The Carboniferous is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period 358.9 million years ago, to the beginning of the Permian Period, 298.9 Mya. Wikipedia
Occurred: 358.9 (+/- 0.4) million years ago - 298.9 (+/- 0.15) million years ago
Era: Paleozoic Mean surface temperature: c. 14 °C; (0 °C above modern)
Good old Yahoo News. Always an impeccable source. From the original Guardian article;
"The study found that this doubling is the result, in part, by an increase in greenhouse gases and water vapor, as well as decreases in clouds and ice.
Researchers also said that a “naturally occurring” shift in the Pacific ocean from a cool phase to a warm one probably had a significant role in amplifying this energy imbalance."
Loeb did say, however, that this research provides only a glimpse in relation to long-term climate change, and, according to Nasa, that “it’s not possible to predict with any certainty what the coming decades might look like for the balance of Earth’s energy budget”.
Oh, what the hell, let's all get upset anyway. Have you noticed how the science deniers do such a wonderful job of ignoring context?