More Words, Deeper Hole

Note having seen any of the books I can't say whether they are compatible with either of the hand waves below.

The earth is fairly close to runaway greenhouse already - on the scale of tens to hundreds of millions of years the sun is warning, and eventually even reducing the CO2 level to zero will keep the temperature at present levels. If an asteroid impact has the effect of triggering an increase in volcanic activity that will result the accumulation of atmospheric CO2 as long as the increased activity continues. Permafrost methane, methane clathrate and ocean-CO2 feedbacks add further CO2 to the atmosphere. If the wrong tipping point is reached rainforest collapse could also add to the CO2 and warming. Ice-albedo feedback also contributes to warming. Add in water vapour feedback, and perhaps the earth is uninhabitable before a new equilibrium is reached.

Alternatively, the impact vaporises a carbonate outcrop adding lots of CO2 to the atmosphere at once. (The ecological severity of the K-P impactor is said to have been increased because it hit carbonate-rich crust). The climate has inertia, so the earth warms steps by step as it approaches the equilibrium associated with the new CO2 level. The feedbacks mention above extend the magnitude and duration of the transition.