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A worker cuts an acacia tree during an area clearing near Bukit Tiga Puluh natural forest in Riau, Indonesia, in April 2008. (Achmad Ibrahim/Associated Press)

In many ways, obviously, the world is a radically different place than it was at the dawn of human agriculture. But perhaps the most visible change over the last 10,000 years has been in the planet's green roof — its forest cover.

There is between one-third and one-half less forest on Earth today. And in recent times, what remains has been disappearing at an alarming rate.

Each year, 13 million hectares of forest is lost, according to 2005 figures. That's an area roughly the size of Nova Scotia and New Brunswick combined.

When forest regeneration is taken to account, the net annual loss is still 7.3 million hectares, the UN Food and Agriculture Organization reports. (Some environmental groups, though, dispute this number as being too low.)

The good news is that the global rate is slowing. The net annual forest loss was higher during the '90s at 8.9 million hectares, according to the FAO.

And in Canada, which has 10 per cent of the world's forests, the rate is stable — no significant gains or losses. Many countries like China, for one, have even begun a trend of "afforestation," which means they plant more trees than they clear.

That's good news for the lungs. A single tree can take between 50 to 100 kilograms of small particles, like carbon dioxide, out of the air in a given year and produce three-quarters of a human's oxygen needs.

Why the trees are cleared

The clearing of forests for agriculture and industry has a long history around the world. Take the case of the eastern United States, which lost almost half of its original forest cover by the end of the 19th century as the land was settled.

Today, the loss of tropical rainforest generates the most attention, and with good reason. Home to half of the world's animals and more than 100,000 plant species, the rainforest's biodiversity is invaluable.

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A barge moves acacia trees near Bukit Tiga Puluh natural forest in Riau, Indonesia, in April 2008. (Achmad Ibrahim/Associated Press)

Brazil's Amazon forest has been a focal point of concern for many years. Since the early 1970s, an estimated 20 per cent of the rainforest has been cleared, much of it for cattle-grazing pasture as Brazilian beef exports skyrocketed.

The rainforest is also being cleared to grow crops. As Brazil has become the world's leading soybean exporter, for example, rapid growth in crop production has led to illegal land-grabs and clear-cutting of forest.

Agriculture plays a somewhat smaller role in the Congo Basin. But the world's second-largest rainforest is also threatened by oil production and logging, which accounts for up to 13 per cent of the economy in the Central African Republic, one of six countries spanned by the forest area.

According to United Nations estimates, 66 per cent of the Congo Basin rainforest will be destroyed by 2040 without immediate action.

A massive illegal logging trade has caused Indonesia to experience some of the world's most radical deforestation in recent decades. Almost 40 per cent of the country's forests were cleared in the last half of the 20th century, and almost two million hectares (two per cent of the total) disappeared each year between 2000 and 2005.

Palm oil production also plays a devastating role. Environmental groups warn that, although many palm trees are grown on land already designated for agriculture, a surge in global demand has led Indonesia to drain wetlands and burn through prime rainforest and peat. The oil is used in foods, cosmetics and as a critical bio-fuel ingredient.

War-torn Afghanistan faces a particularly acute forest loss. About 70 per cent of the country's forests have been cleared during two decades of war, internal strife, drought and environmental mismanagement, according to an Afghan official.

Haiti, Sudan and Zimbabwe have also experienced significant deforestation in recent years.

Less forest, more greenhouse gases

When forests are cleared in a non-sustainable way, the environment receives a nasty double-whammy.

The cleared forests can't take any carbon dioxide out of the air, which means less of a filter for greenhouse gas emissions.

But the act of deforestation actually is a major cause of those emissions as well. Moist, dense rainforest soil contains even more carbon than the tree branches and leaves, and it's all released into the atmosphere when the forest is cut and burned.

The clearing of forests has helped Indonesia become the third-largest producer of greenhouse gases on the planet. Brazil is close behind.

And deforestation contributes roughly 25 per cent of the world's greenhouse gas emissions, according to various estimates.

That's more than 12 times all of Canada's emissions or, as one British newspaper theorizes, the equivalent of eight million people flying from London to New York daily.

Signs of progress

Despite the long-term trend, there is hope to protect the remaining forests.

After years of decline, the trend of deforestation has been reversed in many industrialized countries as forests are regenerated. Europe, for example, added 661,000 hectares of forest a year between 2000 and 2005, according to FAO estimates.

China, as well, added almost four million hectares of forest cover each year during the first half of the decade. Vietnam is adding two per cent annually.

And as awareness of the problem grows, countries are setting aside more land for conservation. The central African country of Gabon, to name one example, set aside 10 per cent of its forest land in 2005.

Former prime minister Paul Martin has joined a $200-million effort by Britain and Norway which aims to protect the Congo Basin, while the United Nations Environmental Program met its Billion Tree Campaign goal and now aims to plant seven billion new trees around the world.

Seven billion new trees: They can't replace the original forests, but they can begin to put a patch on the Earth's green roof.