When crocodiles roamed the Arctic - environment - 18 June 2008 - New Scientist

Oxygen isotopes in the shell fragments show that the waters around Antarctica 100 million years ago were a balmy 15 °C, compared with -2 to 0 °C today.
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It's abundantly clear that both the Arctic and the Antarctic were ice-free and warm from about 100 million to 40 million years ago. But until a decade ago, climate scientists struggled to explain how the Earth could have become so warm at the poles. Their models suggested it could only have happened if levels of carbon dioxide in the atmosphere were very high - turning the Earth into a sweltering greenhouse - but this would also have made the tropics extremely hot. Isotope ratios in marine shells, however, suggested that tropical waters were not much hotter than they are today.

As it turns out, the models were right and the shell studies were flawed.

What caused the Earth to get so warm?  Why didn't the Earth become a permanently uninhabitable fireball?  Note that 100 million years ago is quite "recent" in the context of a planet that may be 4.6 billion years old.

If life on Earth didn't end when Antarctic waters warmed by 15 degrees C, why do we think that warming by 2 degrees C would be catastrophic?  What makes us think we can prevent the Earth from again warming by 2 degrees C at some point?