Could the bluish-green liquid sloshing around in a laboratory beaker save the world from climate change? The liquid is an algae-based bio-fuel, and scientist Steve Mayfield believes it is a sign that a post-carbon future is drawing closer.

If we're going to avoid catastrophic climate change, we will need to move away from fossil fuels as quickly as possible. But which alternatives are the most promising?

POST-CARBON FUTURES, a companion documentary to MR. CO2, examines the options from massive wind and solar projects, to re-engineering the planet itself, to more modest local efforts.

From his office in sunny California, John Woolard of Bright Source Energy sees the future in solar power. The company runs the largest solar plant in the US. Located in Nevada, it produces enough energy to power 15,000 homes. Woolard says we don't have an energy problem; we have a collection and distribution problem. But with 2,000 new cars hitting the road in Beijing every day, and China set to open a new coal-fired plant a week for the next decade, the truth is we will require far more energy than solar and wind energy can produce.

Some believe the solution lies in enormous, continent-altering projects such as a plan to blanket the Sahara in solar panels to produce electricity for Europe. Meanwhile, the developers of the proposed green city of Cao Fei Dian, 150 miles from Beijing, see the future in a city built from scratch on in-filled coastal land.

POST-CARBON FUTURES makes the case that we need a completely different approach to economic growth, and that perusing geo-engineering and huge building projects simply to maintain a consumer society makes no sense.

British environmentalist Tim Jackson, from the University of Surrey, and French writer Paul Aries both argue that our current economic system has trapped us into needing to constantly increase our emissions. Aries, a leading advocate of the "de-growth" movement passionately argues for a re-imagining of our economic system and not just cutting back on emissions but redefining prosperity itself.