Solar energy or growing trees: which is really better for the environment?


What a dilemma for environmentalists: trees, or solar energy? Both are good for the environment. Both are beloved by the environmental community. Both fight global warming.

But sometimes, you just canâ''t have both. Thatâ''s what happened in Sunnyvale, Calif. From 1997 to 1999, Richard Treanor and Carolyn Bissett planted eight redwood trees at the edge of their property. In 2001, neighbor Mark Vargas, installed a 10 kw solar system on his roof and trellis, to power his home and charge his electric car.

The solar system worked just fine. And the trees grew, as trees, especially redwoods, tend to do. And the trees started to cast shade on Vargasâ''s solar panels.

Vargas asked the neighbors to trim the trees back from as much as 12 meters to 4.5 meters. Treanor and Bissett (who drive a Prius) said no. In December, after several years of mediation and, one has to assume, decidedly chilly neighborly relations, solar prevailed; a Santa Clara County Superior Court Judge found Treanor and Bissett in violation of the 1978 Solar Shade Control Act that prevents people from planting trees or shrubs that shade an existing solar system on a neighboring property. Last week, the wood chips flew and at least one of the trees got trimmed back; Treanor and Bissett took pictures and hope the so-called â''poodle cutâ'' will be dramatic enough to satisfy the judge.

In response, California State Sen. Joe Simitian introduced a bill protecting trees planted before solar panels were installed, even if the trees grow and later shade the panels. Sort of a first-come, first-serve solution.

But nowhere in the heavy media coverage of this neighborhood squabble have I read any analysis of the real question. That is, which option is better for the environment, letting the trees grow, or letting the sun hit the solar panels unobstructed.

I took this question to H Scott Matthews, assistant professor of Civil and Environmental Engineering and Engineering and Public Policy at Carnegie Mellon University: considering global warming, is the carbon dioxide absorbed by the growing trees more or less than the greenhouse gases that would be created in generating replacement energy? And, while air conditioning isnâ''t a big concern in California, in many places you would also have to consider the impact of shade on air conditioning use, that is, does losing the trees mean the homeowner has to crank up his air conditioning in the summer?

Matthews did a rough back-of-the-envelope calculation for me. In Northern California, an average household consumes 7 MWh of electricity per year; from the grid, that could represent 3 tons of carbon dioxide. Letâ''s say a solar installation only reduces a householdâ''s consumption of electricity by half, thatâ''s 1.5 tons. Matthews couldnâ''t find numbers on redwood trees (which grow like weeds), but figures Douglas Firs are in the ballpark. An acre of Douglas Firs sequesters about 5 tons of carbon dioxide a year. Douglas Firs are planted at a density of about 400 per acre, so, if Iâ''m doing the math right, Treanor and Bissettâ''s eight trees soak up about 0.1 ton of carbon dioxide a year. So, says Matthews, solar panels win; and, he says, â''Iâ''m not a fan of solar PV technology.â'' (He didnâ''t factor in air conditioning; the difference was big enough to make that it particularly relevant.)

Matthews also pointed out that the trees cut down in Sunnyvale means the carbon sequestered there will be released through decomposition or burning. â''Better build some more solar panels fast!â''

This issue is not likely to go away. This month northern California 4-H club members are madly planting redwood trees to do their part to counteract global warming. Meanwhile, the state of California has earmarked $3.2 billion to subsidize homeowners looking to install solar cells, with a goal of putting solar on a million rooftops.

Photo: Joseph Tringali/iStockphoto


Tech Talk

IEEE Spectrum’s general technology blog, featuring news, analysis, and opinions about engineering, consumer electronics, and technology and society, from the editorial staff and freelance contributors.

Newsletter Sign Up

Sign up for the Tech Alert newsletter and receive ground-breaking technology and science news from IEEE Spectrum every Thursday.