Another element of ideology: the evaluation of very very long term environmental effects

In Economic Cost/Benefit Analysis, there is something called "the discount rate". Things that happen in the future are "discounted", or weighed less heavily than things that happen today. There are some pretty good common-sense justifications for this practice.

1. If we are talking about impacts that can be monetized (that is, treated as equivalent to flows of money - which is sometimes controversial in itself), there exist financial markets that allow you to save or borrow at a given rate of interest to obtain equivalent economic effects over time. Let's say that we have the choice to build a $100 million bridge today, that will deliver $2 million per year in "economic benefits" (time saved, expected number of lives saved, and so on) over the 100 year life of the bridge. Obviously, 100 years x $2 million per year = $200m, that's more than the $100m cost of the bridge, so we should build the bridge, right? Not so fast. We could simply stick the $100m in the bank, and we would wind up with *more* than $200 million after 100 years. (A lot more, in fact). Sticking the money in the bank is judged by cost benefit analysis to be "better" than building the bridge, even though we are comparing apples and oranges (that's what CBA does).
   
2. People are impatient, and they would rather have benefits today than benefits tomorrow.

The practice of discounting is pretty uncontroversial for the sort of 10-20 year projects that governments are likely to get involved in. Where it *really* starts to get problematic is in evaluating environmental regulations, where the costs are immediate but the benefits potentially stretch out over hundreds or thousands of years. Using the standard discount rates invoked in most public projects, any benefit that happens after 100 years or so is discounted right out of existence. What happens in the far future literally *doesn't count*. This really shows up in climate change projects. If you do a standard cost/benefit analysis of Kyoto or any other greenhouse gas initiative, it comes out terribly. The costs are now, the benefits are in 100 years... "don't do it". Stick the money in the bank instead.

Assuming that all costs and benefits can be monetized, do the results of Kyoto CBA imply that it would be better for future generations if we DIDN'T do anything about climate change, but simply "stuck the money in the bank". What does that mean? In other words, what does the discount rate even MEAN when the timescales are so long that the costs and benefits accrue to completely different generations?

Now, for a lot of environmentalists, all this talk of discounting is devil's talk. Sustainability is their mantra, and if environmental impacts can be monetized at all (many think they cannot be) then they should be discounted at rate zero.