Australia's Cardinal George Pell might have shared a relaxing stroll with Pope Benedict XVI yesterday but he and the pontiff appear at odds over climate change and church-related sexual abuse.

As thousands of joyful pilgrims welcomed the cross of World Youth Day to Sydney , the Archbishop of Sydney joined Pope Benedict at the Opus Dei retreat at Kenthurst on Sydney's north-western outskirts where the Pope is resting before his first official public appearance on Thursday. The Pope has signalled his intention to formally apologise, on his first Australian visit, to the thousands of victims of church-related sexual abuse.

But Cardinal Pell avoided revealing yesterday whether the Catholic Church in Australia would follow the Pope's lead, saying only, ''We're keen to make a very difficult situation better [but] it is very hard to know how to do it. The bishops in Australia, individually and collectively, have apologised on a number of occasions.''

Cardinal Pell also appeared to differ with the Pope over climate change, describing himself as a ''sceptic about the claim that human activity is likely to produce a man-made catastrophe''.

His comment came a day after the 81-year-old pontiff spoke of the need to find the ethical capacity to change environmental practices and face the challenge of ''our responsibility towards creation''.

Back in Sydney, the mood of the faithful was jubilant as the Church's version of the Olympic torch the World Youth Day cross was cheered across Sydney Harbour and through city streets on the eve of a week-long event drawing up to 225,000 registered pilgrims from 177 countries. Noisy chants of ''Aussie, Aussie, Aussie'' accompanied choruses of Ave Maria as office workers spilled into the streets to watch the passing parade.

But Cardinal Pell who officially opens World Youth Day today with a mass at Darling Harbour tempered the buoyant mood of the pilgrims.

He said Australia faced two challenges. ''One is the Australian temptation to believe that you can have a good, happy life without God,'' he said. ''And the second challenge revolves around the concept of sexuality, marriage and family.''

Cardinal Pell issued a ''populate or perish'' warning on what he said amounted to a Western world crisis. ''No Western country is producing enough babies to keep the population stable,'' he said. ''In many cases there is an increase in divorces, an increase in serial monogamy. Ruthless commercial forces are telling young people that this is the way forward, this is the modern way, and they remain totally silent on the difficulties and damage this does to family life.'' AAP