And there’s enough room, Swiss scientists say. Even with existing cities and farmland, there’s enough space for new trees to cover 3.5 million square miles (9 million square kilometers), they reported in Thursday’s journal Science . That area is roughly the size of the United States.

The study calculated that over the decades, those new trees could suck up nearly 830 billion tons (750 billion metric tons) of heat-trapping carbon dioxide from the atmosphere. That’s about as much carbon pollution as humans have spewed in the past 25 years.

Much of that benefit will come quickly because trees remove more carbon from the air when they are younger, the study authors said. The potential for removing the most carbon is in the tropics.

“This is by far — by thousands of times — the cheapest climate change solution” and the most effective, said study co-author Thomas Crowther, a climate change ecologist at the Swiss Federal Institute of Technology in Zurich.