Professor Maathai, who won the prize in 2004 for her work on reforestation in Kenya, pledged to plant 2m trees through her Green Belt Movement. She was speaking at the annual UN climate change convention meeting, which is taking place in Nairobi and was described by one delegate as "climate foreplay" because few binding decisions are expected.

"We know the science, we know the data [behind global warming]," said Prof Maathai. "But what is really important is what we do. Planting a tree is something that anybody can do."

The Billion Tree Campaign is being backed by the United Nations Environment Programme, which is asking people to record their contributions online (Unep.org/billiontreecampaign). The campaign is largely symbolic, however, because the problem of deforestation is so acute. Over the past decade 130m hectares (3,235m acres) of trees have been destroyed, according to the UN. Reforesting such an area would require 140bn trees to be planted.

Carbon dioxide released during the burning of forests and the clearing of land accounts for nearly a fifth of all the carbon emissions that contribute to global warming, a bigger share than the transport sector.

Guardian Unlimited © Guardian News and Media Limited 2006

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