On balance, the planet’s forests continue to shrink at an alarming rate, but reforestation campaigns have picked up momentum around the world, recognized as a powerful tool to fight climate change, habitat loss and erosion. After losing much of its forest cover, China has set out to be the world leader in expanding it, and most countries have signed onto an array of ambitious tree-planting campaigns.

The Earth Day Network has called for planting 7.8 billion trees on Earth Day next year — one for every living person.

Worldwide, about 900 million hectares of land — almost 3.5 million square miles, nearly the area of the United States — is not being used by people and could support forests, according to a recent study by researchers at the Swiss Federal Institute for Technology, ETH Zurich, that drew intense interest from environmentalists worldwide.

If trees were planted on all of that land, the study said, when they matured they could store about two-thirds of all the carbon that human activity has pumped into the atmosphere since the Industrial Revolution.

In the early 20th century, about one-third of Ethiopia was covered in forests, according to historical estimates, but that had dropped to just 4 percent by 2000, according to the United Nations. The country’s population has soared to more than 100 million people, about five times as many as it had in 1960 — growth that has increased demand for farmland and timber, contributing to deforestation.

From 1990 to 2015, Ethiopia lost 2.6 million hectares of forest, or more than 10,000 square miles, according to the United Nations’ Food and Agriculture Agency.

Ethiopia is among the nations taking part in the United Nations’ decade-old campaign against deforestation.