William Rees, an urban planner at the University of British Columbia, estimated that it requires four to six hectares of land to maintain the consumption level of the average person from a high-consumption country. The problem is that in 1990, worldwide there were only 1.7 hectares of ecologically productive land for each person. He concluded that the deficit is made up in core countries by drawing down the natural resources of their own countries and expropriating the resources, through trade, of peripheral countries. In other words, someone has to pay for our consumption levels. [Emphasis Added]