeting had not been listed on the official programme of the pope's visit.
Pope Francis arrived in Lisbon on Wednesday for World Youth Day festivities, which is in fact a six-day international Catholic jamboree. It is expected to draw one million people.
Francis, who met with clergy and victim of clerical sexual abuse on Wednesday, will deliver a Mass on Sunday in Lisbon on the last day of his five-day visit to Portugal, when temperatures are forecast to soar to 37 degrees Celsius.
World Youth Day, created in 1986 by John Paul II, is the largest Catholic gathering in the world and will feature a wide range of events, including concerts and prayer sessions.
This edition, initially scheduled for August 2022 but postponed because of the pandemic, will be the fourth for Francis after Rio de Janeiro in 2013, Krakow in 2016 and Panama in 2019.
By Daniel Silva And Clement Melki