One hundred trees were planted at Goddard State Park, in Warren, last month, as part of an Arbor Day celebration.

The Providence Journal / Kris Craig

Over a drink recently, a friend in the horticulture business said I should tell people to plant a tree. I’ve been doing that for years, I said. No, she explained, to reduce one’s carbon footprint. Plant a tree, save the planet. Duh.

Squirming because I hadn’t made the link, I cast my mind back to high school biology class. Trees (and other green plants) take carbon dioxide from the atmosphere during photosynthesis and return oxygen to the air. The carbon molecules are used to make sugars and starches, which in turn feed the growth of cell walls, especially in spring, when everything is going gangbusters.

As long as the tree remains healthy, it just keeps stealing carbon dioxide from the air and storing carbon in its cellulose. Carbon remains there even if the tree becomes furniture or lumber. It is released only when the wood rots or is burned.

Carbon dioxide levels in the atmosphere have increased by a third since the start of the industrial revolution, due mostly to the burning of coal and other fossil fuels, and that buildup has been linked to global warming.

According to a neat calculator on the Web site of Casey Trees (www .caseytrees.org), a fair-size white oak tree with an 18-inch-diameter trunk would reduce atmospheric carbon dioxide by 622 pounds per year. A mature apple tree would lower carbon dioxide by more than 300 pounds. Even the weedy tree of heaven would store (“sequester,” in eco-parlance) 391 pounds once it reached a trunk diameter of 12 inches.

That sounds like a way of enhancing your yard and giving polar bears a future, until you dig a little deeper.

According to the Environmental Protection Agency, a two-person household in the United States is responsible for releasing 41,500 pounds of carbon dioxide into the air annually through the direct and indirect burning of fossil fuels.

To offset that entirely, you would have to plant 483 young trees and wait 10 years, the EPA says. That might be a recurring yearly challenge if you live on a lot smaller than, oh, 100 acres. Another option, while you are thinking of ways to reduce your carbon footprint, would be to donate money to an environmental group that plants trees. American Forests ( www.americanforests.org), based in Washington, D.C., accepts donations for its reforestation projects in the United States and a few other countries, with a $15 minimum. One dollar plants one tree.

So is planting a tree in your yard a futile gesture? No, said the group’s executive director, Deborah Gangloff. “We aren’t going to solve global warming by planting trees, but we can take up a lot of carbon.”

And you can increase the carbon benefits of a tree 15-fold by using it to shade the house in summer to reduce the need for air conditioning. American Forests suggests three trees: one on the east side to block morning sunlight and one each on the south and west sides. If you can plant only one, put it on the west side to counter the fierce afternoon sun. Gangloff points out that a deciduous tree will be bare in winter, allowing the sun to warm the house.

Find a site away from utility lines and buried drainage, and plant at least 15 feet from the house. Study the site conditions; most trees will die in a low-lying area prone to flooding, but some (the bald cypress or swamp white oak, for example) will be happy in such a spot.

Gangloff said homeowners play a vital role in preserving and replanting the urban forest because they own most of it. And the benefits go beyond carbon sequestration. Trees filter pollution, prevent soil erosion, provide habitat for wildlife and can look pretty good.

Mark Buscaino is executive director of Casey Trees, a D.C.-based charity established to protect and restore the city’s urban forest. He said if you plant one tree, you won’t make much of a dent in global warming. But if you plant 10 and “multiply it over their lifetime, they’re going to sequester a heck of a lot of carbon.”

Using Casey Trees’ calculator, I set out to see how much carbon is sequestered in my one-third-acre garden. As Buscaino suggested, it turns out to be quite a lot. The site contains 15 trees that predate my arrival (about half a dozen were taken down over the past 14 years). The largest are two American hollies, each with a trunk diameter of 17 inches; a northern red oak with a 22-inch-diameter trunk; and a saucer magnolia whose four trunks total 35 inches in diameter. The combined carbon sequestration of the 15 trees is 3,827 pounds annually.

I have planted an additional 18 trees, the largest of which is a crape myrtle with five trunks, each four inches across. The smallest and newest are two sourwoods planted 18 month ago, each measuring just 1 inch in trunk diameter. The 18 new trees sequester 1,163 pounds a year, for a total of 4,990. That is a fraction of my annual footprint, but it makes me feel better about burning wood in the fireplace in winter or flying to England to see my mother.

It’s conceivable that a former owner positioned the saucer magnolia to shade a patio, but none of the trees was planted expressly to capture carbon. They were put there to provide privacy, or to yield fruit, or to lend structure to the garden. They were selected for their beauty and multiseasonal interest, and for textures that contrast with each other. They were picked with their eventual size in mind. I wasn’t thinking about all the carbon dioxide swirling about the place.

When I plant more trees I will think about that aspect, though it still won’t be my first consideration. I will continue to ask: Do I have room? Have I picked the right tree for the site conditions, and how will it change the garden?

Casey Trees and American Forests offer online advice on correct tree planting at www .caseytrees.org/programs /treeplanting.html and www.americanforests.org /planttrees/howto.php.