Why of course. All you need to do is change your system of ethics to make reduction of the population the most ethical action no matter the means. . . . . . . . . . . . . . . . . . . . . . .
What could be more ethical than killing off the worlds infidels in the name of God? Let us not forget that under this, life expectancy would drop to late twenties early thirties, Just like the middle ages. You know? The golden years of civilization. Who knows maybe there will be another plague and that will do half the work for us. Yey.
No? No takers?
Okay then I heard the earth is hollow could we just put all the poor people in there? Or send them to the moon and they can live on cheese. Make it voluntary so those who agree won't be missed. I'm not a monster after all. :D
If we cut funding to other environmental programs, we would increase funding to combat overpopulation, by providing family planning education, and free vasectomies for fathers after they have their second child. We got to get environmentalist talking about overpopulation again instead of only going after greedy corporations. We got to get mainstream media talking about overpopulation again, without the fear of backlash.
To reduce the human population, all we need to do is stop trying to interfere with natural selective mechanisms.
Stop trying to cure diseases. Stop vaccinating against fatal diseases. Stop feeding starving populations. Stop trying to reduce rates of infant mortality.
The solution is ugly, and includes incredible rates of suffering, but does not include the ethical violations of anyone trying to contribute to the deaths of anyone.
Arguably, it is ethically perverse to engage in activities that are guaranteed to contribute to the sustainment or increase of human populations in environments that these populations are unable to independently support and sustain themselves. The foreseeable result is even more people suffering and dying at a later date.
Instead of focusing on unnecessary and permanent things such as making fertile people infertile, we'll be able to start spreading ourselves to mars, and eventually out of our galaxy. I would argue that we do not currently have a population issue that needs to be solved, rather that it is inevitable that a over-population will occur if there are no unforeseeable and catastrophic events which solve it for us. I do agree that our attitudes should be as if we do have an over-population issue as one is inevitable, no matter what. There is no viable permanent solution unless we want to start enforcing rules as to who can and cannot reproduce or asking others to voluntarily give up their right to produce children. We just need to keep spreading and focus more on developing technologies which can help us better sustain ourselves. Space travel technology and anti-pollution technologies will be the only truly ethical means to addressing the population issue we have at hand.
Ethics are subjective. If your cause is the environment and overall goodness (at personal sacrifice), your ethics might differ from another person whose ethics might look unethical to you but are tightly coupled with his church due to religion. Ethic is a tricky word. "Dharma" is an even more complicated word from Eastern Philosophy but easier to explain with examples and implement. So I reply "No" to respect the whole question.
But if I reduce the question to is it possible to reduce population (and trying to fill in some blanks) and say that "we can do this humanely" it is possible. It is possible by first letting people understand that it is okay to not have children. While we constantly talk about a lot of people having children accidentally, due to need-of-hands-to-till-the-soil and gobbledegook such as biological-clock etc, most of the population is due to a lack of understanding that there is a choice. It is the same choice that drives you towards one form of lifestyle as the other, or makes you vote or choose between your favourite ice cream.
We need to enlighten top-down about the choice. By top I mean in the society in terms of power. Working bottom-up would also help accelerate the cause. Women being empowered in the poorest countries (the bottom of that pyramid) has always shown results. Its not easy, but then again that was not the question. :) Is it possible? Yes.
Any intentional action that prevents people from reproducing would be seen as unethical by various groups. For instance, various religious groups oppose the use of any form of contraception. A law that prevented their procreation would violate the religious rights. This means that any action to reduce population would always be unethical to them.