I'm a print designer. Knowing how much waste is caused by my work and how it precludes several industries causing harm to the environment in different ways, and considering that I am concerned about having a healthy environment, is it unethical for me to continue my practice? If I stop, others will still continue, and they will be joined by more; and there are plenty of other industries with even more environmentally harmful practices. Although there is a definite environmental impact from my work, there is a social acceptance of, and potential humanistic need for, my work. Does the latter override the former due to its immediacy?

Life is complicated, sure enough... My advice (for what it is worth--not much, I expect) is for you to hang on to your job. Partly, this is precisely because you quitting your job (unless you have some other very clear option available to you in a "clean" occupation) won't make the least bit of difference to the project of ending waste, for the very reasons you gave: someone else will do it, and all you will have done is put yourself out of work. Instead, why don't you consider--and urge your colleagues to consider--"greener" practices at work. For example, designs done on computers (rather than sketched on paper) create less paper waste. (An out-of-date example, I'm sure, but I hope you get the point.) Those with expertise in an industry are in the best position to find ways to cut waste and to come up with processes (and products) that are not so bad for the environment. Find these! And where you can't find better solutions, consider finding "compensations" such as planting more...

Why do people say that some things mankind does are unnatural? Isn't every human development natural because we are part of nature?

I agree with Peter Lipton that most cases of people associating unnaturalness with badness are cases that provide no such connection. Usually, however, I find in such faulty associations a false conception of what is and is not "natural." For example, many people have claimed that homosexual sex acts are "unnatural," for the reason that the biological function of sex is reproduction. So homosexual sex acts would be ones, to use Peter Lipton's expression that appropriated "something that has one function [reproduction] in order to perform a different function [giving and receiving pleasure, for example]." While I concede that homosexual sex acts cannot serve the function of reproduction (at least directly--I can come up with cases where it does so indirectly), I do not at all concede the claim that the natural function of sex is reproduction, since plainly that would make most sexual activities unnatural , including (but not limited to) kissing, caressing, oral or manual sex, and most of the...