Paul Wheaton says don't make compost because carbon and nitrogen will be lost to the air. He says feed waste to animals. That may have some small effect on the gasification rate compared to composting but ultimately when the animal dies it still gases off. I liked what he said about putting waste under mulch, that would potentially have a nitrogen storing effect but the carbon would still gasify somewhat rapidly. He also says that he's not a fan of biochar. I'm wondering if that is because of the emissions produced when making the biochar or because of potential misuse of biochar resulting in crop failure.
Even though gases are produced during pyrolysis these gases are less than what would be produced if the wood where left to rot. Also by adding the products of slow pyrolysis to the soil more nitrogen can then be stored in that soil.
I don't even believe in the whole CO2 based climate change theory but I can see why if you did believe that you would like the idea of biochar if you truly understood the chemistry of it. Also biochar works if you do it right. Loading it with nutrients once it is made, using slow pyrolysis with some extra wood ashes mixed in with the wood that is to be pyrolysed, in order to promote the formation of larger molecules and reduce phytotoxicity.
On our home farm they/we have been making charcoal for use as fuel periodically for 30 years or so. Some of the Charcoal is lost in the soil and the results are obvious. The only negative result I have seen from char is one time we left a lot on the surface and the growth was retarded for a time until the PH/ N returned to normal. The land there produces copious amounts of every fruit and herb and we also get fodder for 6 sheep all from less than 1 acer. Biochar has amaizing potential for nutrient storage and the hydroxyl anions hold water and may help to draw water from the air into the soil and retard evaporation.