Mapping a Decade of Change in the Earth’s Forests
The Gran Chaco in central South America is sometimes called that continent’s other lung. Its forests are second in size only to its better-known northern counterpart of Amazonia. A world center of biodiversity – home to 500 species of birds and scores of large mammal species, from tapirs to panthers – the sprawling Chaco covers large tracts of Argentina and Bolivia, and 60 percent of Paraguay.
Naturalist Gerald Durrell wrote a book titled, “The Drunken Forests,” after visiting the Chaco in 1954. “The whole landscape,” he wrote, “did look as though nature had organized an enormous bottle party, inviting the weird mixture of temperate, sub-tropical, and tropical plants to it.”
But nature’s party in the Chaco may be coming to an abrupt end. Cattle ranching and illegal logging of hardwoods in Paraguay are driving the highest rate of deforestation in the world. The result is an ecological and social disaster. In addition to wiping out biodiversity, deforestation accelerates climate change, and, according to the International Labor Organization, the workers used to convert the Chaco forest into cattle ranches are frequently victims of debt-bondage, a modern form of slavery in which the men clearing the forests are paid wages too low to ever cover exorbitant fees for food and shelter.
The rate of deforestation in the Chaco may be the highest in the world, but the problem is global. And the same map that details the destruction in Paraguay provides scientists with an equally precise picture of how every forest on the planet is changing.
“This is the first map of forest change that is globally consistent and locally relevant,” says Matthew Hansen, a University of Maryland (UMD) professor of geographical sciences who led the research team that produced the first-of-its-kind map.
UMD researchers worked with engineers at Google Earth Engine to assemble a mammoth map that documents changes in all of the world’s forests from 2000-2012. The group used 650,000 images obtained by NASA’s Landsat 7 ETM+ satellite sensor, totaling 20 trillion pixels. Each pixel shows the condition of a section of forest measuring 30 by 30 meters (an area slightly larger than a baseball diamond). With that kind of unprecedented detail, the map reveals forest events such as fires and tornado and hurricane tracks, and makes visible trends like the deforestation that is destroying the Chaco.
According to a study based on the map, between 2000 and 2012, the planet lost 2.3 million square kilometers (888,000 square miles) of forests. This was countered, somewhat, by a gain of 800,000 square kilometers (309,000 square miles) of new forest growth, for a net loss of 1.5 million square kilometers (580,000 square miles) – an area larger than Alaska.
While the map’s most impressive achievement is in helping researchers understand the dynamics of forest change, the map is also a promising technological milestone, leading the way for similar advances in other areas of Earth observation that use satellite imagery.
The massive mapping project would certainly have pleased, but not surprised, the late Dr. James Fletcher, who served as NASA Administrator in 1972 when Landsat 1 was launched. In 1975, Fletcher said, “If I had to pick one spacecraft … to save the world, I would pick ERTS (later renamed ‘Landsat 1’) and the satellites which I believe will be evolved from it late in this decade.”
In the 40 years since that first launch, seven more Landsat spacecraft have been sent into space. Landsat 7, which provided all the images used in the global forest change map, was launched in April 1999, carrying the Enhanced Thematic Mapper Plus (ETM+) instrument, with better resolution and bandwidth capacity than earlier instruments. Landsat 7 is operated from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in a program managed by the U.S. Geological Survey (USGS).
Orbiting the Earth at an altitude of 438 miles and a speed of nearly 5 miles per second, the satellite passes over any given point on Earth every 16 days. Landsat 8, launched in February 2013, uses two newly designed instruments to capture 400 scenes a day – a 62 percent increase over Landsat 7’s capture rate.
Computer scientists at Google played a key role in the mapping project, using Google Earth Engine to mine the voluminous Landsat 7 EMT+ archive which the USGS has made available to the public at no cost.
According to Rebecca Moore, engineering manager of Google Earth Outreach and Google Earth Engine, a single computer would have needed 15 years to perform the analysis that created the forest map. Google Earth Engine completed the task in a few days, logging one million CPU-core hours on 10,000 computers running in parallel.
Researchers hope that this kind of global mapping project will help reverse the global trend of deforestation, and they have evidence to support that belief. “In the last decade,” writes mapping-project leader Matthew Hansen, Brazil “has cut its deforestation rate in half, and historically, they’ve accounted for half of global tropical rainforest loss.”
Brazil’s National Institute for Space Research “use of Landsat data,” Hansen concludes in the Nov. 15 issues of Science, “was crucial to its policy formulation and implementation.”
Experts believe that the deforestation of the Chaco Forest could be abated by applying those same data to land use policies in Paraguay.