But the team’s results aren’t unambiguous, he notes. The krypton and xenon in today’s atmosphere may, for example, be a mix of isotopically light gases delivered by comets and the heavier ...

Small amounts of a substance can have a big impact. For example, the percentage of CO2 in the atmosphere is much higher than the percentage of arsenic per body weight necessary to kill a person.

The Story of the Earth's Atmosphere. By Douglas Archibald, Fellow and some time Vice-President of the Royal Meteorological Society. Pp. 208. (London: George Newnes, Ltd., 1897.

"How much carbon dioxide is the problem," he asks. "How much carbon dioxide is in the atmosphere? It's 0.04%. And of that 0.04%, human beings around the world create 3%. And of that 3%, Australia ...