"No western country is producing enough babies to keep the population stable, no Western country," Cardinal George Pell said yesterday. "Ruthless commercial forces are telling young people that this is the way forward, this is the modern way and they remain totally silent of the difficulties and damage this does to marriage and family life."

Cardinal Pell said the church had a clear "moral obligation not to damage and destroy or ruthlessly use the environment at the expense of future generations". But he disputed pessimistic scientific opinion that human activity had fuelled catastrophic climate change.

Graeme Pearman, a Monash University expert in climate change and sustainability, , said it was ironic to argue for population stability or even growth at the same time as nations were seeking ways to limit carbon emissions and protect the environment.

"The more people we have, the more land that is needed; the more land that is needed to be cleared, the more water you need to capture from rivers," Dr Pearman said.

"And these things are in direct competition with the aims of increasing living standards and a healthy environment."

During his flight to Sydney for World Youth Day, Pope Benedict XVI was asked about climate change and spoke of "our responsibility towards creation".

"We need to reawaken our conscience … I want to give impulse to rediscovering our responsibilities and to finding an ethical way to change our way of life and ways to respond to these great challenges," he said.

"We need to rediscover our Earth in the face of our God and creator and to re-find our responsibilities in front of our maker and the creatures of the Earth he has placed in our hands in trust."

Cardinal Pell said his position was not incompatible with the Pope's because the pontiff had said he would not get involved in the debate on the science or politics of global warming.

The Archbishop's comments came on the eve of the official start to World Youth Day celebrations today at an opening Mass to be attended by 140,000 pilgrims in East Darling Harbour. Overnight, the number of special-event clearways were scheduled to triple, and CBD arteries will close to traffic from 10am.

The closures include all of George Street, which will become a pedestrian walkway for pilgrims for much of the day.

In a highly controversial Easter message two years ago, Cardinal Pell declared that Jesus had nothing to say about global warning, earning a rebuke from the leading Australian Anglican bishop and environment advocate George Browning.

Yesterday Cardinal Pell said: "I think the approach of all of us would be like Francis of Assisi: we admire God's handiwork in nature and through the miracles of modern technology we make it better.

"The Holy Father has said he is not going to enter the scientific debate on particularities especially cause and effect.

"It's even more difficult when you're trying to predict what will happen in five or 10 or 15 or 20 or 100 years. I myself am a little bit of of a sceptic. That means I'm open to the evidence about the claim that human activity is likely to produce a man-made catastrophe.

"I'm well aware over hundreds of years that there have been great changes in the climate, and whether we are going through one of those changes or whether we are contributing to that, I don't know. Whether we are heading slowly towards an ice age or whether we are heading towards significant warming, I don't know but I'm pretty certain if you look at the figures the temperatures have dropped worldwide in the past 12 months."

Dr Pearman said the Pope's environmental message reflected growing recognition among faiths of the need for stewardship "on behalf of people of the Earth for their god or gods". Cardinal Pell's scepticism was not unusual in the wider community but lacked an understanding of the level of consensus among scientists and should not be an excuse for inaction, Dr Pearman said.

"The vast majority of scientists are 90 per cent confident that what we are seeing is caused by greenhouse gases caused by human activities," he said. "Therefore, if we wait for 100 per cent certainty we run the risk of waiting too late and I would strongly argue that it is too late to avoid some of the significant impacts."

The Archbishop of Adelaide and president of the Australian Bishops' Conference, Philip Wilson, said the churches of his city were all too aware of the realities of a severe water shortage in the Murray-Darling basin. Pilgrims had helped plant 20,000 trees on the outskirts of Adelaide.

"We're not looking to enter into the debate about the theories behind this," he said. "What we're saying is that the consequences of the present have very important effects on the lives of people and that decisions and actions have to be taken so we learn to live in this land."

Cardinal Pell declined to comment further yesterday on the case of Anthony Jones, a former teacher who was sexually molested by a priest. Cardinal Pell has referred the case to an independent church panel in light of the Pope's planned apology to abuse victims. He said he and his fellow bishops had already made a personal apology to the victims of sexual abuse but conceded more might be done to help.

"We are open to improving the situation," he said. "We're keen to make a very difficult situation better. It's very hard to know how to do it."