Wherever you’re reading this, you’re being asked to turn off your lights on Saturday night between 8 p.m. and 9 p.m. This is for Earth Hour, an event to encourage energy savings sponsored by the environmental advocacy group World Wildlife Fund.

Delta-Sky’s March cover

The event’s tagline is: “See the difference you can make.” Delta’s in-flight magazine, Sky, attempted to quantify that difference on the cover of its March issue, which claimed that if every U.S. household turns off the lights during Earth Hour it would “prevent more than 16,500 tons of [carbon dioxide] from entering the atmosphere.” But the number appears to be an overestimate.

That’s not an official stat from event organizers; it was calculated by the Delta-Sky cover story’s author, Jim Hackler, using numbers from the U.S. Environmental Protection Agency. The EPA estimated that in 2005 emissions from the residential sector were about 1.2 trillion kilograms of carbon dioxide. Mr. Hackler converted that to tons per hour, assumed that 11% of these emissions were due to lighting, and came up with 16,610 tons of carbon dioxide.

But EPA spokeswoman Roxanne Smith told me the calculations were “off,” because just 70% of household emissions came from electricity, with the rest coming from combustion (such as for heating and cooking). Applying that to the Sky estimate would reduce the potential savings to 11,764 tons of carbon dioxide.

Duncan Christy, the editorial director of Delta-Sky, said the magazine stands by the cover story. “No number is likely to be perfect, I believe we’d all agree, nor is this exactly the point,” he told me in an email. “We’re trying to make our readers understand the positive outcome of a shared gesture. It is more than symbolic — but it is also that.” Mr. Hackler pointed out that his numbers could be “conservative,” because other sources put lighting’s share of the home electricity bill higher, up to 25%. (The Department of Energy, though, has estimated lighting’s share at 8.8%.)

These are projections of Earth Hour’s potential impact, not direct measurements. In the inaugural Earth Hour, last year in Sydney, organizers estimated 2.2 million people participated, and reduction in power consumption, as measured by local utilities, translated into a reduction of 25 tons of carbon-dioxide emissions. If every American participated and carbon dioxide was saved at the same rate, that would mean a reduction of about 3,400 tons of carbon dioxide — barely one-fifth Delta’s estimate. (A spokeswoman for WWF Australia referred me to her U.S. colleagues.)

That may be an optimistic calculation, based on the results of a similar event last October in California. As I wrote at the time, local utilities didn’t detect any reduction in energy consumption. Furthermore, skeptics have questioned whether Earth Hour’s publicity campaign will offset any reductions — for instance, a promotional hot-air balloon has soared over several Australian cities.

With accurate numbers hard to come by in this arena, Earth Hour’s U.S. organizers wisely are avoiding numerical goals for the event, focusing instead on its symbolic value. “Earth Hour is primarily an awareness building event designed to change the way people think about climate change and the things they can do to make a difference and save the planet,” Dan Forman, WWF spokesman in the U.S., told me. “In all reality if every single person in the US was to shut off their lights for an hour it would have an effect, but wouldn’t make a significant dent in our race to reverse the effects of climate change.”

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