- Reducing our reliance on petrochemicals, our carbon footprint
- Reducing our consumption of natural resources, our ecological footprint
- Reducing our consumption of fresh water, our water footprint
A more comprehensive measure is ecological footprint, how many "worlds" worth of earth's resources we consume, or how many "worlds" worth of earth's resources would be consumed if all earths inhabitants lived like us. The science behind ecological footprint isn't as likely to be challenged, because it is less well-understood. Besides, the slogan "Reduce, Reuse, Recycle" is pretty catchy and obvious. We are "sustainable" if we use only as many resources as the earth can renew in a single year. Currently humans worldwide consume resources it takes the earth 1.5 years to produce.
More recently the United Nations has begun focusing on water use. When it comes to water, there is no fossil fuel resource we can tap. The fresh water available is a finite resource, and mankind is not doing a good job of managing this resource. An old saying goes "Beer is for drinking, water is for fighting." Many of us forget that water is limited because all we have to do is turn a knob and water gushes forth. But lack of water is the natural "disaster" responsible for the greatest number of premature deaths across the globe.
When I grow food using aquaponics in my own backyard, I am arguably saving fuel relative to what I'd "spend" driving to the store. But as researchers have carefully measured and I've observed, aquaponics uses only 1/10th the water required for traditional gardens.
Aquaponics might not strike everyone as sustainable, since some of the components must use electricity or at least be made from relatively modern materials. However once you have your infrastructure in place, aquaponics is one of the most sustainable or "green" ways to grow food, particularly from the standpoint of saving water.