EDIT: To FoL ^^
That's true, we don't have another earth and a time machine to be able to run experiments and see the results..
However they do have historical data going back a long time to be able to see what the natural cycles are like and the consequences of those, how much CO2 was in the atmosphere then and what temperatures did, etc.. And we're way outside those levels now, so as I said really the answer is no one knows for sure what all the effects will be, since the parameters are well outside the historical ranges, the ranges we care about anyway. They know when in history the oceans were 5m above where they are now, and what the temperatures and CO2 and other gasses were like then. And we know the excessive CO2 in the atmosphere now is from burning fossil fuels.
It's like astronomy, we don't have a star or a galaxy or a universe to be able to experiment on, however science can still be done.
And you are right, the cost of taking action has to be weighed. Against the cost of not taking action, which is difficult because you can't perfectly predict the future.
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