It has been proved that the earth in other epochs has had global average temperatures higher than now and colder than now.
However none of those periods had human beings interfering with and polluting the atmosphere.
yes, the planet is getting warmer, and the sea levels are rising. Whether this is just part of a natural cycle, being helped along by human activity, or caused by human activity alone cannot be definitely proved using only the data we have at present.
The point is that if it IS human activity causing it, we cannot put things back again, once the damage is done. It's like saying let's not bother putting emergency exits in a theatre as you cannot prove that this particular theatre is definitely going to burn down.
We cannot take such risks with the only habitable planet we know of. What if it is our fault? Laissez faire in this situation could turn out to be mass suicide by dithering over details.
HB