NOAA, Earth's Past Tropical Climate🌴🍍 The episode known as the Paleocene-Eocene Thermal Maximum (PETM) occurred 55-56 million years ago. During the PETM, the GLOBAL TEMPERATURE HAD RISEN BY AS MUCH AS 5-8°C (9-14°F) to an average temperature as high as 73°F. AROUND THIS TIME MUCH OF THE CONTINENTAL UNITED STATES HAD A SUB-TROPICAL ENVIRONMENT. This fossil palm is from Fossil Butte National Monument, Wyoming. Image courtesy U.S. National Park Service. Earth’s hottest periods—the Hadean, the lat