Biodiversity refers to diversity within the biosphere, including genetic, species, and habitat diversity. The concept emerged in the 1980s with the rise of biotechnology—and the consequent commodification of biological resources—and conservation biologists' growing awareness of an increasing species extinction rate. Increases in the numbers of extinct species and species threatened with extinction present a global biodiversity crisis, which some scientists believe constitutes the Earth's sixth episode of mass extinction, and the first caused by human activities. Causal explanations vary, but often include disparities of wealth and poverty, insecure property rights, overpopulation, habitat loss, and climate change. Geography is critical to understanding biodiversity, including its global distribution, our scientific knowledge about it, and its vulnerability. Biodiversity touches nearly every aspect of human affairs in the 21st Century, including environmental sustainability, poverty and social justice, medicine and healthcare, agriculture and food, and economic globalization. As a consequence, a wide range of critical assessments of the concept has emerged. Three areas of concern stand out: social justice and proprietary rights; uncertainties in the study of biodiversity; and the effectiveness of biodiversity protection efforts.