The Nairobi-based agency said the tree planting campaign, inspired by Kenyan Nobel Peace laureate Wangari Maathai, will help mitigate the effects of pollution and environmental deterioration.

The campaign launched in 2006 saw two billion trees planted, double the original target, with Ethiopia leading the count at 700 million, Turkey at 400 million, Mexico at 250 million and Kenya at 100 million trees.

The campaign set a new target of seven billion by late 2009, when governments gather in Copenhagen for a crucial climate change conference.

"The goal of planting seven billion trees -- equivalent to just over a tree per person alive on the planet -- must therefore also be do-able given the campaign's extraordinary track record and the self-evident worldwide support," UNEP chief Achim Steiner said in a statement.

"It is a defining issue of our era that can only be tackled through individual and collective action. I am convinced that the new target will be met -- one tree at a time," he added.

Heads of state participated in the campaign, as did corporations, cities, faiths and communities, but individuals accounted for over half of all participants, UNEP said.

Experts say that trees are the most cost-effective way of containing carbon that accumulates the heat-trapping gases blamed for climate change.

"Trees and forests play a vital role in regulating the climate since they absorb carbon dioxide," UNEP said.

"Deforestation, in turn, accounts for over 20 percent of the carbon dioxide humans generate, rivaling the emissions from other sources."

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