The concept of sustainability was originally defined as « meeting the needs of the present generation without compromising the ability of future generations to meet their needs » (United Nations Brundtland Commision, 1987). Following this definition, something is either sustainable or not sustainable; sustainability cannot be “improved” or “worsened” (or it is merely the time before depletion that changes).
We are faced to the issue of sustainability because we live in a finite world, with limited amounts of the natural resources we depend upon. When we get rid of rubbish, we often condemn useable material to idleness, which brings us to constantly extract raw material.
Recycling means using raw material that has already been extracted; this reduces the need to extract even more of it to meet consumption wants. Simply enough, to be sustainable or to avoid depletion, the rate of extraction must be brought lower or equal to the natural replenishment rate.
Our use of forests is a good example of unsustainability : world forest cover has been shrinking quickly in the last decade, jeopardizing future generations' ability to enjoy its services (not only generations of humans, but also generations of animal forest dwellers). By recycling forestry products, such as paper and cardboard, we reduce the pressure put on forests and do not impede the latter's ability to replenish. In addition, making paper products form recycled fibers is much less energy demanding.
However, here's the drawback :
Products made from recycled materials are often of lesser quality than the object for which the material was previously used. This is called downcycling; the idea that raw material cannot be reused infinitely is intrinsic to it. Also, recycling is an unsustainable activity if it engenders the excessive use of other resources (e.g. Oil for transportation, or energy for transformation). This may occur for certain materials that are shipped overseas to be managed in developing countries at a lower [financial, not environmental] cost.
Still, most of the time, recycling is effective in reducing the demand for raw products and energy. This is especially true when the recycled objects are made from substances that takes very long to regenerate, such as oil (of which all plastics are made).
For recycling to be unquestionably sustainable, it is necessary to review the way the material is transformed and re-processed. And for that reason, reusing before recycling is even better as we aim for sustainability.
For more information :
Recycle Everything: Why We Must – How We Can (Janet Unruh, 2010)
The Story of Stuff (Annie Leonard, 2010)