Globally, roughly 13 million hectares of forest fall to the blade or fire each year. Such deforestation has long been driven by farmers eking out a slash-and-burn living or loggers using new roads to cut inroads into pristine forest. But now new data appears to show that, at least for the first five years of the 21st century, big block clearings that reflect industrial deforestation have come to dominate, rather than smaller-scale efforts that leave behind long, narrow swaths of cleared land.

Geographer Ruth DeFries of Columbia University and her colleagues used satellite images from Landsat, along with the MODIS instrument on Aqua to analyze tree-clearing in countries ringing the tropics, representing 98 percent of all remaining tropical forest. Instead of the typical "fish bone" signature of deforestation from small-scale operations, large, chunky blocks of cleared land reveal a changing driver for cutting down woods: large enterprises feeding urban demand, according to a new paper published in Nature Geoscience on February 7. (Scientific American is part of Nature Publishing Group.)

In fact, a statistical analysis of 41 countries revealed that forest loss rates are most closely linked with urban population growth and agricultural exports from 2000 to 2005—even overall population growth was not as strong a driver. "In previous decades, deforestation was associated with planned colonization, resettlement schemes and local farmers clearing land to grow food for subsistence," DeFries says. "What we're seeing is a shift from small-scale farmers driving deforestation to distant demands from urban growth, agricultural trade and exports being more important drivers."

In other words, the increasing urbanization of the developing world—as well as an ongoing increase in consumption in the developed world for products that have an impact on forests, whether furniture, shoe leather or chicken fed on soy meal—is driving deforestation, rather than containing it, as populations leave rural areas to concentrate in booming cities. "One of the really striking characteristics of this century is urbanization and rapid urban growth in the developing world," DeFries says. ""People in cities need to eat."

"There's no surprise there," observes Scott Poynton, executive director of the Tropical Forest Trust, a Switzerland-based organization that helps businesses implement and manage sustainable forestry in countries such as Brazil, Congo and Indonesia. "It's not about poor people chopping down trees. It's all the people in New York, Europe and elsewhere who want cheap products, primarily food."

To help sustain this increasing urban and global demand, agricultural productivity will need to be increased on lands that have already been cleared, such as the many degraded and abandoned lands in the tropics, DeFries argues, whether through better crop varieties or better management techniques. And the Tropical Forest Trust is building management systems to keep illegally harvested wood from ending up in, for example, deck chairs, as well as expanding its efforts to look at how to reduce the "forest footprint" of agricultural products, such as palm oil. "The agricultural stuff, that's where the deforestation happens," Poynton says. "The point is to give forests value as forests, to keep it as a forest and give it a use as a forest. They're not going to lock it away as a national park, that's not going to happen."

Of course, tropical deforestation has allowed forest regrowth in other areas, including tropical lands previously cleared. And forest clearing in the Amazon, the world's largest tropical forest, has dropped from roughly 1.9 million hectares a year in the 1990s to 1.6 million hectares a year over the last decade, according to the Brazilian government. "We know that deforestation has slowed down in at least the Brazilian Amazon since the data we have for this study," DeFries says. "We looked very broadly over 41 countries. Every place is different. Every country has its own particular situation, circumstances and drivers."

Regardless, cutting down forests is one of the largest sources of greenhouse gas emissions from human activity—a double blow that both eliminates a biological system to suck up CO2 and a new source of greenhouse gases in the form of decaying plants. The United Nations Environment Programme estimates that slowing such deforestation could restore some 50 billion metric tons of CO2, or more than a year of global emissions, and international climate negotiations continue to attempt to set up a system to drive that, known as the U.N. Development Programme's fund for reducing emissions from deforestation and forest degradation in developing countries (REDD). "If policies [like REDD] are to be effective, we need to understand what the driving forces are behind deforestation," DeFries argues, and there are some new pressures looming. "Competing land uses for other products such as biofuels will exacerbate these pressures on tropical forests," the researchers wrote.

But millions of hectares of pristine forest remain to potentially save, according to this new analysis—60 percent of the remaining tropical forests are in countries or areas with little agricultural trade or urban growth. "The amount of forest area in places like central Africa, Guyana and Suriname," DeFries notes, is huge. "There's a lot of forest that has not yet faced these pressures."