It is pitiful that your article, " Tax the Knickers Off Your Grandchildren," can pass for economics. Your key assumption is that environmentalists and the Sierra Club get in the way of economic growth.
Your one example of this process is that Oregon lumberjacks are unemployed because the Sierra Club has obstructed their right to "confiscate your rich grandchildren's view of the giant redwoods." Well, to begin with, there are no redwoods in Oregon. Beyond that, the biggest factor creating unemployment for Oregon lumberjacks is that the previous generation of timber owners cut down almost all of the aboriginal inheritance of forests in this country--over 90 percent (by the most conservative estimates) was logged out in one century. Yet this impoverishment of the next generation through the mismanagement of natural resources is exactly what you argue can't happen.
Or look at the even more impoverished fishing communities of California, Oregon, and Washington, where overlogging, overdamming, and overfishing have wiped out more than 90 percent of the jobs once supported. The sons and daughters of the Mendocino salmon fleet are having a much harder time making ends meet than their parents did because we destroyed the natural resource upon which they depended.
You also question whether we know that future generations will value clean air, wild forests, and natural beauty. But you seem willing to extrapolate the future of economic growth from the immediate past. It's very clear that for a much longer time the importance that Americans attach to wilderness and environmental values has been increasing, and there is every reason to expect this to continue to be true. As our population and wealth increase, and our supply of wild places and wildlife decreases, the price is almost certainly going to go up. As we have more people, and if you are right, more rich people, they will want to do with their wealth what rich people have always done: buy beauty, peace, solitude.
Indeed, economic theory dictates that if most of those who would like to enjoy something don't have the right to purchase it in the market, the price will be artificially depressed. Most of the generations that would like to enjoy the ancient forests and wildlife of Oregon have no opportunity to shape their future, so it's almost a foregone conclusion that whatever their current price in the marketplace is, it's artificially low, and the real value is much higher than our economy takes into account.
So the Sierra Club is actually making the prices of the market closer to their true value, and in so doing, helping the economy be more efficient and ensuring that this generation, as well as future generations, will be more prosperous.