[jbattle]

Yeah there are concerns about methane being released from melting permafrost. I can't find information about how old the permafrost is - but my understanding is tens of thousands of years old. I also can't tell if the methane itself is old, or if the methane just forms rapidly with all this newly thawed rotting vegetation.

https://www.vox.com/2017/9/6/16062174/permafrost-melting

There's evidence of times in the distant past where climate has changed very quickly, but all those predate history. There are some that even predate humanity. Here's an interesting one from ~12k years ago:

https://en.wikipedia.org/wiki/Younger_Dryas

[paypalcust83]

20 megatonnes/year is small potatoes compared to methane hydrates' 1-5 teratonnes storage that could go within a decade. There is at most 1.5 teratonnes of methane in arctic permafrost, but that will take much longer to release (50-100 years at a very conservative minimum) and it will degrade with a 7 year half-life.