Trees Help Climate Without Counting Toward Colleges’ Goal

This guest blog was authored by NRDC intern Smriti Kumble, who recently graduated from American University in Washington, D.C.

More than 90 colleges across the country have pledged to become carbon-neutral in three years, and while they’re taking important steps to reach their Climate Leadership Commitments, they may be ignoring a simple way to offset climate-warming carbon emissions: carbon sequestration by the thousands of trees on their campuses.

My college—American University (AU)—is among them. That’s why I joined a group of Environmental Science seniors to measure the carbon removed from the atmosphere and stored each year by the 2,294 trees on our main Washington, D.C., campus.

We wanted to see if carbon sequestration was a viable way for AU—and other universities—to reach carbon neutrality since the carbon removed from the atmosphere by trees offsets the schools’ emissions. Increasing the number of trees that sequester carbon should offset more of our emissions and help AU achieve its goal—right?

The Rules of the Game

I’ll acknowledge upfront that “officially” we were wrong—not because carbon sequestration doesn’t matter, but because the rules of the game won’t let us use it in the data that AU reports each year. Here’s why.

The Climate Leadership Commitments, an initiative implemented by 91 colleges to make higher education more sustainable through the reduction of carbon emissions, come with rules. The one that matters is additionality: for any action to count toward our 2020 goal, it must be new rather than something the university was already doing. For example, American University recently announced that it will purchase carbon offsets for emissions caused by students traveling to study abroad. On the other hand, our trees—existing and those scheduled to be planted—would have sequestered carbon even if AU wasn’t trying to offset emissions, so they don’t count and weren’t studied by the university.

That’s a shame because urban trees like ours have historically been ignored. Most research focuses on forest trees; in fact, a lot of data on D.C. trees comes from just one team of researchers at a nearby Forest Service outpost. So even though carbon sequestration can’t officially count toward AU’s goal, we studied it to fill gaps in AU’s data and to contribute our work to the data pool on D.C. trees. (Note: It may be possible to credit carbon emission savings in an updated plan in the future).

What We Found

After a few mishaps (we tried to measure a 75-foot-tall tree with a ruler and a tape measure—it didn’t go well), we collected height and width data on a representative sample of campus trees and used a complex U.S. Forest Service model to estimate carbon sequestration.

Our findings: the 2,294 trees on our main campus sequester 15.8 tons of carbon each year; on average, each tree sequesters only 6.6 kilograms annually. In comparison, AU emitted 6,484.9 tons of carbon in 2015. Therefore, AU would need over a quarter-million trees to be carbon neutral by 2020 solely through carbon sequestration. We currently have a few thousand.

But all is not lost! Our project looked only at AU’s 84-acre main campus; it ignored four satellite locations (East campus, Tenley campus, Spring Valley, and most importantly, the Airlie Center with over 300 tree-lined acres in Virginia). Though our study didn’t include carbon sequestration by trees on all of AU’s campuses, the emissions total of 6,484.9 tons/year represented all AU campuses since the AU emissions available data aren’t disaggregated by location. That contributes to the ridiculous number of 327,520 trees to be planted on our main campus for AU to be carbon neutral by 2020. How did we get that number? Divide total emissions by the average amount sequestered each year by a tree and you’ll reach the same conclusion. But if we’d counted all the trees, the total certainly would be lower.

While AU can’t be carbon- neutral solely through carbon sequestration, this doesn’t mean our urban forest should be overlooked as part of the solution. Increasing tree populations is an easy way to absorb carbon dioxide from the atmosphere (as opposed to expensive geosequestration, where carbon dioxide is captured and pumped deep underground) and AU, like other colleges, plants hundreds of trees annually. It makes sense to choose species based on how well they sequester carbon.

Other Benefits

In addition, the trees on campus play a range of other environmental roles, including:

  • Reducing temperatures exacerbated by the urban heat island effect, which makes urban areas warmer than their surroundings thanks to human activities like energy usage and the buildings we construct. Trees are known to reduce ambient air temperature (and thus the need for cooling-related energy use). Our study did not look at the impact of trees on building energy use, but AU tracks its energy consumption using tools like ENERGY STAR®’s Portfolio Manager—so a future study is made easy. 
  • Reducing runoff that would otherwise end up being treated at a wastewater plant: DC’s wastewater plant uses about 30 megawatts of electricity to treat 300 million gallons of wastewater on average every day. As for AU’s benefit: main campus trees prevent 980 m3 of runoff every year, which is equivalent to saving over $23,000 (based on the District of Columbia’s stormwater fee of $2.67 per 1,000 square feet of impervious surface, which is the amount the city charges landowners to clean the stormwater that runs off their land).

Trees provide more than just shade, but without data there’s no way to know how much you benefit from them. If you’re a homeowner, plant trees and they’ll help cut your energy bills. If you’re a researcher, explore how cities can alter their landscaping practices to increase the amount of carbon they sequester. At a time when we’ve crossed another climate threshold of the amount of carbon dioxide in our atmosphere, why not use every tool we have to fight the effects of climate change?

Credit for some of the research mentioned in the blog also goes to Heather DeLucia, Taytum Sanderbeck, Corey Amann, and Will Dominguez.

About the Authors

Peter Miller

Director, Western Region, Climate & Clean Energy Program

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