Earth's climate from 60 million years ago


Global deep ocean temperature for past 60 million years
Global deep ocean temperature for past 60 million years
The history of atmospheric CO2 through the Cenozoic Era (66 million years ago to present day) provides a perspective for assessing an appropriate target for future CO2 levels. The paleoclimate record provides good evidence that Earth's temperature started cooling about 50 million years ago and that about 34 million years ago it had dropped by about 5 °C when Antarctica began to glaciate. Since then Antarctica has remained glaciated. The temperature 52 million years ago was about 14°C higher than the last ice age 20,000 years ago. CO2 concentration, which was about 180 ppm in glacial times, was much higher, about 1500 ppm, at the beginning of the cooling period leading to the glaciation of Antarctica. This is in qualitative agreement with evidence from a wide range of sources that has revealed a close correlation between temperature and atmospheric CO2 concentration. Quantitatively it has been determined that when the atmospheric CO2 level doubles, the Earth's surface temperature increases by about 3°C, with an uncertainty range of 1.5–4.5 °C.

Atmospheric CO2 over the past 60 million years


Greenhouse gas focing during the last 60 million years
Greenhouse gas forcing during the last 60 million years
Knowledge of Cenozoic CO2, in contrast, is limited to imprecise proxy measures except for recent ice core data. There are discrepancies among different proxy measures, and even between different investigators using the same proxy method. Nevertheless, the proxy data indicate that CO2 was of the order of 1000 ppm in the early Cenozoic but less than 500 ppm in the last 20 million years. Between 60 and 50 million years ago the Indian subcontinent moved north rapidly, averaging about 18-20 cm/year, through a region that long had been a center for carbonate and organic sediments. It is suggested that the subduction of carbon-rich crust provided a large source of CO2 outgassing. The warming climate peaked 50 million years ago when the Indian subcontinent collided with Asia. It is expected that CO2 then decreased because there was less subduction. Since then, the Indian and Atlantic Oceans have been major centers for the deposition of carbon, but subduction of carbon- rich crust has been limited mainly to small regions near Indonesia and Central America. Thus atmospheric CO2 declined following the Indo-Asian collision in parallel to the cooling climate.

Conclusions


Temperature and greenhouse gas forcing for last 400,00 years
Temperature and greenhouse gas forcing for last 400,00 years
It is argued that decreasing CO2 was the main cause of a cooling trend that began 50 million years ago and led to Antarctica glaciation about 34 million years ago. Evidence is presented that Earth was nearly ice-free until CO2 fell to 450 ± 100 ppm. Based on this paleoclimate evidence it is argued that atmospheric CO2 level of 350 ppm or larger, if maintained for a long period of time, would push the Earth toward an ice-free state. Because of its inertia the Earth's climate system has not yet responded significantly to anthropogenic inputs. The authors argue that the long term effects of elevated CO2 are more serious than has been thought. If the present elevated CO2 levels persists, there is a possibility of irreversible catastrophic effects. Target Atmospheric CO2: Where Should Humanity Aim?, James Hansen, Makiko Sato, Pushker Kharecha, David Beerling, Robert Berner, Valerie Masson-Delmotte, Mark Pagani, Maureen Raymo, Dana L. Royer, James C. Zachos, NASA Goddard Institute of Space Studies, The Open Atmospheric Science Journal , 2008, 2: 217-231