The Catholic Church has declared Sunday "Save Creation Day" as Pope Benedict joined about 300,000 young Roman Catholics for an eco-friendly festival.
Nearly everything at the festival is biodegradable or recyclable
The youth festival is being held at the shrine of Loreto, in Italy.
In his main homily the Pope urged young Catholics to take better care of the planet and called for "courageous decisions" to safeguard creation.
The weekend gathering is a run-up to the 2008 Catholic World Youth Day in Sydney, Australia which he will attend.
Wearing green vestments, the pontiff told the crowd it was up to them to save the planet from development which often upset "nature's delicate equilibrium".
"Before it's too late, we need to make courageous choices that will recreate a strong alliance between man and Earth," he said.
"We need a decisive 'yes' to care for creation and a strong commitment to reverse those trends that risk making the situation of decay irreversible."
He focused on water, which he described as a "precious" resource and a potential source of tension and conflict.
On Saturday, the Pope told the crowds to have faith that they can succeed in marriage and decried what he called a "failure of love" and the decline of family values.
But the BBC's David Willey in Rome says the emphasis of the festival as a whole is on the environment, with each participant receiving a free knapsack made out of recycled plastic.
It contains a hand-cranked battery charger, plates and cutlery made from bio-degradable plastic and bags for tying up their rubbish when they leave.
The Pope is also practising what he preaches, our correspondent adds.
He is installing solar panels on the roof of the main audience hall at the Vatican and is paying for a forestry project to offset the Vatican's carbon emissions.