The Northern Hemisphere cycle of ice ages began 2.6 million years ago. In the geologic time scale this corresponds to the Quaternary Period as defined by the International Commission on Stratigraphy. During this time the Earth was characterized by alternating glacial and interglacial periods. Initially the period of these cycles was about 40,000 years, seemingly linked to the 41,000 year period of Earth’s tilt (Milankovitch cycles). About 1.2 million years ago this began to change. During the mid-Pleistocene transition (MPT), the periods of the glacial-interglacial periods became longer and since 800,000 years ago average roughly 100,000 years. Evidence from Antarctic ice cores has revealed a close correlation between surface temperature and atmospheric carbon dioxide concentration for the past 800,000 years excluding the immediate present.
Until recently it was thought that 800,000 years was the limit of the evidence that could be provided by Antarctic ice cores. In 2017 research showed that the so-called blue ice in the Allan Hills of Antarctica extends the ice record to 2.7 million years ago. This ice has been exposed as the result of complicated processes and does not represent a continuous record, but snapshots of temperature and atmospheric conditions can be measured and dated using Argon-40 dating. The latest research has measured CO2 and CH4 concentrations from the MPT to about 1.55 million years ago as well as a temperature proxy based on deuterium (D).
The analysis reveals that throughout the MPT and preceding period of 40,000 year glacial/deglacial cycles, CO2 concentration and temperature as measured by deuterium proxy were closely correlated. Furthermore the analysis reveals that surface temperature maxima during interglacials prior to 1.2 million years ago were about the same as after 800,000 years ago. However, during thermal minima (ice age maxima) neither temperatures nor CO2 concentrations were not as low during the 40,000 year periods as during the 100,000 cycles. Thermal minima were 28% lower after 800,000 years ago. During the 40,000 year glacial/deglacial cycles the observed range of CO2 concentration was 204 to 289 ppm. During the last 800,000 years prior to the present the range was larger from 180 to 290 ppm. This analysis suggests that there has not been a noticeable long term decrease in atmospheric CO2 concentration over the past 1.5 million years.