The amount of carbon dioxide in Earth's atmosphere has reached the highest level in human history, exceeding 400 parts per million in April. Ice cores taken from Antarctica with air bubbles as old as 800,000 years has not revealed a level higher than 300 ppm. "If we fail to heed this warning, our children will end up living in a world that is much hotter than any human being has ever experienced," says scientist Ken Caldeira.
