May 21 2009

Give Me Shade

Posted by: Jonathan Marshall

While scientists and engineers spend billions of dollars seeking "clean tech" solutions to our environmental woes, experience shows that it's sometimes hard to improve on Mother Nature's own remedies. Example: shade trees that cut air conditioning bills in summer while letting the sun in to heat homes naturally during the winter.

The simple act of planting of a shade tree on the western or southern side of a house in one of the warmer areas of California can reduce its summertime electric bill about 5 percent, or $25 a year, according to a new study by researchers from the National Institute of Standards and Technology and the U.S. Department of Agriculture.

The findings--based on the first large-scale study to use utility billing data (from Sacramento)--won't win anyone a Nobel Prize, the authors admit. But co-author Geoffrey Donovan of the U.S. Forest Service's Pacific Northwest Research Station, says "this study gets at the details: Where should a tree be placed to get the most benefits? And how exactly do shade trees impact our carbon footprint?"

A London plane tree, planted on the west side of a house, can reduce carbon emissions from summertime electricity use by an average of 31 percent over 100 years, the authors found. But trees planted on the east side had no effect and those on the north increased summertime electricity use.

PG&E has long studied tree planting as an energy efficiency measure. Back in 1991, PG&E launched a shade tree program in Fresno, offering a $10 rebate to each customer who planted approved trees to shade residential buildings.

A detailed computer model of the program concluded two years later that each mature yard tree would save a projected 346 kilowatt-hours per year (mainly from reduced air conditioning) and absorb significant emissions of polluting gases and particulates. Each tree was also estimated to lock up 103 pounds of carbon in tree biomass and reduce CO2 emissions from power plants by 153 pounds a year.

Unfortunately, under existing regulations, the utility could not cite all of these social benefits to justify the cost of the program and it was dropped.

Last year, PG&E revisted the idea by launching a pilot "Shade and Save" program in San Jose, with an emphasis on covering western exposures. PG&E also created pilot tree planting programs in Stockton and the Woodland-Winters-Davis area. Studies of cost effectiveness are now under way. 

As we noted yesterday, protection of tropical rainforests is high on the list of tactics for slowing the pace of global warming. It's worth remembering that our less exotic urban trees are a powerful antidote as well.

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