Last year, Pope Benedict XVI raised eyebrows when the Vatican announced it was installing 1,000 solar panels on the roof of a football field-size building that is the main auditorium in Vatican City.

Then the pope led an eco-friendly Catholic youth rally in Loreto, Italy, where the faithful received backpacks made from recyclable material and crank-powered flashlights.

Last month, the Vatican added polluting the Earth to the church's list of sins, and the pope has issued a string of increasingly strong statements on global climate change. "I think the pope recognizes that for this and the next generation, it may very well be that global warming is the most important international moral issue that faces humankind," said the Rev. Thomas Reese, former editor of the Jesuit magazine, "America."

The pope and other church officials have said that good stewardship of the earth, as they see it, has theological underpinnings, and they often cite Genesis 2:15: "The Lord God took the man and settled him in the Garden of Eden to cultivate and take care of it."

Pope Benedict XVI is not the first pope to talk about the environment — his predecessor, John Paul II, was an outdoorsman who also expressed alarm about global warming.

Nor is he the only religious or secular leader to focus on the issue. But experts say Pope Benedict XVI is taking on the issue from a pulpit no one in the world can match — leader of the 1.1 billion member Catholic Church — and with a seriousness that is outdoing even John Paul II.

"His vocal support particularly for climate solutions could really tip the balance in world action," said Melanie Griffin, national director of environmental partnerships for the Sierra Club. "He's really not mincing words. He's walking the walk."

Sierra Club and other environmental organizations have struck what not long ago would have been considered an unlikely alliance with the Catholic Church.

Pope Benedict XVI appeared to get on the "green" bandwagon from the start of his papacy in April 2005. In his first homily, he declared that "the Earth's treasures no longer serve to build God's garden for all to live in, but they have been made to serve the powers of exploitation and destruction."

The pope presents climate change as a moral issue, warning that environmental neglect especially hurts the poor and vulnerable. Besides Genesis, Benedict and others in the church pushing for environmentalism have pointed to St. Francis of Assisi, who lived a simple life respectful of the planet.

"The Catholic Church and Benedict have never been called trendy, but their concern for the environment is an extension of what we believe about creation and what we believe about the creator," said John Carr, executive director of the U.S. Conference of Catholic Bishops office of justice, peace and human development.

The church and environmental movements were not allies previously, mainly because environmentalists often cast the issue as a matter of population control — something that went against Catholic Church teachings on birth control and abortion.

But they have found common ground in protecting the earth. "The Catholic Church is not the Sierra Club at prayer, but we do share a commitment to the earth that is based on a commitment to creation," Carr said.