The evolution of latitudinal temperature gradients from the latest Cretaceous through the Present

Latitudinal temperature gradients are a defining characteristic of the climate system. Using thermometric indicators, including δ18O, plant and animal fossils, glycerol dialkyl glycerol tetraether (GDGT) proxies, and clumped isotope estimates, we document marine and terrestrial temperature gradients for the latest Cretaceous, Late Paleocene-Early Eocene, Early Oligocene, Pliocene, and Recent. The changes in gradients reflect the transition greenhouse to icehouse conditions. The evolution of latitudinal temperature gradients in marine and terrestrial realms are similar but has some distinctive differences. Marine temperatures are generally warmer than those on land. Except for the Late Paleocene-Early Eocene, the marine records show distinct inflection points at ~30° and ~50° latitude indicating the existence of frontal systems in the ocean. Except for the Late Paleocene-Early Eocene, the marine records show an increasingly steeper trend, from latest Cretaceous through Recent, being most pronounced after the Early Oligocene greenhouse-icehouse transition. This trend reflects the increasing intensity of high-latitude and polar cooling as the icehouse state developed. During the Late Paleocene-Early Eocene the oceans were characterized by slightly warmer tropics and much warmer higher latitudes than at present. The continents have generally had tropical temperatures like those of today, varying by <5 °C. Higher latitude temperatures cooled during the latest Cretaceous, became much warmer during the Late Paleocene-Early Eocene, then cooled during the Early Oligocene and have become increasingly colder since then. The results suggest that there is a climate thermostat mechanism, probably related to greenhouse gas concentrations, that ameliorates tropical warming by redistributing warmth to the poles in the greenhouse world. That mechanism broke down as greenhouse gas concentrations declined resulting in the conversion from greenhouse to icehouse conditions.