The "carbon tax" is an example of understanding this. There are some industries and businesses which, by their very nature, will have a negative impact on the Earth. Yet, they may do all they can to limit their impact and it is still not enough. So they offset the damage they cause by preserving forests or helping to pay for research into methods which promise to reduce greenhouse gas emission over time.
Zero population growth is another scenario. By limiting your own family to two children seems as if it would help to bring population levels to sustainability, but not really. There will be people, for one reason or another, have as many children as possible. Even if every one in the world was part of a couple and only had two children you would be in an expanding population simply because of generational overlap.
And, let's face it, living your life simply to create a balance sheet that totals zero seems rather limiting. Why live a life directed to nothing? There have to be things an individual can do to add to the world...to its knowledge, its creativity, its spiritual sense. As long as a person uses air and water and resources of the Earth, one must feel obligated to not only affect the world so its a better place when leaving it.
So, one can have both a positive and negative effect on the environment. As a whole, because of the laws of physics, we all can never have a positive effect. But as individuals we may be able to have a net positive effect on the Earth by doing much more than the average Joe. Going for zero impact is great, but we should go farther...as not everyone will take this mission to heart. We must serve to offset the negative impact of others.
Some can do a lot themselves, others maybe not so much. Still others could care less...and only want as much as they can get while they are living.
I, for one, know I can never do enough, but I will keep thinking and re-directing toward limiting my impact on the Earth, justifying my existance and offsetting what resources I take with what I can give back (and then some).