Relationships of the Environmental Performance Index to Six Interrelated Variables in Nations Around the World

In recent years, scholars at Yale's Center for Environmental Law and Policy and its Data‐Driven Environmental Solutions Group, with others at the Columbia Center for International Earth Science Information Network, have devised an Environmental Performance Index (EPI) to compare different nations. Their 2018 report (Wending et al. 2018) evaluates performance of EPI under two objectives—Ecosystem Vitality and Environmental Health—that are divided into 10 issues that have one or more indicators, as follows, and are shown in Fig. 1. Under the issue of Climate and Energy, the EPI is based on climate‐change performance indicated by trends in carbon intensity and emissions of methane and other compounds. Under Biodiversity and Habitat, there are six indicators employed. Two relate to terrestrial protected‐area metrics: national biome weights proportional to their areas and global biome weights weighted by their global abundance. These evaluate a country's contribution to protecting habitats that are rare or threatened worldwide. A species protection indicator reflects efforts by countries to protect mammals, birds, and amphibians in their actual ranges. Marine protected areas are also evaluated, as are species habitat and protected areas. The Fisheries issue measures the proportion of a country's total catch, within its exclusive economic zone, that comes from fish stocks that are overexploited or collapsed as well as the mean trophic level of fish caught by a country. The Forests issue is based on loss of tree cover. Areas considered must have at least 30% canopy cover. The Agriculture issue is evaluated by an index that gauges nitrogen use efficiency and crop yield. Water Resources are measured as the proportion of wastewater collected from all sources, weighted by the number of people in the sewage network. Air Pollution combines two indicators, one measuring nitrogen oxide and another addressing sulfur oxide. This objective has three issues. The first, Water and Sanitation, reflects the proportion of a country's population with access to a facility that protects water from outside contamination, particularly by fecal material. It also reflects the proportion of a country's population that has access to toilets providing safe treatment of human fecal material. The second, Air Quality, is treated under three indicators. Two indicators focus on fine particulate matter, one on chronic exposure and the other on acute exposure that exceeds WHO thresholds of 10, 15, 25, and 35 micrograms per cubic meter. The third issue, Heavy Metals, is represented by lead