Tropical deforestation increased by 62% over the last two decades and did not decrease as assessed by the UN, says a new study spanning the entire tropics.

A region equivalent to Sri Lanka is being lost to deforestation annually in the tropics. Tropical forests not only act as vital carbon sinks (or lungs) that take in carbon dioxide but also regulate interactions between the earth and the atmosphere.

The new estimate is based on vast amounts of Landsat image data which directly record the changes to forests over 20 years.

The previous estimate of a 25% slowdown, from the UN's Food and Agriculture Organization's (FAO) Forest Resource Assessment, was based on a collection of reports from dozens of countries.

"Several satellite-based local and regional studies have been made for changing rates of deforestation [during] the 1990s and 2000s, but our study is the first pan-tropical scale analysis," explains University of Maryland, College Park, geographer Do-Hyung Kim, lead author of the new study accepted for publication in Geophysical Research Letters.

Kim and colleagues looked at 34 forested countries which comprise 80% of forested tropical lands.

They analysed 5,444 Landsat scenes from 1990, 2000, 2005 and 2010 with a hectare-scale (100 by 100m) resolution to determine how much forest was lost and gained. Their procedure was fully automated and computerised to minimise error.

Net forest loss

They found that during the 1990-2000 period the annual net forest loss across all the countries was four million hectares per year. During the 2000-2010 period, the net forest loss rose to 6.5 million hectares per year - a 62% increase is the rate of deforestation.

Tropical Latin America showed the largest increase of annual net loss of 1.4 million hectares (5,400 square miles) per year from the 1990s to the 2000s, with Brazil topping the list at 0.6 million hectares (2,300 square miles) per year.

Satellite images captured between August 2012 and July 2013 showed 5,891 sq km of forests were cleared in the Brazilian Amazon, with a rate of destruction up by 29%.

Tropical Asia showed the second largest increase at 0.8 million hectares (3,100 square miles) per year, with similar trends across the countries of Indonesia, Malaysia, Cambodia, Thailand and the Philippines.

Tropical Africa showed the least amount of annual net forest area loss despite a rise in Democratic Republic of Congo and Madagascar.

Rodney Keenan, a University of Melbourne, Australia, forest science researcher who participated in the FAO's assessment says that it was based on ground based surveys of trees, often supplemented by imagery.

While the new study is "an important contribution to the overall picture," Keenan added, it "should not be seen as contradicting the FAO figures." But Kim and colleagues do not agree.

The FAO missed on deforestation in many areas. For instance it reported no change of deforestation rate for 16 of 34 countries from 1990 through 2010, whereas Landsat images show otherwise.

The new satellite-based estimates are particularly important for those trying to understand the global amounts of carbon being released into the air or being taken up by plants, soils and waters.

Experts see the increased deforestation is a logical consequence of the mechanisation of the process from the saws to chainsaws to tractors.

Clearing of tropical rainforests not only increases carbon dioxide in the air but also affects rainfall patterns and raises temperatures across the globe, said a recent study by University of Virginia.

Deforestation in South America, Southeast Asia and Africa can alter the climate as far away as the US Midwest, Europe and China, with disastrous effect on crops.

Tropical forests move more water than any other ecosystem on land and are central to the process of generating and regenerating moisture.