An anonymous reader shares a report: If we do nothing to reduce our carbon dioxide (CO2) emissions, by the end of this century the Earth will be as hot as it was 50 million years ago in the early Eocene, according to a new study out today in the journal Nature Communications. This period -- roughly 15 million years after dinosaurs went extinct and 49.8 million years before modern humans appeared on the scene -- was 16F to 25F warmer than the modern norm. [...] During the Eocene, it took more atmospheric CO2 to influence temperatures than it does today. In fact, if we don't change our behavior, 2100 will be as hot as the Eocene with much less atmospheric CO2 than was present at the time. A hotter sun means we get more bang for our CO2 buck. "Climate change denialists often mention that CO2 was high in the past, that it was warm in the past, so this means there's nothing to worry about," said lead study author Gavin Foster, a researcher in isotope geochemistry and paleoceanography at the United Kingdom's University of Southampton. "It's certainly true, that the CO2 was high in the past and that it was warm in the past. But because the sun was dimmer, the climate wasn't being forced as much [as it will be] in the future if we carry on as we are."