Geologic temperature record

The study of past temperatures provides an important paleoenvironmental insight because it is a component of the climate and oceanography of the time.

[1] The last 3 million years have been characterized by cycles of glacials and interglacials within a gradually deepening ice age.

[1] Similarly, the initiation of this deepening phase also corresponds roughly to the closure of the Isthmus of Panama by the action of plate tectonics.

This recent period of cycling climate is part of the more extended ice age that began about 40 million years ago with the glaciation of Antarctica.

In the earliest part of the Eocene period, a series of abrupt thermal spikes have been observed, lasting no more than a few hundred thousand years.

Geologists and paleontologists think that during much of the Paleocene and early Eocene, the poles were free of ice caps, and palm trees and crocodiles lived above the Arctic Circle, while much of the continental United States had a sub-tropical environment.

[6] This is likely to be the result of a favorable configuration of the continents during this period that allowed for improved circulation in the oceans and discouraged the formation of large scale ice sheet.

[citation needed] The Phanerozoic eon, encompassing the last 542 million years and almost the entire time since the origination of complex multi-cellular life, has more generally been a period of fluctuating temperature between ice ages, such as the current age, and "climate optima", similar to what occurred in the Cretaceous.

According to standard solar theories, the sun will gradually have increased in brightness as a natural part of its evolution after having started with an intensity approximately 70% of its modern value.

The initially low solar radiation, if combined with modern values of greenhouse gases, would not have been sufficient to allow for liquid oceans on the surface of the Earth.

This is known as the faint young sun paradox and is usually explained by invoking much larger greenhouse gas concentrations in Earth's early history, though such proposals are poorly constrained by existing experimental evidence.