Atlanta has more than nine million trees which soak up 46,345 tons of carbon every year. By contrast, Calgary, Canada, has nearly 12 million trees, but they only sequester 21,422 tons per year. The difference is due largely to different species -- many conifers in Calgary, many mature, broad-leafed trees in Atlanta.
According to the Forest Service, large diameter, long-lived, leafy trees tend to be the most beneficial.
This is still a relatively new field, and scientists are finding that it's more complex than it seems on the surface.
Colorado State University researchers studied a eucalyptus plantation in Hawaii to see how good those popular trees are at sequestering carbon. Surprisingly, they found that the eucalyptus performed better if they were interspersed with another local tree, the mimosa. They think the mimosa enriched the soil with nitrogen, stimulating the growth of the eucalyptus. Carbon is absorbed in the new growth.
Not everybody, however, is on this band wagon. Some researchers point out that trees require maintenance, which frequently involves power tools, which gunk up the air.
And water is a big factor. Many plantations around the world now grow trees as part of the fight against global warming, but researchers at Duke University warned recently that there's no free ride here.
"We believe that decreased stream flow and changes in soil and water quality are likely as plantations are increasingly grown for biological carbon sequestration," the Duke scientists concluded.
And, by the way, trees are only a short-term fix. Eventually they die, which is why the Forest Service recommends trees that are expected to live at least 50 years. Dead trees rot, releasing all that sequestered carbon.
So it's not the only answer, and it won't end the threat of global climate change. But it could buy time.