This study analyzes the extent to which greenhouse gas (GHG) emissions may be affected by a plan to purchase private forestland for the expansion of carbon sinks, focusing on how changes in forestland ownership affect deforestation and urbanization and how subsequent changes in deforestation and urbanization affect GHG emissions, using South Korea as a case study. The results from ex ante simulations imply that carbon dioxide equivalent emissions could increase between 17.4 and 19.2 million tons with private forestland purchases from a constrained budget of $750 million, compared with an increase of 34.5 million tons without the purchasing plan.