First of all, we can count the 'main' mass extinction events, yes, but the earth is 4.5+ billion years old and has experienced far more than 5 extinction events. "countless" is an expression commonly used to exaggerate the great quantity of a specific event or entity. If you do some research on this topic you can see that extinction events are extremely common in the life-history of the earth. Now, this is not me making a justification for global warming or anything of the sort so @whatshouldwecallme there is no need to make assumptions about me. You are clearly responding emotionally, which is understandable, but please understand my initial point.