Perhaps the single greatest threat to the environment is overpopulation, because it acts as a contributory factor to all other environmental problems. Whether you are concerned about habitat destruction, deterioration of fresh water, desertification, carbon levels in the atmosphere, or biodiversity, human population pressure is either a root or contributory cause.
Although in developed nations family sizes have fallen to or below replacement levels, in many parts of the developing world, population is still growing. Two things that can reduce population growth are access to affordable contraception and female education. The more educated and economically secure women are, the fewer children they have.
As a marketer, you can offer pro bono services to NGOs doing important work to improve quality of life and access to contraception in the developing world. Your marketing skills can be put to good use in fund-raising for non-profit organizations.