Ironically what needs to be sustained, individual human life, is fundamentally unsustainable.  Society, however, as made up of communities can be sustainable. 

In order to survive into and beyond the 21st century in an age of declining natural resources we need to change the way we live. This does not mean developing new technology.  We need to begin organizing ourselves as small communities in order to become self-sufficient by: getting to know our neighbors, growing our own food or buying locally, reducing our dependence on non-renewable resources, protecting our environment and creating local job markets.