Are Easterbrook's oxygen-isotope paleo-temperature reconstructions credible?

Question

In this video of a presentation given at a climate-skeptic Heartland Institute conference, Don Easterbrook presents his view of recent climate history. He compares his own projections of future climate with IPCC forecasts and claims them to be superior since about 1999.

This comparison has been debunked (see this SkepticalScience post) as a misrepresentation of IPCC results.

But, more interestingly, Easterbrook also presents paleoclimate reconstructions based on oxygen-isotope analysis of ice cores which he claims show both much higher and much lower temperatures in the past 15,000 years than other, more widely accepted paleoclimate reconstructions (the key discussion starts around 3:32 in the video). In particular he claims that the ice-core data are better at showing the well-known medieval warm period and the little ice age than tree ring proxies for temperature but also show periods over the last few thousand years that are significantly warmer than present.

This is clearly a contrarian view compared to the current mainstream consensus on historic climate (often dominated by tree-ring temperature proxies for the last millennium-ish). So are his ice-core based temperature reconstruction credible? and if not, why not?

NB please let's keep the discussion to the question of O₂ isotope analysis of ice-core data and not get diverted into ad-hominem attacks on Easterbrook's other opinions.

Answer 1

Easterbrooks main argument here has already been discussed at Skeptical Science, the article is here. There are two main errors, the first is to treat Greenland as if it were representative of global temperatures. The second, more important error is that apparently it is conventional in this field to treat 1950 as the "present" in this kind of study.

There are three main timescales used on the CD-ROM. For both GRIP and GISP2, these timescales are in years before present (yr BP) where year 0 refers to northern hemisphere summer of the year 1950 A.D. For example, year 1989 (summer) is be referred to as the year "-39 yr BP" on this CDROM, following the convention of the radiocarbon dating method (see e.g. Stuiver and Reimer, 1993). It should be noted that, in general, GRIP publications have not used the A.D. 1950 convention. (source)

Thus the most recent datapoint in the GISP2 core is 95 years prior to that (i.e. 1855). This means that none of the warming since 1855 in Greenland is shown on the chart, an hence some of the comments made by Easterbrook in his talk (at about 10:25) about a cooler periods from 1880 to 1915 and 1945 to 1977 are obviously nonsense, the ice core didn't "end there" as Easterbrook says, it ended in 1855.

The author of the SkS article updated the plot of the GISP2 ice core temp series, to add observed temperatures for 1855 and 2009 from another greenland temperature reconstruction (updated) from

Box, Jason E., Lei Yang, David H. Bromwich, Le-Sheng Bai, 2009: Greenland Ice Sheet Surface Air Temperature Variability: 1840–2007. J. Climate, 22, 4029–4049. doi:10.1175/2009JCLI2816.1)

which clearly suggest that temperatures there now are warmer than they have been for the last 10k years, and that temperatures have been rising there very fast. See the SkS article for details of how the two datasets were reconciled.

The main canard implied in Easterbrook's video is the idea that because there have been large changes in (especially regional) temperature in the distant past, when there were no anthropogenic carbon emissions, then carbon emissions now are not causing the changes in temperature. This is a non-sequitur as both anthropogenic and natural forcings affect the climate, and climatologists know this and take it into account in their studies. They have also studies past changes in climate, such as the Younger Dryas, indeed such events are helpful in the understanding of climate.