While carbon dioxide occurs naturally in the environment, and, indeed, is essential to plant life, too much of even a good thing can be bad. Increased carbon dioxide levels, of course, are now infamous for leading to global warming (see the chapter on climate change for more on this), but near ground level this gas can also be poisonous to plants and animals. This became readily apparent during a 1990 incident in which carbon dioxide emissions from volcanic faults in California’s Inyo National Forest killed trees and caused tourists to become dizzy and lightheaded. The tourists became ill while inside cabins, where the levels of carbon dioxide had increased to 25 percent of the air content.