A recent study (summarized here) concludes that the biggest contributors to our ecological footprint are population size and consumption.
The ecological footprint is a rough and somewhat abstract measurement of the impact humans individually and collectively have on Earth. It is, as defined in the aforementioned article, "a quantitative measurement of the stress placed on the environment by demands for available lands and resources to meet the need for food, housing, transportation, consumer goods and services." There is strong evidence that we are currently overstressing the planet by leaving an ecological footprint that is larger than the productive area of the poor overworked planet.
The study is rather optimistic, suggesting, for example, that it is within the realm of possibility that we will reach the Millineum Development Goals of the UN. I won't pretend to know what that means. But I will suggest that this news should not encourage us to sink comfortably into our SVUs and go about our Sunday shopping. Right now the U.S. has a footprint of 1.4, that is, 1.4 times the sustainable level. It is the country with the largest footprint, and though the study's authors do not expect that to be true in 2015, this is not because the U.S. is expected to dramatically reduce its footprint by then. Rather, China and India are expected to outpace the U.S., and to help bring our global footprint to 1.6. This is depressing. Even bringing our global footprint down to 1 means we would be using exactly the land and resources our environment can sustain. Meanwhile, the population grows and global warming reduces the amount of land we have to live on.
I am, however, encouraged by one aspect of the study: the two most significant contributors to our downward-spiraling global environmental dilemma are the two things we can each easily contribute less to on an individual level, by simply reducing our daily consumption and limiting our procreation.