As a biosphere evolved that supports life on planet earth, toxic gases were locked away in various places, such as ice and permafrost.
The atmosphere is in delicate balance. Animal life absorbs oxygen and emits carbon dioxide. Trees absorb carbon dioxide and emit oxygen. This balance has been under assault for 150 years. Deforestation has reduced the ability of the planet to process CO2, while a carbon-based existence of 7 billion people pour CO2 into the atmosphere.
The CO2 buildup is believed to be the cause of the warming that is melting the polar ice caps and the permafrost. A sudden release of methane equivalent to 1,000 gigatons of CO2 could be the consequence. This is about as much CO2 as industrial civilization has released in 150 years.
There are many feedbacks, and once the process begins it feeds upon itself regardless of human measures. For example, the more polar ice is lost, the faster the warming.
Previous events that destroyed the balance in the biosphere resulted in life extinctions. The belief that a 7 billion population in a carbon-based culture cannot alter the balance in the biosphere seems to be wishful thinking not supported by science.