Although we have only been directly measuring temperature for the last 150 years and have been measuring atmospheric carbon dioxide (CO2) for only the last 50 years, scientists have found ingenious methods of measuring these values globally for hundreds of millions of years into the past. For detailed information on these proxy methods of measurement see this post.
The current post considers temperature and CO2 reconstruction over the last 400,000 years, as illustrated in the graphic below. As always on this blog click on the graphic to open a larger version of it in a window or tab. The top pane of the graphic shows temperatures (in Antartica), the middle pane CO2 and the bottom one dust levels. The left hand side is the present, the right hand side is 400,000 years ago. The data for these graphs was produced from ice cores drilled into the ice sheet. The video later in this post graphically shows the process of drilling and analysing ice cores.
One of the things that stands out most in the graphic is that the planet has been quite cold over most of the last 400,000 years. There are only five short lived warmer periods, called interglacials. We are currently living in the latest interglacial, in fact the whole of human civilisation has occurred during the current interglacial - called by geologists the Holocene.
There is a fairly obvious cycle in the temperature data of a little over 100,000 years. The generally accepted explanation for this periodicity involves the Earth's orbit around the sun, and is called Milankovitch Cycles, after the theory's originator.
The interglacial preceeding the current one is called the Eemian. For a detailed description of the Eemian Interglacial follow this link. The Eemian probably lasted from 127,000 to 118,000 years ago, but there is still debate on its exact length as indicated by the previous link.
The graphic below shows temperature levels for the last 150,000 years. The Eemian, Holocene and the glacial period between them can be seen clearly. It is obvious that the peak temperature of the Eemain was higher than that of the Holocene (so far). The Eemian also lasted for a shorter period of time. A likely explanation for these differences is in the values of the Milankovitch forcings.
Given that most experts think that global temperatures will increase over the next century, the Eemian is an important period of time to study. The North Greenland Eemian Ice Drilling project (NEEM) is one large scale project that is studying this important period of Earth's history.
Ice Core Drilling at NEEM
The video below describes the NEEM project and introduces some to the techniques that are used to extract information from the ice cores.
How to Drill an Ice Core
The following notes and graphic come from this link. (Note that there are two noisy graphics that automatically start on this page.)
The drill used for the NEEM ice core drilling is based on a design that goes more than 3 decades back to the DYE-3 deep drilling in South Greenland. The drill is a self-contained unit only connected to the surface by an 7.3 mm thick steel cable. Inside the cable, power for the drill is transmitted together with control commands from the operators.
The drill is about 13.5 meters long, and is shown here with the outer barrel (indicated by green) removed. When operational, only the drill head and the anti-torque section (lower right) are visible.