The number of trees on Earth has almost halved since the beginning of human civilisation, a new assessment has found.

There are more than three trillion trees worldwide - around eight times more than some previous estimates - according to the study led by researchers at Yale University in the US.

But around 15 billion trees are currently being lost each year as a result of deforestation, forest management and changes in land use, the research published in the journal Nature showed.

Since the start of human civilisation around 11,700 years ago the total number of trees has fallen by around 46%, it estimates.



The researchers collected on-the-ground data for the number of trees in more than 400,000 plots of forest from all continents except Antarctica.

They used satellite imagery to assess how the density of trees in the plots was related to local characteristics such as the climate, vegetation, soil conditions and the impacts of human activity, and used the information to build models for the number of trees in various regions.

The global map generated suggests there are around 3.04 trillion trees, or around 422 for each person, on Earth.

A country-by-country breakdown reveals there are 709 million trees in Ireland, equating to 154 for each person.

The highest densities of trees are found in the forests of the sub-Arctic regions of Russia, Scandinavia and North America, but the largest forest areas were in the tropics, which are home to around 43% of the world's trees.

The information on tree populations will help efforts to model global systems such as carbon storage, the changing climate and the distribution of animal and plant species, the researchers said.

Lead author Thomas Crowther, post-doctoral fellow at Yale School of Forestry and Environmental Studies, said: "Trees are among the most prominent and critical organisms on Earth, yet we are only recently beginning to comprehend their global extent and distribution.

"They store huge amounts of carbon, are essential for the cycling of nutrients, for water and air quality, and for countless human services.

"Yet you ask people to estimate, within an order of magnitude, how many trees there are and they don't know where to begin," he said, adding he was "certainly surprised" to find the estimate was in the trillions.

He said: "We've nearly halved the number of trees on the planet, and we've seen the impacts on climate and human health as a result.

"This study highlights how much more effort is needed if we are to restore healthy forests worldwide."

The study was prompted by a request by Plant for the Planet, a youth initiative leading the United Nations Environment Programme's "billion tree campaign", for baseline estimates of tree numbers to help set targets for tree-planting initiatives.

The previous global estimate was of just over 400 billion trees worldwide.