Cenozoic deep-sea stable isotope record derived from analyses of  benthic foraminifera. The oxygen isotope record primarily reflects on changes in deepwater temperature and ice volume.

Source

Global temperatures in the Cenozoic

Detailed studies of deep ocean cores over many years have revealed the general pattern of temperature change through the Tertiary period. Scotland had a humid tropical climate until around 30 million years ago. Then global temperatures dipped sharply, triggering the onset of ice sheet growth in Antarctica. Temperatures recovered to subtropical levels in the early and middle Miocene but gradually declined thereafter into the Pleistocene. Although synonymous with the Ice Age, the Pleistocene perhaps should be seen equally as a period of dramatic climate shifts, changes which intensified around 1 million years ago with the first build up of massive ice sheets across the Northern Hemisphere.