Scientists have determined that all the crude oil we pump from the earth came from ancient algae. That's right, even petroleum is a solar energy! It took eons for algae to slowly grow and absorb carbon from earth's atmosphere and then die and become geologically lock away until we exploited it. In so doing, we've essentially released millions of years worth of carbon into our atmosphere in just the last 150 years. This continuous release of ancient carbon into our atmospheric is not environmentally sustainable.
Many now believe we can grow algae and extract the oil ourselves. The act of growing algae absorbs carbon, while the act of burning algal oil releases that same carbon. We in effect will create a very short environmentally sustainable carbon cycle, of just a few weeks.