Playing the devil’s advocate here, but intellectual honesty requires thought about the following.
Human powered foot (and ski) travel, as practiced in the real world, has plentiful impacts. As does motorized recreation. To say that one foot traveler has less impact than one motorized backcountry recreationist is an over-simplified view that obfuscates reality. For example, if a backcountry skier starts a huge avalanche that knocks down trees, possibly kills animals, and possibly endangers other people (for example, hitting homes and taking lives), he’s having a lot more impact that day than a guy out for a trail ride on his snowmobile. If a hiker brings a dog, and said dog chases an elk calf, that impact is greater than a sight-seeing person on a 4-wheeler. And consider backcountry ski huts: Hundreds of cords of firewood are burned to heat the huts, maintenance is done by snowmobile and 4×4 truck, the hut users drive to trailheads, the hut was built with and is maintained with resources, etc.
Another thing to consider is the fact that travel by foot and ski takes people to much more pristine areas than travel by road, even if the road is a jeep trail. Thus, the impact of one non-motorized user can be huge compared to the guy driving…
Now before you foot travelers start screaming, let me say that I’m perfectly aware that a muscle powered traveler who is not using a hut and not traveling with a dog is probably having less impact than a guy on a snowmobile. (That is unless said traveler flew by jet to a far away country for a vacation.) By foot is how I do most of my ski trips, and I’m certain I have minimal impact on the environment.
Yet give us skiers a hut, a fireplace, etc., and the issue gets clouded. Or take a mountain bicycle rider or hiker, send them off the trails, and the issue becomes ever more “muddy,” especially if such behavior is practiced in pristine wilderness — or yes, on muddy trails that are easily damaged. In truth, I’d love to see the exact amount of firewood and gasoline that an outfit such as 10th Mountain Huts uses per guest, then compare that to the amount of fuel burned by a snowmobiler for the day. One would “win,” but they both pollute and have an impact.