Dating tree cores
Figure 6 shows the temperature for both the EPICA and the Vostok sites, which indicated that the temperature records are in good agreement for most of the time.Before approximately 200,000 years ago there was some divergence in the estimate age but as can be seen in Figure 6 these differences are well within the range of likely errors.Ice cores are one of the most effective, though not the only, methods of recreating long term records of temperature and atmospheric gases.Particularly in the polar region, but also at high elevations elsewhere, snow falls on an annual cycle and remains permanently.
Whilst ice cores allow direct measurement of atmospheric gases, like CO2 and Methane, some care is needed in interpreting the results.
Because the gases in the atmosphere are mixed and decay over time this adds another element of uncertainty.
In effect, the data represent the average over a period of time, which can be several decades; a corollary of this is that data calculated from ice cores, for temperature of CO2 for example, will have less variation than the measured record.
Orbital tuning refers to calibrating the times against the Milankovitch cycles and the associated changes in radiation (Figure 1).
In the case of temperature no direct measurement is possible.