Energy, naturally

Plants and trees do it. Algae do it. Even primitive bacteria do it. They capture energy from the Sun and use it to convert water and carbon dioxide into oxygen and carbohydrates, such as glucose. 

6CO₂ + 12H₂O + Energy → C₆H₁₂O₆ + 6O₂ + 6H₂O  

This is photosynthesis, and began on Earth over two billion years ago, changing our atmosphere from an oxygen-poor to an oxygen-rich one. In a two-stage process, water is first split into hydrogen and oxygen (reaction (i), sometimes referred to as the 'light' reaction because light is needed) and the hydrogen then reduces carbon dioxide to carbohydrates and other organic molecules (reaction (ii), the 'dark' reaction because no light is needed).  

H₂O → O₂ + H₂ (i) 

n  CO₂ +  n  H₂n → (H-COOH)  carbohydrates + organic molecules (ii) 

Today many scientists around the world believe that if this process could be reproduced in the laboratory and developed on an industrial scale, we would have the perfect solution to our energy problem. By 2050 scientists estimate that there will be an energy shortfall of 14TW (1TW = 10¹² W). With over 100,000 TW falling on the Earth as sunlight, the potential of 'artificial photosynthesis' is huge. Not only does it offer a route to sustainable hydrogen production and other useful fuels such as methane and ethanol, but it also removes CO₂ from the atmosphere, a key process with the potential to build other useful chemicals.