At any given time, 10,000 to 15,000 people are plugged into Second Life, the virtual world populated by users' digital alter-egos. As Scott mentioned last week, those avatars take up real space on real servers, and demand electricity--lots of it--from San Francisco natural-gas generators. Back-of-the-envelope calculations done last month by Nicholas Carr suggest that "your average Second Life avatar uses about as much electricity as your average Brazilian." A reader pointed out that these virtual people might as well be emitting the same amount of CO2 every year that an SUV does in the course of driving 2,300 miles. The analogy is thought-provoking, a shocking reminder of how much more energy Americans, and apparently their imaginary friends, use than about 85 percent of the world (about 11 times more).

PGE provides electricity to San Francisco, mostly by burning natural gas. The carbon-dioxide calculation in Carr's blog is fuel-neutral, which means it assumes a steady coupling between kilowatt-hours and emissions. My guess would be that the avatars use somewhat less energy than he indicates, since their electricity is probably generated from natural-gas, not oil or coal. Generic electricity-to-emissions calculations usually assume a fuel mix reflective of the nation's total, which is more than 50 percent coal, 20 percent nuclear, 20 percent natural gas, and less than 10 percent oil, hydro or renewables.

Carr made headlines in May 2003 when the Harvard Business Review published his provocative tract, "IT Doesn't Matter."