A new study concludes warm climate is more sensitive to changes in CO2 / November, 2016

"Using these estimates based on Earth’s paleoclimate sensitivity, the authors computed the warming over the next 85 years that could result from a human-induced, business-as-usual greenhouse gas emission scenario. The researchers project that by the year 2100, global temperatures will rise 5.9°C (~10.5°F) above pre-industrial values. This magnitude of warming overlaps with the upper range of estimates presented by the Intergovernmental Panel on Climate Change."

From "What would 3 degrees mean?" / September, 2010:

"Six degrees

• At the end of the Permian period, 251 million years ago, up to 95% of species were extinct as a result of a super-greenhouse event, resulting in a temperatures rise by six degrees, perhaps because of an even bigger methane belch than happened 200 million years later in the Eocene.
• One scientific paper investigating “kill mechanisms” during the end-Permian suggests that methane hydrate explosions “could destroy terrestrial life almost entirely”. Acting much like today’s fuel-air explosives (or “vacuum bombs”), major oceanic methane eruptions could release energy equivalent to 10,000 times the world’s stockpile of nuclear weapons."