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This summer, the cardinals at the Vatican accepted an unusual donation from a Hungarian startup called KlimaFa: The company said it would plant trees to restore an ancient forest on a denuded stretch of land by the Tisza River to offset the Vatican’s carbon emissions.

The young trees, on a 37-acre tract that will be renamed the Vatican climate forest, will in theory absorb as much carbon dioxide as the Vatican will produce through its various activities in 2007: driving cars, heating offices, lighting St. Peter’s Basilica at night.

In so doing, the Vatican announced, it would become the world’s first carbon-neutral state.

“As the Holy Father, Pope Benedict XVI, recently stated, the international community needs to respect and encourage a ‘green culture,'” said Cardinal Paul Poupard, leader of the Pontifical Council for Culture, who took part in a ceremony marking the event at the Vatican. “The Book of Genesis tells us of a beginning in which God placed man as guardian over the Earth to make it fruitful.”

In many respects, the program seems like a win-win-win proposition. The Vatican, which has recently made an effort to go green by installing solar panels, sought to set an example by offsetting its carbon emissions.

Hungary, whose government scientists are consulting on the project, will take over large swaths of environmentally degraded, abandoned land that have been restored as forests. That will have a beneficial effect on the climate here and provide jobs in an economically depressed area.

KlimaFa, an 18-month-old company based in Budapest, gets the Vatican’s seal of approval and free publicity for its first project. In addition to the Vatican, several European governments as well as Dell, the computer maker, have bought carbon offsets that will be backed by planting trees on the 555-acre island.

But creating and selling carbon “offsets” or “credits” is still a novel concept for both business and science, and much debate remains. The calculation for planting trees is especially complicated.

Planting forests is only “a partial solution, and a temporary one,” said Laszlo Galhidy, a forestry officer for the environmental group WWF Hungary. Young forests — dominated by growing trees — soak up a lot of carbon dioxide, but once the forests mature, they absorb far less, he said.