This article originally published on Yale Environment 360.

People have lived in Castro Laboreiro, where northern Portugal borders Spain, long enough to have built megaliths in the mountainous countryside and a pre-Romanesque church, from 1,100 years ago, in the village itself. But the old rural population has dwindled away, leaving behind mostly elders yearning for their vanishing culture.

Roughly half the area once grazed by sheep, goats and cattle is unused and reverting to nature, meaning that wolves, bears, wild boars and other species have rebounded in their old habit. Iberian ibex and griffon vultures thrive where they were extinct, or nearly so, as recently as the 1990s. So what feels like loss to some village residents looks to others like a great recovery.

Places such as Castro Laboreiro are, of course, everywhere. Abandonment of rural lands has become one of the most dramatic planet-wide changes of our time, affecting millions of square miles of land. Partly it’s a product of rural flight, and the economic, social and educational appeal of cities. Partly it’s about larger forces such as climate change and globalization of the food supply chain. But the result, according to a new study in Nature Ecology and Evolution, is that the global footprint of agriculture has "started decreasing in size during the past two decades, with more land now being abandoned from agriculture than converted to it, especially in Western Europe and North America." (This change doesn’t appear to have affected global food supply, at least not yet, because the land lost was marginal to start with and farming elsewhere has become more productive.)

The study, led by researchers from the University of Minnesota, found that abandoned lands can take decades or even centuries to recover their original biodiversity and productivity. But it termed land abandonment "an unprecedented opportunity for ecological restoration efforts to help to mitigate a sixth mass extinction and its consequences for human wellbeing." Indeed, by some accounts, a more aggressive — and evidence-based — approach to restoring abandoned lands could bring about major progress in both the climate and extinction emergencies. Researchers termed land abandonment 'an unprecedented opportunity for ecological restoration efforts to help to mitigate a sixth mass extinction and its consequences for human wellbeing.'

A study earlier this year in Science calculated the potential tree cover on "degraded" lands worldwide and found, according to senior author Thomas Crowther of ETH Zurich, that a massive program to plant trees and grow them to maturity "could cut carbon dioxide in the atmosphere … to levels last seen almost a century ago." That study, which elicited sharp criticism from other researchers, called for planting at least 6.6 million square miles of degraded land not currently used for urban or agricultural purposes. More than half the planting would take place in six countries that are, conveniently, also major contributors to climate change: Russia; the United States; Canada; Australia; Brazil; and China.

Crowther calls it "the best climate change solution available today," with the potential to remove 25 percent of the carbon dioxide emissions humans have added to the atmosphere. But critics have characterized the proposal as a distraction from the immediate priority of reducing greenhouse gas emissions. They also questioned the suitability of land in the study for reforestation.

"These plans have been developed by scientists who do a lot of remote sensing and don’t understand the social context of why these lands are in transition, or if they are in transition," says Mark Ashton, a forest ecologist at the Yale School of Forestry & Environmental Studies. "This is much more complex than looking at a map and thinking you can plant trees, without understanding the human context around that land."

The study came with major caveats of its own: The authors could not determine whether the available land is publicly or privately owned. Moreover, some lands suitable for regeneration could become much less so as climate change advances.