The idea sounds simple: When trees are cut down and decompose or are burned, they release the carbon dioxide they soaked up from the atmosphere. Keeping them intact can help slow the pace of global warming.

Yet many experts have long been skeptical that such “payment for ecosystem services” programs actually work. How does anyone know the money is not just flowing to people who would have preserved their forests anyway? What if people take money to protect their own land but then go cut down trees elsewhere?

“Unless you set up a randomized trial, where you’re carefully comparing people who take part in the program with people who aren’t, it’s hard to know if you’re having any effect,” said Seema Jayachandran, an economist at Northwestern University and the lead author of the study.

Her team randomly selected 60 villages in the Hoima and northern Kibaale districts of Uganda and offered owners of forested land $11.20 an acre a year if they did not cut their trees. Using satellite imagery, they then compared what happened in those villages to 60 “control” villages that were not offered payments.

Over the next two years, they saw forest cover drop 9.1 percent in the control villages. But in the villages where voluntary payments were offered, one-third of the landowners signed up and forest cover declined just 4.2 percent. Further analysis showed that deforestation was not simply shifting to nearby lands, as feared.