Cenozoic Era

The Cenozoic era is the most recent of the nine eras in geological time, and extends from 65 million years ago to the present. The Cenozoic era catalogues extensive climate changes, ranging from hothouse climates with warm-temperate to sub-tropical forests near both the north and south poles, to icehouse climates with ice sheets kilometers thick covering much of the high latitudes in both hemispheres. The first 30 million years of the Cenozoic were the warmest of the era and were characterized by several extreme warmth events.

The first of these climate maxima was the Initial Eocene Thermal Maximum, which occurred approximately 55 million years ago. Geological evidence suggests that at this time, atmospheric CO2 concentrations soared to nearly 20 times current levels, and temperatures in the Arctic Ocean approached those of a comfortable swimming pool, approximately 68 degrees F (20 degrees C). This climate maximum persisted for 50,000—100,000 years, before the climate cooled and then continued a gradual warming trend through the longest period of sustained hothouse warmth in the Cenozoic, the Early Eocene Climatic Optimum.

Early Eocene Climatic Optimum

The Early Eocene Climatic Optimum lasted for over 2 million years, and was characterized by warm and equable (meaning the climate was relatively similar everywhere) conditions. Deciduous, temperate forests covered Antarctica, and palm trees marched north across Wyoming and into Arctic Canada. Summer temperatures in the Arctic Ocean were approximately 59 degrees F (33 degrees C), almost 30 degrees F (17 degrees C) warmer than today, while ocean surface temperatures in the tropics were hardly different (at most, 9 degrees F, or 5 degrees C warmer) from those at present. This low equator-to-pole temperature gradient, with tropical and subtropical climate zones spanning much of the globe, was a notable characteristic of early Cenozoic hothouse climates, and understanding the mechanisms by which such a low temperature gradient would be maintained is one of the greatest challenges in paleoclimate science.

Research on the early Cenozoic era, when an extensively changing climate caused tropical forests to grow in Wyoming and ice sheets to cover a large amount of the high latitudes of both hemispheres, is revealing new insights into this extreme era.

Long thought to be a time of gradual cooling, scientists have recently discovered additional [Page 213]thermal maxima during the later stages of the Eocene. The occurrence of all Eocene thermal maxima appear to be modulated by the eccentricity of Earth's orbit (how much the shape of a planet's orbit deviates from a circle), and these new insights into early Cenozoic climate suggest that hothouse climates are as dynamic as the icehouse climates of the later Cenozoic.