Hey Mr. Green,

It seems hypocritical for the Sierra Club to voice concern about the environment while promoting air travel to distant destinations. How bad is plane travel? Would traveling by blimp help? Is it ethical? --Robert in Rochester, New York

Commercial flights account for 9 percent of U.S. transportation-fuel consumption. But given air travel's effects on the upper atmosphere, its global-warming impact may be far greater. According to the Intergovernmental Panel on Climate Change, aviation's share of U.S. transportation's global-warming impact could be 20 percent, or up to 5.5 percent of total U.S. emissions. (Air travel can be very efficient, however. Domestic flights move a passenger 50 miles on a gallon of fuel, compared with 30 miles per person for cars. International flights do even better.) 

Blimps are like many American males: great at sitting still and watching football, but slow and inefficient. The legendary Hindenberg got about 17 miles per gallon per ride, and the new models aren't much better. Also, the expensive helium needed to float 'em is a product of natural gas wells.

As for ethics, the Sierra Club's mission is "to explore, enjoy, and protect the wild places of the earth." The Club learned long ago that people who actually visit places become their staunchest defenders--even if they fly there. In many areas, sustainable tourism provides a crucial economic alternative to natural resource extraction. Plus, you can offset emissions from air travel at sierraclub.org/outings/national/offsets. --Bob Schildgen

Read more advice from Mr. Green and submit your own questions for him at sierraclub.org/mrgreen.