Oceanic Thermal Inertia
The primary reasons we are not heating up faster include oceanic thermal inertia and industrial negative aerosol forcing.
This is another two edged sword. Similar to the aerosol dilemma but different in its context. In the case of oceanic thermal inertia, the good news is that because the oceans are so large, and take so much time to absorb the thermal energy, we are warming more slowly than would otherwise occur.
The bad news is that the oceans not only take up heat slowly, the also dissipate heat slowly. So even if we are able to reduce the greenhouse gases in the earth atmosphere to reasonable levels (closer to 300ppm CO2) the thermal inertial of the oceans will still take quite some time to respond and cooling down the earth will take considerable time.