I found an interesting article over at Slashdot regarding one of the worst Ice Ages ever. This happened around 2.3 billion years ago when blue-green algae, or cyanobacteria, gained the ability to break down water. This resulted in a huge flood of oxygen into the atmosphere which broke down atmospheric methane.
Temperatures plunged to minus 50 degrees Celsius, and ice at the equator grew to 1 mile thick. Although this process took several million years, substantial damage to the methane layer could have occurred in the first 100,000 years.
Life-forms only recovered after microorganisms, clinging then to thermal vents or living underground, evolved the ability to consume oxygen and turn it into carbon dioxide.