The study of the environmental impact of war focuses on the modernization of warfare and its increasing effects on the environment. Tactics such as scorched earth methods have been used for much of recorded history; however, the methods of modern warfare cause far greater devastation on the biophysical environment. The progression of warfare from chemical weapons to nuclear weapons has increasingly created stress on ecosystems and the environment. Environmental toxicants like heavy metals linger for numerous decades in the environment, and in dry areas can be transported across large distance. Genotoxic effects as in the case of Agent Orange can affect multiple generations.