They aim to grow trees and plants to absorb CO2 and then trap the carbon by turning the resulting biomass into “biochar”, a fine-grained form of charcoal that can be buried in the soil, keeping it safely locked up for thousands of years. The pre-Columbian Indians used biochar to make the poor soils of the rainforest – which otherwise quickly become exhausted – productive for harvest after harvest. It is still there today, many hundreds of years later, forming islands of black fertile earth in the otherwise unpromising ground. … Trees and plants soak up carbon dioxide as they grow, but release it again as they are burned or left to rot. But burning them largely in the absence of oxygen, through pyrolysis, reduces the amount of the gas emitted by 90 per cent, and stores the carbon in the charcoal instead. It also gives off energy that can be used as an efficient biofuel. If the resulting biochar is then buried in the ground it will stay there for some 5,000 years, keeping the carbon out of the atmosphere, and nourishing the soil while it is there. It also cuts down on the use of fertilisers; reduces the emission of methane and nitrous oxides, which are also greenhouse gases, from the ground; filters out pollutants; and retains water, thus combating flooding.