The problem with some modern science is that it is not unequivocal. Quite frequently proxies have to be used to quantify parameters, hence there is scope for argument and rejection. There is also inertia, coupled with peer pressure, and of course no one is a prophet in their own land.
Examples Wegener Continental drift   
         KT boundary and impact
         Bretz and the Missoula floods
         Firestone et al Cycles of cosmic catastrophe
Some of the above has largely been accepted, some is hotly contested.
Peer pressure and funding sets the agenda. There is a lot of wood to sift through before the trees can be seen.
Climate science is no different. Parameters can be measured reasonably accurately, the problem arises with the subsequent interpretation. Mix politics into the equation and the result is a witche's brew.
But past CO2 levels have never reached existing levels previously.

https://cdiac.ess-dive.lbl.gov/images/air_bubbles_historical.jpg