50 million years ago, the Green River region was a lush, subtropical environment inhabited by tapirs, crocodiles, palm trees, and other organisms that are restricted to equatorial regions today.
© National Park Service, mural by Robert Hynes.
Fifty million years ago, our world was a different place. An atmosphere rich in greenhouse gases meant that global temperatures averaged around 82 degrees Fahrenheit (28 degrees Celsius), compared with just 57º Fahrenheit (14º Celsius) today. There were no polar ice caps. Forests inhabited by frost-sensitive plants and animals grew far north of the Arctic Circle. Much of what is now the United States was covered in dense tropical and subtropical rain forests.
The area of the Green River, a tributary of the Colorado River, is today cold sagebrush desert. Precipitation averages between 9 and 12 inches (23 to 30 centimeters) per year; most of it falling as snow. Winters can be extreme, with temperatures sometimes falling to –30 °F (–34 °C) or less. The mean frost-free period is only 59 days.
The fossils preserved in the ancient lake sediments show that 50 million years ago this environment was very different. Evidence of palm trees, tropical vines, alligators and crocodiles, and fish that today are found only in tropical regions suggest a much warmer and wetter climate, with rainfall from 30 to 40 inches (76 to 102 centimeters) per year and an overall average temperature of 60 to 70 °F (16 to 21 °C).
During the early Eocene, Earth’s atmosphere was much warmer and damper than it is today. There are two main reasons why this was so. First, the concentration of “greenhouse gases” in the atmosphere was much higher than today. Greenhouse gases absorb and emit infrared radiation, which tends to have a warming effect on our planet’s atmosphere. In the Eocene, the most important of these gases was methane, also called swamp gas or natural gas.
Second, the positions of Earth’s continents at the time allowed warm water to circulate into high latitudes, which prevented the formation of polar ice caps. Ice caps are important to the maintenance of Earth’s cooler climate today because they reflect sunlight that might otherwise warm the planet’s surface. The ocean-cooling effect of the ice caps also helps to keep methane trapped in ocean sediments and ice caps trap water that might otherwise enter the atmosphere and fall as rain. Around 60% of Earth’s freshwater is held in the Antarctic ice cap. This water was circulating freely through rivers, oceans and the atmosphere 50 million years ago.