There's no single culprit responsible for deforestation: around the world, forest cover is lost because of fires, disease, logging, clear-cutting, and myriad other factors. And the environmental consequences threaten to be severe, especially given that deforestation causes an estimated 15 percent of global greenhouse gas emissions.

And before experts can effectively mitigate the problem, they need to know where it's happening — and to what extent. Now, a collaborative effort led by the University of Maryland (and including both Google and NASA) has created the first-ever high-resolution map that tracks forest gains and losses over time. Described this week in the journal Science, the map's creation depended on more than a decade of satellite imagery provided by Landsat — a satellite program operated by the US Geological Survey to capture and store images of Earth — combined with the processing prowess of Google Earth Engine.

"The Earth Engine takes a job that would have taken two weeks, and turns it into a few days," says Matthew Hansen, a professor of geological sciences at the University of Maryland and a co-author of the Science paper. Hansen and his colleagues developed the algorithms necessary to process the massive quantities of data, which included more than 650,000 satellite images. "We had the knowledge, and they had the technology to iterate that into something meaningful."

The resulting maps offer robust resolution and show the changes in forest cover worldwide from 1999 to 2012. Prior to this map's creation, experts like Hansen largely relied on self-reporting from various countries to keep tabs on deforestation: until around 2011, the United Nations monitored international forest changes by asking countries to submit their own information. "You didn't know if all that data was high-quality and reliable, which made it problematic," says Jeffrey Masek, a research scientist specializing in satellite monitoring of forest dynamics at NASA. "These maps help resolve any of those inadequacies on a global scale."

Right now, the maps don't offer up many surprises, but they do highlight an ongoing dilemma: the rapid acceleration of forest losses in tropical regions. In the decade tracked by the maps, forest losses in the tropics increased by 811 square miles a year — despite the fact that Brazil (which once had the highest deforestation rate in the world) drastically reduced the extent of its losses. Those improvements, unfortunately, were offset by accelerated deforestation in countries like Malaysia, Indonesia, and Bolivia. In Indonesia alone, for instance, the maps indicate that nearly 8,000 square miles of forest were lost between 2011 and 2012. "Forest losses can occur for many reasons, but most of the changes we see here are humans impacting the landscape," Hansen says. "And that impact is really quite remarkable in its extent."

Hansen and his colleagues anticipate the maps (which they plan to update annually) will be harnessed by myriad experts and organizations, from climate scientists to economists. And while the research team is primarily focused on creating transparent data — rather than making any political statement with their work — Hansen notes that the implications of the maps are tough to ignore. "If you're a treehugger, you can use this data. If you're a logger, you can use this data," he says. "But it's not good news."