When my wife and I bought our house, the yard was typical for our neighborhood: a mostly barren plain of lawn so sunbaked that you could bounce a tennis ball off it. So being eco-groovy types, we’ve tried to improve the place: we put in a rain barrel, built a natural drainage system, and added topsoil planting berms. But I’m most proud of the trees we’ve planted: a pair of akebono cherries in the parking strip and a white-star magnolia in the front yard; and in the backyard, a shore pine, a Chinese dogwood, a couple of vine maples, a Japanese maple, and a limelight cypress.

I recently began wondering how much carbon our new trees are soaking up. Since tree planting is the sine qua non of carbon offset programs, how much of my emissions are offset by my yard? Enough, perhaps, to justify moving from a dense, highly walkable neighborhood to a still-urban but less foot-friendly place? (My Walkscore dropped from 92 to 80.)

The answer, I’m afraid, is "no."

I estimate that in an average year my nine trees will soak up right around 100 pounds of carbon-dioxide combined. (Brief methodology note at the end of this post.) That’s the emissions equivalent of burning five gallons of gasoline — or actually just four gallons, if you consider the “lifecycle” emissions of gas. In other words, my tree planting allows me to burn about one-third of a tank of gas guilt-free each year.

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That’s certainly better than nothing. But then again, the average American is responsible for about 45,000 pounds of yearly CO2 emissions from energy use alone. Nine trees like mine offset about 0.2 percent of those emissions — and much less when nonenergy sources are considered.

Even giving myself a big benefit of the doubt — my electricity is carbon-free hydropower and I take other steps to reduce my climate footprint — it’s highly unlikely that my trees are offsetting more than half a percent of my annual emissions. Plus, half of those tree offsets belong to my wife. So that means at the very, very most I’m offsetting about one-quarter of one percent of my own emissions.

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I could do more for the climate by simply avoiding a couple of trips in my car.

Admittedly, we could fit a few more trees into the yard, though things would start to get crowded. And, in fact, we do have a few trees besides those I mentioned — an apple, a yew, and two thickets of arbor vitaes — which are also adding their own biomass and carbon each year. I didn’t count their carbon sequestration because I can’t take credit for them. They’d be around whether or not I owned the property.

In any case, it’s hard to see how small-scale tree planting can be a viable solution for carbon offsets. After all, not everyone has an empty yard, just waiting to be stocked with trees. The better choice for the climate might be to do without a private backyard in the first place.

What’s worse is that the carbon stored in my trees is only temporary. Someday, they’ll die and they’ll be chipped, or maybe they’ll decay, and most of the CO2 will be released back into the atmosphere. It’s possible that a portion will remain trapped in the soil (my backyard soil is vastly richer than it was before), but most will be lost. So when I take the long view, I haven’t even effectively offset my emissions, I’ve just delayed them. In the accounting of global warming, the delay won’t help at all.

Now, in defense of trees, it’s worth mentioning that they do have some other climate benefits. Properly placed trees can shade buildings in the summer, lessening the energy needed for cooling. In the winter, they can act as wind breaks and slow the heat escaping from buildings.

And I should say this too: I love trees. I love all kinds, and I love them for all different reasons. In our yard alone, some have springtime flowers that dispel the winter’s angst, while others have autumn foliage so brilliant it’s heartbreaking. Still others provide food and shelter for the juncos, chickadees, flickers, and finches that are now common visitors to our property. They help screen our yard from the neighbors and the street, and they give our outdoor space more depth and appeal. In time, they’ll help buffer city noises, shade our exposed lot, and soak up water in places that are inclined to be soggy. But the one thing they don’t do, I’m sorry to report, is offset my carbon emissions.

A note about methodology. Calculating carbon storage in trees is notoriously tricky for many reasons. For this post, I tried to make a decent estimate — about 100 pounds of CO2 per year — but I’m not claiming great precision. I made some educated guesses about about my trees’ growth rates, mature sizes, and lifespans, but I could be off by dozens of pounds of CO2 per year, and perhaps more. If others are interested in ballparking trees’ carbon benefits, here’s a metric tree carbon calculator based in Australia. I wasn’t able to find any calculators from Canada or the U.S., but I may simply have overlooked them. Please share, if you know of others.