Core samples from glaciers in Greenland and Antartica. The gases caught in the ice enable researchers to measure the ratio of gases in the atmosphere.
Anyway - You are right natural effects will eventually overwhelm anything humans can do but that wasn't the issue.
The issue was that Fyron wrote - as it if were self-evident - that:
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we just did not cause the hole in the ozone layer over Antarctica. That was caused by massive volcanic eruptions.
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Even your two links refute that myth.
[ February 13, 2004, 20:34: Message edited by: rextorres ]