WASHINGTON — The most effective way to fight global warming is to plant lots of trees, a study says. A trillion of them, maybe more.

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.

Six nations with the most room for new trees are Russia, the United States, Canada, Australia, Brazil and China.

Before his research, Crowther figured that there were other more effective ways to fight climate change besides cutting emissions, such as people switching from meat-eating to vegetarianism. But, he said, tree planting is far more effective because trees take so much carbon dioxide out of the air.

Thomas Lovejoy, a George Mason University conservation biologist who wasn’t part of the study, called it “a good news story” because planting trees would also help stem the loss of biodiversity.

The researchers used Google Earth to see what areas could support more trees, while leaving room for people and crops. Lead author Jean-Francois Bastin estimated there’s space for at least 1 trillion more trees, but it could be 1.5 trillion.

That’s on top of the 3 trillion trees that now are on Earth, according to earlier Crowther research.

In February The City of Cullman was recognized as a Tree City USA for the 31st year. At a ceremony, Mayor Woody Jacobs said trees are important to the community because they can reduce erosion of topsoil, cut heating and cooling costs, clean the air and provide habitats for wildlife while beautifying the city.