At a red-brick factory in the German port city of Hamburg, cocoa bean shells go in one end, and out the other comes an amazing black powder with the potential to counter climate change. The substance, dubbed biochar, is produced by heating the cocoa husks in an oxygen-free room to 600 degrees Celsius (1,112 Fahrenheit). The process locks in greenhouse gases and the final product can be used as a fertiliser. This is what Silvio Schmidt, a 45-year-old farmer based near Bremen, to the west of Hamburg, is experimenting with in his potato field. After spreading biochar over part of his field at the beginning of April, he hopes the fertiliser will help "give more nutrients and water" to his sandy soils and enable him to make transition to organic farming. IMAGES AND SOUNDBITES ARRANGED IN SEQUENCES