Law Dome, Antarctica is a coastal ice core site. It has high annual accumulation which means that new layers of snow are added every year (which does not happen at low accumulation sites). As a result, the age of the ice can be "accurately" determined by simply counting the layers. (In some sources, this "accuracy" is disputed because it is possible to accumulate more than one layer per year.)
There is a controversy about the Law Dome icecore data. There are bore hole measurements of the actual ice temperature and an 18O isotopic analysis (a temperature proxy) of the ice. According to Steve McIntyre, both of these temperature related datasets were omitted from the IPCC reports. His analysis of the Climategate emails indicates that there was much discussion about whether or not to include the data.
I checked at NOAA Paleoclimatology Ice Core Gateway - Law Dome Ice Core and neither of these datasets is available (or even mentioned). In addition, the cores (there are several) extend back about ~80,000 years .. but only ~1,000 years of CO2 and CH4 data are available.
Temperatures actually measured in the Law Dome bore hole are available from the Niels Bohr Institute Centre for Ice and Climate. This is the data that the Climategate "scientists" wanted to hide. (McIntyre only shows about 3,000 years of this data.)
Of course, it gets better - using all the data in ddjtemp.txt, this is what is shown.
Blue - GRIP .. Red - Dye-3 .. Black - Law Dome
It is an interesting exercise to assume that the Law Dome peak at ~1,000 BCE is actually at the same time as either the MWP at 1,000 CE or that it coincides with the stone age optimum at about -2,500 BCE. Remember, these cores are dated by simply counting the number of visible layers and that there is no guarantee that one layer represents one year. It is far more likely that one layer represents one storm, or one season. I am not showing the plots (you should make them yourselves), but there is good agreement between the 3 series if you assume that Greenland accumulates 2 layers per year, and somewhat better correlation if you assume that Law Dome accumulates 2 layers per year.
Neither correlation proves anything .. but both indicate that more research is necessary.
By the way, I know that dating cores is not as simple as just counting layers .. but the references specifically say that that is what was done. A complete dating will also look at other markers - dust, volcanic gases, and so forth - in order to validate the dates.
Blue - GRIP .. Red - Dye-3 .. Black - Law Dome .. Green - Law Dome CO2
This is the same data, zoomed and with markers displayed. Notice that the CO2 series increases almost linearly from 1860 to 1955. At that point, there is an obvious discontinuity and the slope changes (increases) until 1975 .. the end of the data. (The points are 5 years apart.) I have also included the points on the Dye-3 temperature curve. (Notice that they are much farther apart.)
|Steve: borehole data at http://www.gfy.ku.dk/~www-glac/data/ddjtemp.txt . I got the isotope data by an email from Tas van Ommen in 2006 after a couple of years. At the time, he said that he anticipated making the data public (with update to 2004) in the near future. I just checked and he replied that the data remains non-public. There is a short data set to 2000 at the Australian|