Abrupt cooling at the Eocene/Oligocene transition (35 million years ago) marked the appearance of the first continent-spanning ice sheet on Antarctica, potentially the first Cenozoic Northern Hemisphere ice sheets, and the transition from hothouse to icehouse climate in the Cenozoic. However, much of the planet continued to experience climatic conditions warmer than the present, with cool temperate
forests characterizing Arctic highlands, and warm temperate forests covering much of northern North America and Eurasia.
Following the abrupt initial cooling, Oligocene climate continued the overall Cenozoic cooling trend as atmospheric CO2 levels fell to present values by the latest Oligocene. During this time, the driving influence of Earth's orbital parameters became increasingly evident in the waxing and waning of the Antarctic ice sheet, with frequencies that suggest modulation by not only eccentricity, but also the obliquity (variations in the axial tilt) of Earth's orbit.
Atmospheric CO2 concentrations remain near modern levels throughout the subsequent Miocene epoch and the variability in both ice volume and climate evident in the first half of the Miocene is also attributable to the action of Earth's orbital parameters. Orbital parameters are also believed to be responsible for the last gasp of the warm climates, the approximately 3-million-year-long (14-17 million years ago) Miocene Climate Optimum.
During the Miocene Climate Optimum, ice volume on Antarctica was low, subtropical forests flourished as far north as central Europe, and the Arctic Ocean returned to a seasonally ice-free state. The Miocene Climate Optimum ended with 1.5 million years of rapid cooling, after which global cooling continued more gradually. In the late Miocene, several notable climate events took place, undispu-table evidence for Northern Hemisphere glaciation, the establishment of largely modern vegetation zones, the establishment of a near modern latitudinal temperature gradient, and the inception of the Asian and Indian monsoons.
Global cooling continued through the Pliocene epoch (5.3-.8 million years ago), though global average temperatures in the Pliocene were approximately 5.5 degrees F (3 degrees C) warmer than at present. Peak Pliocene warmth occurred in the middle Pliocene from 4-3.5 million years ago, with Northern Hemisphere boreal forest extending to the Arctic Ocean, and deciduous hardwood forests of southern beech covering coastal Antarctica. In spite of this relative warmth, the global climate was cooling, and the late Pliocene, approximately 2.7 million years ago, is marked by the first major Northern Hemisphere cycle of glaciation and deglaciation. This first glacial cycle was modulated by a 41,000-year frequency that is characteristic of climate forcing by the obliquity and precession (the wobble of Earth's spinning axis) of Earth's orbit and a frequency that paced glacial cycles well into the Pleistocene epoch.
The Pleistocene (1.8 million-10,000 years ago) is the epoch commonly known as the Ice Age and was characterized by repeated waxing and waning of Northern Hemisphere ice sheets. For the first 1.2 million years of the Pleistocene, ice ages were paced by variations in solar energy associated with the 41,000-year obliquity cycle. Approximately 900,000 years ago, Northern Hemisphere ice sheets began to grow significantly larger and by 600,000 years ago, those larger ice sheets were waxing and waning with a periodicity of 100,000 years—a frequency that is not easily explained by simple combinations of Earth's orbital parameters. The reasons for this shift and the exact mechanisms driving this frequency in glacial cycles are two of the more challenging questions in ice age climate dynamics; neither is fully explained.
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