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Re: "If this is true, then at least one Global Warming event occurred without human intervention, and given that there is a warming event going on now, what is unique about this event that makes it possible to conclude it is caused exclusively by human activity?"
In terms of global warming and cooling trends, there is nothing that unique about it. The Earth has warmed and cooled many times in the past 3 billion years. this time humans are causing contributing the greatest amount to the warming, but it has happened from other causes as well. And each time the climate changed in the past many species went extinct.
The "uniqueness" about this time is that Humans may be the ones to go extinct this time.
The Earth will go on just fine without us, of course. Other species will evolve to fit the niches left open, producing many wonderous and unique living beings. Just humans won't be around to appreciate them.
Nothing unique at all.