Special Series: What's the deal with offsets?Photo illustration by Tom Twigg / GristImagine a system that can: (potentially) store billions of tons of carbon in soil for centuries; dramatically reduce agricultural waste, forest debris and some municipal solid waste, thus eliminating the production of greenhouse gases that result from their decomposition; generate energy to both power itself and a surplus for use in surface transportation or electricity generation; and greatly increases the productivity of agricultural soil, thus reducing the need for expensive and polluting fertilizers. This is the promise of biochar -- the carbon-rich remains of "burning" organic matter via …