I’ve written a coupletimes about ways that drivers can offset their global warming emissions by buying carbon credits—usually by paying for renewable electricity projects.  For folks who can’t afford a Prius (and even for folks who can) this seems to me to be a pretty good way to reduce some of the pollution impacts of your driving habits.

Alert reader Dave Manelski from Earth Share of Washington pointed out this post from the blog of environmental publisher and writer Joel Makower, covering 5 different carbon offset programs.  All of them seem worthwhile, and even though their costs vary, they’re all cheap compared with the cost of a new hybrid. 

But what’s particularly odd:  the estimates of how much carbon you create when you drive varies with the program.  One carbon calculator estimated that Joel’s driving habits produce 5,577 pounds of CO2 per year; another, 4,409.  Not that this matters all that much, really—but it is interesting that carbon accounting seems to confound even the people who’d seem to be most likely to have a handle on it.