A new study shows a 15-year increase in forests, which give off oxygen and absorb carbon dioxide, which scientists believe causes global climate change.

Carbon dioxide released from cars and power plants collects in the upper atmosphere and prevents heat from escaping Earth.

"This great reversal in land use could stop the styling of a `Skinhead Earth' and begin a great restoration of the landscape by 2050, expanding the global forest by 10 percent - about 300 million hectares, the area of India," said Jesse Ausubel, an environment expert at Rockefeller University in New York.

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"Forest area and biomass are still being lost in such important countries as Brazil and Indonesia, but an increasing number of nations show gains," said the report in the journal Proceedings of the National Academy of Sciences, chartered by the United States Congress to advise the US on science policy.

"The forests of Earth's two most populated nations no longer increase atmospheric carbon concentration: China's forests are expanding; India's have reached equilibrium - these changes are due in large part to urban migration, agricultural yield increases and reforestation policies," the report said.

Among the 50 countries studied, forests shrank fastest in percentage terms in Nigeria and the Philippines and expanded fastest in Vietnam, Spain and China during the 15 years covered in the study - 1990 to 2005.

Forested areas fell fastest in Indonesia and Brazil, while gains were highest in China and the US.

The study was written by six experts in forestry, environmental technology, ecology, geography, resource economics and agronomy in China, Finland, Scotland and the US.

The study said the scientists, "following independent lines of thinking, came to agree that forest transition on a major scale is under way and have now collectively demonstrated it."

Pekka Kauppi of the University of Helsinki said: "Without depopulation or impoverishment, increasing numbers of countries are experiencing transitions in forest area and density.

"While complacency would be misplaced, our insights provide grounds for optimism about the prospects for returning forests."

Why are the forests returning? Co- author Paul Waggoner of the Connecticut Agricultural Experiment Station noted a set of events that seem to feed on each other.

When countries protect forests, they can grow; at the same time, when farmland is preserved, farmers are less likely to encroach on forests.

In Europe, timber imports, energy technology, and economic development that sent country people to the cities also played a role, Waggoner said. As farm technologies improved, farming concentrated on fertile lands and left marginally fertile forests alone, even as urban migration depleted rural populations.

The study also said tree planting for materials such as paper is an improvement over cutting old-growth forests.

The authors predict the share of industrial wood production in forest plantations will grow from an estimated one-third today to half by 2025 to three- quarters by 2050.

"Plantations and the trade to make them effective reduce the impact of industrial pressures on the expanse of natural forests, which may be rich in soil carbon and biodiversity," added Roger Sedjo, an economist at Resources for the Future, a Washington think-tank.

The study used data from the UN Food and Agriculture Organization, which measured area covered by forests, the volume and tonnes of biomass - organic material.

By this standard, the researchers found that despite widespread concerns about deforestation, the number of timber-size trees increased between 1990 and 2005 in 22 of the 50 countries with the most forest.

And almost every country with a per capita gross domestic product over US$4,600 (HK$35,880) has moved to reforestation. AGENCE FRANCE-PRESSE, REUTERS