Okay, TheU, I think you're confused about something. The issue is not that the climate is changing. You're right - the earth is 4 billion years old and the climate has varied hugely in that time.
The issue is the rate at which the climate is changing. In the past, increases and decreases of temperature would take place over thousands of years. More than enough time for the biosphere to adjust, migrations to occur, evolution to work its magic, etc.
Now we're talking about exactly the same kind of temperature fluctuations occurring in the span of decades. Now, it's possible that these kinds of rapid fluctuations have happened in the past, but as far as I know there isn't much or any evidence to support that.
So that's the problem - not that we are changing the climate, because you're right the earth will survive, - but how fast we're changing the climate.
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