From Science Matters
By Ron Clutz
Jørgen Peder Steffensen, of Denmark’s Niels Bohr Institute, is one of the most experienced experts in ice core analysis, in both Greenland and Antarctica. In this video he explains a coincidence that has misled those alarmed about the warming recovery since the Little Ice Age. And if you skip to 2:25, you will see the huge error we have made and the assumptions and extrapolations based on that error. Transcript below is from closed captions with my bolds and added images. H/T Raymond
What do ice cores tell us about the history of climate change and the present trend?
This ice is from the Viking age around the year one thousand, also called the medieval warm period. We believe that in Greenland the Medieval Warm Period was about one and a half degrees warmer on average than today
NorthGRIP the Greenland ice core project is being reopened to drill the last few meters through the ice sheet to the rock beneath the research station. The ice core over three kilometers in length has been hauled up to the surface piece by piece, and contains important data on the history of the climate of the earth. It bears the fingerprints of climatic conditions over more than 120 thousand years.
When we remove a drilled ice core we leave a hole, and we insert a thermometer in the hole. We are able to map out the temperature through the three kilometer ice sheet. Now that temperature, if we do it precisely enough with a thousandth of a degree accuracy, then the ice has not forgotten how cold or warm it was on the surface at the time the snow fell.
So using those temperatures we have been able to reconstruct the temperatures of the last ten thousand years. Here we have our picture of the temperature reconstructions that we have from Greenland in the graphical form. And if we go back along this axis it’s going back in time. This is the last 8 000 years we have plotted here. On this axis we have the temperatures at the site in Greenland, and we can see that if we go back from now about 4000 years ago, we would have temperatures up here for about 4 000 years in the past that on average were two and a half degrees warmer than today.
Reconstructed temperatures for Greenland ice cores Dye3 and GRIP. (A) The temperature from 8000 BP to present; (B) the temperature from AD 0-2000 (after Dahl-Jensen et al., 1998). The observed difference in amplitude between the two cores is a result of their different geographic location in relation to the variability in atmospheric circulation. Source: Reconstructed-temperatures-for-Greenland-ice-cores-Dye3-and-GRIP-A
Now as we go forward to approach our time, we can see that in the period after four thousand years ago and up to the two thousand years ago (which is actually the Roman Age) the temperatures have been decreasing in Greenland by two and a half degrees. Then temperatures increased gradually up to a maximum point around a thousand years ago, we call it the Medieval Warm Period. And then temperatures declined and go down to minimum around 1650 a.d., before coming back up a little in the 18th century.
And then around 1875 we have right here the lowest point in the last 10,000 years.
And that matches exactly the time when meteorological observations started.
Other core samples from elsewhere in Greenland confirmed that the little ice age ended about 140 years ago at the coldest point in the last 10,000 years. The natural pronounced alteration of warm and cold periods back in time has also been confirmed elsewhere in the northern hemisphere. Carbon 14 dating of organic matter from peat bogs and tree rings confirms the pattern. Also the data from stalactite caves in China and measurements from North Africa.
The problem is that we can all agree completely that we have had a global temperature increase in the 20th century. Yes, but an increase from what? It was probably an increase from the lowest point we’ve had for the last 10,000 years. And this means it will be very hard indeed to prove whether the increase of temperature in the 20th century was man-made or it’s a natural variation. That would be very hard because we made ourselves an extremely poor experiment when we started to observe meteorology at the coldest time in the last ten thousand years.
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