There's solar, wind, fuel cells, nuclear, and more. Many of these seem like viable alternatives to fossil fuels and were part of the sweeping proposals unveiled on January 11 by the European Commission.
But some experts have a simpler, more direct way to combat global climate change: planting trees.
"We humans are only part of this ecosystem," 2004 Nobel Peace Prize laureate Wangari Maathi said. "And when we kill part of this system, we are killing ourselves.
"To plant a tree for future generations is a simple gesture, yet a strong symbol of sustainable development," Prince Albert II of Monaco said in a statement on the UN website. "The 2007 aim of Plant For The Planet -- The Billion Tree Campaign is to create an unprecedented mobilization in favor of the environment."
Prince Albert II is the patron of an initiative begun this year by the United Nations Environment Programme (UNEP) called The Billion Tree Campaign. It's goal is to at least 1 billion trees in 2007 in a bid to help stem climate change.
Deforestation accounts for more greenhouse-gas emissions per year than does the world's transport sector. That's because trees act as "sinks" for the environment -- they naturally absorb carbon dioxide, a by-product of the burning of fossil fuels and the main greenhouse gas blamed for global warming. And that carbon dioxide is released when forests are cleared by burning.
Inspiration From Kenya
The initiative was inspired by 2004 Nobel Peace Prize laureate Wangari Maathi, the founder of Kenya's Green Belt Movement, which has planted more than 30 million trees in 12 African countries since 1977.
A forest fire near the central Russian city of Kostroma in May 2006 (TASS)
When she first began her project, Maathi wanted to address the consequences of deforestation on Kenya's rural population. People no longer had firewood, and topsoil and water resources were being depleted by from deforestation.
So Maathi, as she tells UNEP's website, started a campaign to plant trees -- an effort that eventually restored workable land to thousands of rural people around Africa.
"We humans are only part of this ecosystem," Maathi said. "And when we kill part of this system, we are killing ourselves. That's what we have to understand: that this ecosystem must remain as it is, it must be sustained. Because in its survival, depends our own."
Just A Beginning
UNEP admits its goals remain modest compared to the world's needs. It says millions of hectares and billions of trees have to be planted to stabilize soil and water resources and to meet fuel-wood needs.
An area roughly twice the size of Ukraine, or 130 million hectares, would have to be planted just to make up for the loss of trees over the last decade. UNEP says that would take 14 billion trees a year for 10 years. That is, each person on Earth would have to plant two seedlings a year and care for them.
So far, the Billion Tree Campaign says more than 75,000 trees have been planted this year, but that it has received pledges for another 150 million to be planted.
Hope In Armenia
That's only a dent in the goal of 1 billion trees, but it's a beginning that is certain to grow if more organizations like the Armenia Tree Project step forward.
That nongovernmental organization has made perhaps the most significant pledge to date -- a promise to plant 500,000 trees this year in Armenia.
The UNEP website offers plenty of practical advice to would-be planters, including what types of trees to grow and a step-by-step guide to planting.