Al Gore compares carbon composition of Earth with Venus, advising that
carbon pumped into Earth's atmosphere will result in a feedback loop that
could turn the Earth into a Venus-like hothouse. "The habitability of this
planet for human beings really is at risk" he warns.

The Paleocene-Eocene Thermal Maximum (PETM) is believed to have been caused
by a huge release of carbon into the Earth's atmosphere. Already-warm global
temperature was quickly raised 4-8°C, with CO2 levels believed to have been
considerably higher than today. During this period the Arctic was ice-free,
with oceanic surface temperatures 22°C. Subtropical life flourished where
there is now an ice cap. Modern mammals evolved on land.

Assuming Gore's Earth-Venus analogy is correct, what factor(s) kept Earth
from becoming an uninhabitable hell during the PETM that is(are) no longer
relevant today?