The term 'ecotourism', like 'sustainable tourism', is considered by many to be an oxymoron. Tourism in general depends upon and increases air transportation, contributing significantly to greenhouse gas emissions from combustion placed high into the stratosphere where they immediately contribute to the heat trapping phenomenon behind global warming and climate change. Additionally, "the overall effect of sustainable tourism is negative, where, like ecotourism, philanthropic aspirations mask hard-nosed immediate self-interest."[5]