Geologists generally agree that about 33.5 million years ago Earth cooled significantly during a period known as the Eocene–Oligocene transition. How this cooling occurred and its prevalence throughout the globe is still a matter of debate.

Geologists investigating this transition typically gather data from marine environments, but two independent groups wanted to understand the changes on land.

Alessandro Zanazzi and his colleagues (see page 639) looked for clues in fossil bones and teeth from the Toadstool Geologic Park in northwestern Nebraska. They found that the fossils' composition of oxygen isotopes, which correlates with the temperature at the time the animals were living and during fossilization, revealed a temperature drop of 8 °C.

Guillaume Dupont-Nivet and his co-workers (see page 635) wanted to find the cause of the disappearance of lakes around this time in the region that is now the Tibetan plateau. Using known reversals of Earth's magnetic field to date rocks from the plateau, they found that the lakes' disappearance was recorded in sediments deposited in different parts of the region. This made it unlikely that tectonic-plate-induced mountain building caused the lakes to vanish. They surmise instead that the aridity during the Eocene–Oligocene transition dried up the lakes.