An amount of photosynthetic biomass such as wood is converted into coal such as charcoal and a substantial amount thereof is permanently stored until the atmospheric CO.sub.2 level is reduced to an extent which is unable to cause a greenhouse effect. While a main portion of the charcoal remains in permanent storage, a fraction thereof may be retained prior to or retrieved during or after achievement of the reduction in the atmospheric CO.sub.2 level. Such fraction is converted into energy or an energy source, for example hydrogen, in a manner such that thereby the atmospheric CO.sub.2 is maintained at a level which does not cause an undesired greenhouse effect.