The main key to proliferation is concern about forest certification among consumers of timber and paper products. The annual average volume of emissions resulting from the reduction of forest and land-use change was 1.6 Gt of carbon ± 0.8 Gt of carbon, which is thought to be mainly due to deforestation of tropical forests. Furthermore, net capture by land ecosystems was estimated at 0.7 Gt of carbon ± 0.8 Gt of carbon, but this was considered mainly the result of an increase in forest stock in the Northern Hemisphere (Kobayashi, 2005: 30).