Q: If everyone in the country switched to a hybrid car, how much less carbon would be emitted into the atmosphere?

– Sanford, Alton, IL

A: That's a pretty simple question — with a potentially complicated answer. A lot would depend on how you define "hybrid." Would you include plug-in electric hybrids (PHEVs) which can get as much as 140 miles per gallon? What about European "microhybrids" which boost fuel efficiency by 10 percent? And would you factor in the stricter mileage standards that would take effect in 2011? And can you predict when and if the SUV trend will pass?

Here's a very rough, back-of-napkin estimate from Bradley Berman, editor of Hybridcars.com. Considering that the typical hybrid is capable of getting about double the mileage of its comparable, non-hybrid counterpart, if the hundreds of millions of vehicles on the road in the U.S. right now could be magically transformed into hybrids, their total resulting emissions would drop from about 300 million metric tons per year to about 150 million. That's a hefty chunk — especially considering that passenger vehicles are responsible for about 10 percent of total greenhouse gas emissions. Still, hybrids aren't necessarily the silver bullet that that calculation might imply. Even if factories started producing only hybrids this very minute, it would take about 20 years for all of the current vehicles to run their course and be replaced by hybrids. And pie-in-the-sky fantasies aside, automakers have been disappointingly slow to increase their production capacity by even a modest amount. Despite ever-increasing consumer demand and ever-present waiting lists, a measly three percent of the 2008 models rolling off the assembly line will be hybrids — just a smidgen more than the 2.5 percent in 2007. Would a nudge from the federal government help? Barack Obama seems to think so, having recently proposed an incentive plan to put a million PHEVs on the road by 2015. A small fraction of cars, true, but still, the fact that the president is even throwing that out there may be a good sign.

This article originally appeared in Plenty in August 2008. The story was added to MNN.com in November 2009.

Copyright Environ Press 2008

What if everyone drove a hybrid?
The answer to this depends on the definition of hybrid, and how long it would take to "convert" each car on the road to a hybrid. One thing's for sure -- emissi