Without geo-engineering solutions, a certain amount of warming is irreversible. Even if we stop emitting CO2 today, the energy is in the system and it will cause a certain amount of warming.
Dr. James Hansen of NASA Goddard has talked a lot about this. He often says that there is already warming "in the pipeline" that is irreversible.
In a 2005 paper in Science Magazine he says:
Earth's energy imbalance. We infer from the consistency of observed
and modeled planetary energy gains that the forcing still driving
climate change, i.e., the forcing not yet responded to, averaged ∼0.75
W/m2 in the past decade and was ∼0.85 ± 0.15 W/m2 in 2003.
This imbalance is consistent with the total forcing of ∼1.8 W/m2
relative to that in 1880 and climate sensitivity of ∼2/3°C per W/m2.
The observed 1880 to 2003 global warming is 0.6° to 0.7°C,
which is the full response to nearly 1 W/m2 of forcing. Of the 1.8
W/m2 forcing, 0.85 W/m2 remains, i.e., additional global warming of
0.85 × 0.67 ∼ 0.6°C is “in the pipeline” and will occur in the future even if atmospheric composition and other climate forcings remain
fixed at today's values.
from Earth's Energy Imbalance: Confirmation and Implications (DOI: 10.1126/science.1110252)
There is inertia in the Earth System. The CO2 has been emitted, and the radiation balance is off. It will take a while for that warming to show up in the climate, but it will happen. The Earth System is out of equilibrium and is working towards getting to it, and it will be a warmer equilibrium.
However, this doesn't mean we can just forget about trying to reduce CO2 emissions because we're screwed anyway. This image from the IPCC AR4 shows the model predictions of warming for different CO2 emission scenarios. The yellow line represents if we could manage to get our atmospheric CO2 concentrations down to 2000 levels, and the red line represents a business as usual approach where we make no changes whatsoever.
IPCC Fourth Assessment Report, Working Group I: The Physical Science Basis
Projections of Future Change in Climate
You can see from this chart that the scenario we choose, and how much CO2 that we emit in the future, does make a difference in considering how much warming will occur. Reducing CO2 emissions is crucial for being able to mitigate the amount of warming that will occur, but at this point we've already locked in a certain amount of warming becuase of past CO2 emissions.