I get my water from the tap
Life cycle assessment refers to calculating the environmental impact of the entire life of a product or service, from manufacture to disposal ("cradle to grave"). Life cycle calculations are usually more of an estimation, because there are always uncertainties with the numbers used in the calculations - some assumptions have to be made.
The side bar of an article on the trendiness of the environmental movement lists some of the life cycle costs of a few items, in terms of oil. For example, it notes that if 10,000 people
"stopped buying bottled water for one year, enough oil would be saved to fuel four cars for that entire year. "Just some of the assumptions made for this estimation would have included the number of bottles of water being purchased, the brands of the four cars being fueled, and the distance the four cars were driven during the year such that that much oil was enough. Oil here is obviously representing energy. Therefore, according to the assumptions, it takes the same amount of energy to manufacture and transport those bottles of water as it does to power four cars.
But what if you assume that water was manufactured and sold locally, in recycled containers? The estimated environmental impact would probably be a lot smaller than if the same assumptions were used as for the above example.
If you're interested in doing some estimating based on assumptions yourself, I found an interesting article and accompanying spreadsheet that helps determine the most sustainable beer receptacle, the aluminum can or the glass bottle.
In the example of a car, an estimated ten percent of the total energy impact comes from its manufacture. Along the same lines, alternative fuel is usually a lot less efficient than it seems at first glance before the entire life cycle is examined, including the energy it took to create and distribute it.
Environmentalists use estimations like the bottled water example to get a point across, and get people thinking, and it usually works to serve this important purpose. But it is very important for mindful individiuals to think about the life cycle assessments, and the assumptions and uncertainties that go into them, before making environmental decisions. Most of the hard numbers you read really aren't that hard after all.

