I first encountered the "ecological footprint" fifteen years or twenty ago in a Boston Globe article, and was immediately hooked. The article referred me to Redefining Progress (which now requires a modest subscription). There are so many environmental problems, from habitat destruction to water pollution, to over-fishing, to global climate change--and it's hard to keep them all straight in your mind at once: they all seem so different. The ecological footprint solves this problem--and quantifies them--by putting them all on the same footing: LAND AREA.
Land area is needed for farms to feed the world's growing population (and much more is needed to satisfy the developed world's love of meat). Land area produces our lumber, our paper, our reservoirs for freshwater. Land area (in forests) is needed to neutralize our air pollution and to soak up the vast amounts of carbon dioxide that comes from our power plants and cars. And the wildlife habitat we destroy in pursuit of picturesque places to plant our over-sized homes is also land area. All of the land area needed in all these ways to support your way of life is your Ecological Footprint. Voila! our environmental problems are tied together: we can compare them, and add them up, and--best of all--know how far we have to go.
(There are, of course, issues not so easily converted into this currency of land area; still, a great many are. Also, the differences land in land quality in different geographic locations must also be factored in.)
Right now I'm a long-term substitute teacher in Environmental Science, and rekindling my interest as I teach it, and watch the kids find out their own "footprint."
It is enlightening and sobering to take a "footprint quiz" that will evaluate your lifestyle and determine you personal footprint. More sobering still to realize that our collective human "footprint" is totally unsustainable--bigger, in fact, than the land area of Earth itself! As a species, we are living unsustainably: we are using up the earth--consuming the "capital" instead of only the "interest." Looking at the data nation by nation, we find that Americans have the biggest per capita footprint of any nation on Earth--such that it would take FIVE EARTHS to sustain humanity if everyone lived as we do! We are head-and-shoulders higher livers than western Europeans, despite similar quality of life. To make matters worse, the other ten million-odd species on earth must try to subsist on our one species' leavings--often unsuccessfully.
Footprint quizzes usually have a feature that allows you to recalculate your footprint if you were to make positive lifestyle changes (such as downsizing to a more fuel-efficient car, insulating your home, etc), but of course the real impact comes in collective action and political involvement. A sobering lesson is that even the most committed "green" American cannot live a sustainable life: there is simply too much greed, consumption and waste built into the very fabric of our society, and we must address it together.
To that end, it is well worth your time to look more deeply into the "ecological footprint" concept, the relative impact of different aspects of our society (no, I'm afraid recycling barely registers!), and the systemic changes we need to make to have any chance of turning the human juggernaut around. It is also interesting (for me, at least) to know a little of the ins and outs of how the numbers are calculated, the data sets used, and the assumptions that underlie the models.
Free versions of footprint quizzes include that of the Global Footprint Network, which includes a lot of comparative information.