At one point in time, the goal was to remain below 1.5C heating (I forget over which time frame), with the worst effects kicking in at 2.0C. I believe one of the recent IPCC reports suggest to stay near 2.0C, we have to sequester carbon using a process that's not invented yet.
I believe that current thought is that we will reach 2.0 C of heating even if we stop fossil fuel usage, entirely, tomorrow.
My post was pretty pessimistic, but the reality is pretty bad. The reason that all that carbon was sequestered prior to burning it is that plant life existed before fungi for a significant amount of time. Plants would sequester carbon, die and fall, then remain and not rot.
Today, sequestering carbon can only be done by adding biomass. Trees sequester carbon until they die, then release all of the carbon back into the atmosphere (either quickly in a forest fire or slowly as they rot). Existing forests really aren't doing much sequestering once they reach steady-state biomass (growing trees balanced by rotting trees).
I have no idea what the cycle is in the ocean, though. I know it's 70% of Earth's photosynthesis. Maybe the situation is not quite as dire.
The future is uncertain, perhaps humanity will figure out methods to mitigate things. There are thoughts that injecting synthetic volcano ash into the atmosphere might be feasible with today's technology. This would emulate the cooling effect seen with volcanic eruptions that reach high enough. The effect can last a couple of years.