By Andy May

Last week, I posted a global temperature reconstruction based mostly on Marcott, et al. 2013 proxies. The post can be found here. In the comments on the Wattsupwiththat post there was considerable discussion about the difference between my Northern Hemisphere mid-latitude (30°N to 60°N) and the GISP2 Richard Alley central Greenland temperature reconstruction (see here for the reference and data). See the comments by Dr. Don Easterbrook and Joachim Seifert (weltklima) here and here, as well as their earlier comments.

Richard Alley’s (Richard Alley, 2000) central Greenland reconstruction has become the de facto standard reconstruction and is displayed often in papers and posts. And, truth be told, I’ve often used it. See here for an example. But, it is a central Greenland reconstruction, uncorrected for elevation differences over time, and all of Greenland is north of 60°N. A better comparison is with my Arctic reconstruction that goes from 60°N to the North Pole. This comparison is shown in figure 1.

Figure 1

Alley’s reconstruction is based upon trapped air in ice cores taken from central Greenland and his proxies are calibrated to air temperatures on land. My Arctic reconstruction is based upon nine proxies, five are marine proxies and 3 are land proxies. Only one of the land proxies is a Greenland ice core and I used a composite of two Greenland area ice cores, Agassiz and Renland, by Vinther, et al. (2009) and not the better-known Alley reconstruction. The Vinther reconstruction and the Alley reconstruction are compared, using actual temperature, in figure 2.

Figure 2

As can be seen in figure 2, the Vinther Agassiz and Renland reconstruction is less erratic and has a more prominent Holocene Climatic Optimum (HCO) than the Alley reconstruction. In addition, the Vinther Medieval Warm Period is older and the Roman and Minoan Warm periods are far less prominent and offset in time. Notice the reconstructions match in the Little Ice Age (LIA) and that the Vinther Holocene Climatic Optimum (HCO) from 8000 BC to 4500 BC is more prominent. The HCO doesn’t really show up in the Alley record. Below we compare our Arctic reconstruction to the Vinther record in Figure 3.

Figure 3

Vinther’s record shows a more prominent HCO than ours, more detail and a deeper LIA. Finally, let’s compare both Vinther and Alley to our Northern Hemisphere mid-latitude reconstruction in figures 4 and 5.

Figure 4

It is interesting that Vinther agrees with the mid-latitude Northern Hemisphere reconstruction in the Neoglacial period (roughly 5700 BP or 4300 BC to the present), but agrees better with the Arctic reconstruction during the HCO. I’m not completely sure why that is.

Figure 5

Comparing figure 4 to figure 5, we can see that Alley has a very flat trend and is more active than Vinther. Vinther is a better match to our Northern Hemisphere mid-latitude reconstruction. Alley’s reconstruction starts to show the HCO and then fizzles at about 1,000 years in to it. Figures 4 and 5 are anomalies from the mean temperature from 9000 BP to 500 BP, however, which distorts the picture a bit given the two reconstructions differ on the temperatures of the HCO and the LIA. I refer you to figure 2, where we compare Vinther to Alley in actual temperature and not in an anomaly form. Here the two reconstructions agree on the temperature of LIA, but the Alley reconstruction does not see the HCO. We see that the key difference between the two is the degree of warming during the HCO.

Why are Alley and Vinther different?

The short answer is that Vinther, et al. (2009) corrected their ice core records, including GISP II and GRIP, for elevation differences and Alley did not. In Vinther’s words:

“The previous interpretation of evidence from stable isotopes (δ18O) in water from GIS [Greenland Ice Sheet] ice cores was that Holocene climate variability on the GIS differed spatially and that a consistent Holocene climate optimum—the unusually warm period from about 9,000 to 6,000 years ago found in many northern latitude palaeoclimate records—did not exist. Here we extract both the Greenland Holocene temperature history and the evolution of GIS surface elevation at four GIS locations. We achieve this by comparing δ18O from GIS ice cores with δ18O from ice cores from small marginal icecaps [Agassiz and Renland]. Contrary to the earlier interpretation of δ18O evidence from ice cores, our new temperature history reveals a pronounced Holocene climatic optimum in Greenland coinciding with maximum thinning near the GIS margins. Our δ18O -based results are corroborated by the air content of ice cores, a proxy for surface elevation.”

In figure 6 we see a summary of the Vinther, et al. (2009) data, it is their figure 1.

Figure 6 (Source: Vinther, et al. 2009)

The six cores are well distributed across Greenland, with Agassiz on Ellesmere Island very close to Greenland. Agassiz and Renland are both coastal cores and have similar profiles. It is possible to reconstruct the elevation histories for these two locations with confidence, so they are used to develop corrections for the remaining 4 ice cores. All six core records shown were included in the Vinther, et al. (2009) reconstruction after adjustment for elevation and ice thickness changes, but the Agassiz and Renland cores are the key cores. The corrections to these cores are shown in 6D. The δ18O profiles for these cores, after the uplift (or elevation) correction has been applied, is shown in 6c. Considering that Agassiz and Renland are on opposite sides of the GIS and 1,500 km apart, the agreement between the two corrected records is astounding, as Vinther, et al. (2009) described it in their paper.

Alley’s reconstruction focused on the GRIP and GISP II cores, these two cores are 30 km apart in central Greenland, they are combined into one point…