The temperatures required to melt the ice in a short timespan, and then the temperatures required to re-freeze it in a short timespan after that, would leave marks in other places in the world as well.
We also know for sure that such extreme events have not happened in recorded history, so at the very least you can trust the method for atmospheric carbon content in the last few thousand years - more than enough to prove man-made as the only explanation of these carbon levels, and to compare for climate effects.
We also know for sure that such extreme events have not happened in recorded history, so at the very least you can trust the method for atmospheric carbon content in the last few thousand years - more than enough to prove man-made as the only explanation of these carbon levels, and to compare for climate effects.