Costanza et al. (1997) estimated the current economic value of 17 ecosystem services for 16 biomes. The value of the entire biosphere, most of which exists outside of the market, is estimated conservatively to be between $16–54 trillion per year. By comparison, global GNP is approximately $18 trillion per year. The size of this figure demonstrates the significance of ecosystem services on human welfare and income generation, and the importance of identifying and recognizing this value. The valuation techniques used by the authors were often based on estimations of individuals' "willingness-to-pay" for ecosystem services.