Human emissions of the potentially harmful trace metal vanadium into Earth’s atmosphere have spiked sharply since the start of the 21st century due in large part to industry’s growing use of heavy oils, tar sands, bitumen and petroleum coke for energy, a new Duke University study finds.“Human emissions of vanadium to the atmosphere now exceed those from all natural sources combined — by a factor of 1.7,” said William H. Schlesinger, James B. Duke Professor Emeritus of Biogeochemistry at Duke’s Nicholas School of the Environment, who led the study.“Less than two decades ago, the ratio of human to natural emissions was 0.59 to 1, or less than half the current level,” Schlesinger said. “Our analysis suggests that much of this rapid rise can be traced to the increased use of unconventional heavy-petroleum fuels.”