Most carbon "transactions" in the usual carbon cycle cancel out. The CO2 that photosynthesis takes out of the air is eventually returned to it, whether the farmer’s green products are burned immediately (say, corn ethanol as fuel) or eaten. We exhale CO2 and excrete the stuff that sewage treatment plants bubble air through, so as to quickly make CO2. (This shortcuts the usual rotting process, which tends to generate unwelcome odors before releasing CO2 or methane.)

The timber industry would have you believe that one-cancels-the-other reasoning applies to trees as well. But there is a big difference between farming and forestry. For crops, it only takes a year until that released carbon is recaptured by another round of photo¬synthesis. For forestry, this loop may take fifty years.