Unit 12: Earth's Changing Climate // Section 4: Past Warming: The Eocene Epoch
Scientists have looked far back in time to find a period when atmospheric GHG concentrations were as high as they could rise in coming decades if current emission trends continue. The Eocene epoch, which lasted from 55 million to 38 million years ago, was the most recent time when scientists think that CO2 was higher than 500 parts per million.
Fossil evidence shows that Earth was far warmer during the Eocene than it is now. Tropical trees grew over much larger ranges to the north and south than they occupy today. Palm trees grew as far north as Wyoming and crocodiles swam in warm ocean water off Greenland. Early forms of modern mammals appeared, including small creatures such as cat-sized horses whose size made them well adapted to a warm climate (Fig. 6). Without ice cover at the poles, sea levels were nearly 100 meters higher than today. The deep ocean, which today is near freezing, warmed to over 12°C.
Figure 6. Phenacodus, a sheep-sized herbivore found in the Eocene era
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Source: Courtesy Wikimedia Commons. Public Domain.
Scientists cannot measure CO2 levels during the Eocene—there are no ice cores because there is no ice this old—but from indirect measurements of ocean chemistry they estimate that atmospheric CO2 levels were three to ten times higher than pre-industrial levels (280 ppm). These concentrations were probably related to a sustained increase in CO2 released from volcanoes over tens of millions of years. Because this climate persisted for tens of millions of years, living species and the climate system had time to adapt to warm, moist conditions. If humans release enough GHGs into the atmosphere to create Eocene-like conditions in the next several centuries, the transition will be much more abrupt, and many living organisms—especially those that thrive in cold conditions—will have trouble surviving the shift.
A troubling lesson from the Eocene is that scientists are unable to simulate Eocene climate conditions using climate models designed for the modern climate. When CO2 levels are raised in the computer models to levels appropriate for what scientists think existed during the Eocene, global temperatures rise but high latitude temperatures do not warm as much as what scientists measure, particularly in winter. Some scientists believe that this is because there are unrecognized feedbacks in the climate system involving types of clouds that only form when CO2 levels are very high. If this theory is correct, future climate could warm even more in response to anthropogenic release of CO2 than most models predict.
The beginning of the Eocene also hosted a shorter event that may be the best natural analogue for what humans are doing to the climate system today. Fifty-five million years ago a rapid warming episode called the Paleocene-Eocene Thermal Maximum (PETM) occurred, in which Earth's temperature rose by 5 to 6°C on average within 10,000 to 30,000 years. Several explanations have been proposed for this large, abrupt warming, all of which involve a massive infusion of GHGs into the atmosphere, resulting in a trebling or perhaps a quadrupling of CO2 concentrations, not unlike what is predicted for CO2 levels by 2100 (footnote 4).