I don't think you need any discounting. Your effect on the year 2012 is somewhat predictable. It is possible to choose a course of action based on known effect's on the year 2012.

You effect on the year 3000 is unpredictable. You can't even begin to predict what effect your actions will have on the human race in the year 3000.

Thus, there is an automatic discounting effect. An act is only as valuable as it's expected outcome. The expected outcome on the year 1,000,000 is almost always ~zero, unless there is some near-future extinction possibility, because the probability of you having a desired impact is essentially zero.