Forest loss in Sumatra's Riau province, Indonesia, 2000-2012. [Courtesy of Hansen, Potapov, Moore, Hancher et al ., 2013]
Global forest losses have outstripped gains since 2000, according to a detailed new map built with the help of satellite data and cloud computing. The map is the first one with a resolution high enough to allow researchers to explore both global and regional forest changes in the 21st century.
Between 2000 and 2012, the world lost 2.3 million square kilometers of forest, an area slightly smaller than the entire country of Argentina. During that time, only 0.8 million square kilometers were reforested, according to Matthew Hansen of the University of Maryland, College Park and colleagues, who published the new map in the 15 November issue of Science.
The detailed map could reveal much more about the global environment beyond changes in tree cover, the researchers noted.
"Losses or gains in forest cover shape many important aspects of an ecosystem including, climate regulation, carbon storage, biodiversity and water supplies, but until now there has not been a way to get detailed, accurate, satellite-based and readily available data on forest cover change from local to global scales," said Hansen in a press release.
Hansen and colleagues noted several trends of forest loss and gain all over the globe, but they found the strongest signal of forest cover change coming from the tropics. There have been both high losses and gains in tropical forest cover in the past twelve years, with losses outweighing the gains.
The 30-meter resolution scale of the map allowed the researchers to delve deeper into this trend, and explore why the tropics have experienced both high gains and losses. They discovered that Brazil was the country with the largest annual decline in forest loss over this period, while Indonesia had the greatest increase in forest loss.
In this sense, Hansen said, the map can also reveal where conservation efforts have been successful and where there is a need to step up conservation efforts. The maps shows that "Brazil in the last decade has cut its deforestation rate in half, and historically, they've accounted for half of global tropical rainforest loss," he noted.
In the United States, the most noticeable trend occurred in the southeastern U.S., where 30 percent of forest land was either lost or re-grown in twelve years."Trees are used as crops here, so you might want to rethink a definition of forest, because it's a different thing here," Hansen said. "It's not really natural."
Data for the map come from the Landsat  program, the longest continuously collected database of global satellite imagery. Hansen said the new map was made possible when Landsat made its data archives open to free access in 2008. At the same time, the researchers were able to analyze the massive amounts of data by collaborating with Google and using its cloud computing capabilities.
"If we did this project on one CPU, it would have taken 15 years," Hansen said, "but if we do it in the cloud it's a matter of days."