## 2 Ecological Footprint and Levels of Ecological Consumption  
Despite some criticisms of its rationale and methodology, the EF is one of the most popular and inclusive indicators of ecological consumption [12; 13]. Some authors argued that the EF is now the most widely used indicator in sustainable development research [14]. The EF measures ecological consumption by calculating the area of biologically productive and mutually exclusive land and water that is required to provide the resources a population demands and to absorb the corresponding wastes in a given year [8]. The EF consists of grazing land footprint (providing animal-based food and other animal products), cropland footprint (providing plant-based food and fiber products), fish product footprint (providing fish-based food products), forest product footprint (providing timber and other forest products), carbon footprint (providing carbon uptake land for absorption of anthropogenic carbon dioxide emissions), and built-up land (representing ecological productivity lost due to occupation of physical space for shelter and other infrastructure) [15]. The EF measures humanity's final demand on a wide range of ecological resources and services from the consumption side (EF\({}_{consumption}\) = EF\({}_{production}\) + EF\({}_{imports}\) - EF\({}_{exports}\)) [16]. Therefore, the land and water to be calculated are not only within national borders but also outside national borders. Because EF values vary greatly with consumption behaviors and habits, it is not difficult to understand global ecological impacts of individual daily lives with the use of the EF [17; 18]. The EF is a biophysical rather than monetary accounting approach to measuring ecological consumption. The measuring unit of the EF is global hectares (gha) per capita. A global hectare represents an ecologically productive hectare with global average biological productivity. Another prominent advantage of the EF is that it has a counterpart, i.e., the biocapacity (BIO), which measures the theoretical maximum capabilities of ecological systems to meet humanity's demands for ecological consumption. The measuring unit of the BIO is also gha per capita. Comparing national EF to globally available BIO provides a quantitative criterion to assess whether national ecological consumption exceeds globally average ecological capacities. For countries, if their EF values are higher than globally available BIO values, they are countries with an ecological deficit; otherwise, they are countries with an ecological surplus. Data of the EF and BIO were obtained from the National Footprint Account results (2019 Edition) provided by Global Footprint Network (GFN). Figure 1 depicts temporal trends of the global EF and BIO from 1961 to 2016. The globally available BIO values declined continuously from 3.12 gha to 1.63 gha per capita. The global EF values increased from 2.28 gha per capita in 1961 to 2.87 gha per capita in 1973 and fluctuated between 2.54 gha and 2.87 gha per capita for the time period 1973-2016. Following 1970, the global EF values were larger than the globally available BIO values, which demonstrates that humanity's ecological consumption exceeded the regenerative and absorptive capacities of natural capital and that humanity is living in a state of an ecological deficit. By calculating the ratio of global EF to globally available BIO, how many Earths are needed to support humanity to be ecologically sustainable could be obtained. Figure 2 depicts evolution of the \"number of Earths\" needed. In 1961, 1970, 1980, 1990, 2000, 2010, and 2016, 0.73, 1.00, 1.19, 1.29, 1.38, 1.67, and 1.69 Earths were needed to support humanity to be ecologically sustainable, respectively. Before 1970, one Earth was sufficient to support humanity to be ecologically sustainable. Since 1970, we needed more than one Earth and, in general, more Earths. Figure 1: Temporal trends of global ecological footprint (EF) and biocapacity (BIO) (1961–2016).