LONDON • There are about three trillion trees on the earth, roughly 422 for every person and eight times more than previously estimated, researchers said on Wednesday, admitting surprise.

A 15-nation team led by Yale University experts used a combination of old-fashioned headcounts and state-of-the-art satellite and supercomputer technology to produce what they claim is the most comprehensive tree census ever.

"I don't know what I would have guessed, but I was certainly surprised to find that we were talking about trillions," said the study's lead author, Dr Thomas Crowther, of the Yale School of Forestry and Environmental Studies in Connecticut, the United States.

The new estimate will form the baseline for a wide range of studies, such as those that consider animal and plant habitats for biodiversity reasons and climate modelling.

But Dr Crowther cautioned that the number did not change anything, telling the BBC: "It's not like we've discovered a load of new trees; it's not like we've discovered a load of new carbon. So, it's not good news for the world or bad news that we've produced this new number.

"We're simply describing the state of the global forest system in numbers that people can understand and that scientists can use, and that environmental practitioners or policymakers can understand and use."

And there is bad news too, the researchers reported in the journal Nature. The calculation revealed that tree cover had nearly halved since the start of human civilisation. And the pace of deforestation has not abated: Our species is currently felling some 15 billion trees every year, the study found.

The team based its research on verified tree counts from some 400,000 forest plots. It then used satellite imagery to determine how factors like climate, topography, vegetation, soil conditions and human impact affected tree density.

Developing models to estimate tree numbers at regional levels, they then drew a global map of the earth's estimated 3.04 trillion trees.

"The highest densities of trees were found in the boreal forests in the sub-arctic regions of Russia, Scandinavia and North America," a Yale statement said. "But the largest forest areas, by far, are in the tropics, which are home to about 43 per cent of the world's trees."

The team's calculations revealed that of all the factors that had an impact on tree numbers, human activity had by far the biggest effect, largely through deforestation and land-use change.

There has been a 46 per cent drop in tree numbers in total since humans began to clear land to plant seeds, the study found. "In short, tree densities usually plummet as the human population increases," said the statement.

"We've nearly halved the number of trees on the planet, and we've seen the impact on climate and human health as a result," said Dr Crowther. "The study highlights how much more effort is needed if we are to restore healthy forests worldwide."

Meanwhile, Indonesia on Wednesday unveiled an ambitious new target for reducing carbon emissions, promising to slash its greenhouse gas output by 29 per cent by 2030.

The increased commitment by one of the world's largest greenhouse gas emitters will be officially submitted to the United Nations later this month ahead of a major climate change summit in December.

The pledge goes beyond Indonesia's 2009 agreement to slash emissions by 26 per cent by 2020. The final draft submission states that Indonesia has set aside 12.7 million ha of forest for conservation to help realise its target.

The government also hopes to derive nearly a quarter of its vast energy needs from renewable sources within a decade.

AGENCE FRANCE-PRESSE