Misuse of the GISP2 Ice Core

Central Greenland Temperature Reconstruction

In 2000, R. B. Alley published the results of a proxy analysis of a single ice core in Central Greenland as part of the GISP2 project.[1] The resulting time series reconstructs local temperature fluctuations over the last 50,000 years. The graph above includes only the last 15,000 years; you may see versions of this graph showing the last 10,000 or 12,000 years. The proxy reconstructs temperatures at the Greenland Summit (the GISP2 project) using the ratio of two different oxygen isotopes: 18O and the more abundant 16O. Lower the 18O/16O ratio indicates lowers temperatures, since as temperatures decrease, a more 18O precipitates out at lower latitudes, making the 18O/16O ratio smaller over Greenland. Alley's time series ends 95 years BP, or 95 years before 1950, so the last data point in this analysis is 1855. The IPCC and other organizations are interested in modern warming frequently consider modern warming as measured against the 1850-1900 mean. Currently, global temperatures are about 1.2 C warmer than this baseline. But since Alley's data ends in 1855, it contains virtually none of modern warming. Anyone using this graph to make claims about modern warming is misusing this graph.

Alley's reconstruction is also limited to the temperature in Central Greenland. It does not represent global temperatures or even Greenland temperature. Other proxies, even within the same GISP2 ice core, yield different results. There are uncertainties in the proxy evidence, and Alley's reconstruction does not correct for elevation changes in the time series. Consequently, it misleading and wrong to make conclusions about global temperatures from the Alley's study, or for that matter, any single proxy.

Greenland Temperatures from Vinther 2009

As an example, above I show a graph of Greenland Temperatures from Vinther 2009[2] produced by CarbonBrief. The graph is a reconstruction of Greenland temperatures from 6 different ice cores. Using more proxies from different parts of Greenland allows for a better reconstruction of paleo temperatures. This version from CarbonBrief also includes the Berkeley Earth temperature data from Greenland in black with 20-year smoothing ending in 2018. A comparison of the Vinther reconstruction to Alley's reconstruction clearly shows that adding more proxies significantly improves our understanding of Greenland temperatures, removing the amplitude of the warmer and cooler portions of the GISP2 reconstruction. It's completely unreasonable to think that the Alley's reconstruction accurately represents global temperatures, and it's is not just me saying this. This is what Alley says as well:

...although the central Greenland ice-core records may provide the best paleoclimatic temperature records available, multiple parameters confirm the strong temperature signal, and multiple cores confirm the widespread nature of the signal, the data still contain a lot of noise over short times (snowdrifts are real, among other things). An isotopic record from one site is not purely a temperature record at that site, so care is required to interpret the signal and not the noise. An extensive scientific literature exists on this topic, and I believe we are pretty good in the community at properly qualifying our statements to accord with the underlying scientific literature; the blogospheric misuses of the GISP2 isotopic data that I have seen are not doing so, and are making errors of interpretation as a result.

So, what do we get from GISP2? Alone, not an immense amount. With the other Greenland ice cores… and compared to additional records from elsewhere, an immense amount… Using GISP2 data to argue against global warming is, well, stupid, or misguided, or misled, or something, but surely not scientifically sensible.[3]

It should be pretty obvious that we should be suspicious of anyone attempting to argue against global warming from a single ice core in central Greenland. It's simply not sound practice, and those who have scientific expertise (or claim to have scientific expertise) should know better. By way of example, imagine you have a very accurate weather station in your back yard. You maintain the station well so that it provides the best and most accurate temperature data possible. What does this data tell you about other temperatures around the globe? Practically nothing. Even if you were able to maintain this record for thousands of years, there is very little reason to think this single thermometer will correspond to trends in GMST. There is simply too much variability in local temperatures. This is why reconstructions of global temperatures in paleoclimate studies use a wide variety of proxies from all over the globe.

So why do contrarians so frequently use the GISP2 ice core and make the errors that even Alley identifies as "stupid?" Let's look at a few contrarian misuses of Alley's study to see. I'll examine a few representative misuses from popular contrarians.

Gregory Wrightstone

Wrightstone's Misuse of GISP2

Gregory Wrightstone includes this graph in his self-published book, Inconvenient Facts.[4] The book has this to say about the above graph from Alley's GISP2 data. "The damning data show that, for more than 6,100 years (or 60%) of the current interglacial warm period, the temperature was warmer than it is today. Of the nine earlier significant periods of warming since the end of the last ice age, five had higher rates of temperature increase (Figure I-31) and seven had larger total increases in temperature. Moreover, each of the previous warming cycles experienced significantly higher temperatures than today." Here's his Figure I-31.

Wrightstone's Continued Misuse of GISP2

A few obvious things should be noted here about Wrightstone's misuse of this graph:

1. If you compare these "warming periods" from the GISP2 data to Vinther's reconstruction of Greenland temperatures, these "warming periods" all but disappear. If these warming period don't survive the addition of 5 more proxies in Greenland, why would we imagine that they would be maintained in global temperatures? 
2. We have good reconstructions of global temperatures covering this time frame, and even those showing the most variability in temperatures do not preserve these warming periods or the rates of warming. They do show current warming to be exceptional and likely unprecedented for the Holocene.
3. The temperature from Box 2009 is misplotted. The 2000-2009 average temperature at the GRIP site was -27 C, and that was 1.44 C warmer than the 1850-1859 average, according to Box 2009.[5] The problem is that there’s a mismatch between the Alley’s 2004 temperature for 1855 and the the one reported by Box. But even adjusting for that mismatch, the 2000-2009 average is still warmer than what is reported in Wrightstone’s graph.
   
4. Even taking the data in Wrightstone's graph at face value for the first 9 warming periods, warming rates are always less than 0.01 C/year or 0.1 C/decade. Current warming rates globally exceed 0.2 C/decade with the Arctic warming exceeding 3x faster than the global average.[6]

Mr. Wrightone has made multiple errors here. He misuses GISP2 data, incorrectly plotted current Greenland temperatures, and miscalculated Greenland's warming trends.

Andy May

Andy May's Exercise in Cherry Picking

Andy May's use of this graph is my personal favorite.[7] He's a frequent contributor to the WUWT blog, and he maintains his own blog as well. Back in 2016, May tried to correlate world events with the proxy data from Central Greenland, as if when the Greenland summit is warm good things happen across the globe. However, he misunderstood Years BP to mean Years before 2000 instead of years before 1950, so his entire graph was off by 50 years. Then he added modern warming to the end from HadCRUT. But because the end date of his data was wrong (1905 instead of 1855), the thermometer record was also wrong. The events he used to show a correlation between warm temperatures and the flourishing of civilization were rather comical. He even included "Witch hunts reach maximum in 1600, 50,000 witches burned" as if it was caused by cold temperatures in Greenland.

Andy May Cherry Picking with a Partially Corrected Graph

Later he was corrected on the scale of the graph.[8] He was also shown another reconstruction from the same ice core using a different proxy from Kobashi.[9] This reconstruction sometimes had very different  warm and cool periods, the most notable being at a.d. 700. But the imagined correlation between Greenland temperatures and witch hunts remained. May's graph shows extreme cherry picking. For times that were warm in Greenland, he chose examples of the flourishing of civilization. For times that were colder in Greenland, he chose examples of bad things happening.

Joanne Nova - David Lappi

This graph is also kind of hilarious, though I'm not sure which errors belong to each of the names on the graph. Here we get the same graph (maybe adapted from Easterbrook's below?) with a curved "trend" added to the graph. It has the same mistake of assuming present is 2000 to make the faked trend look more believable. It also has projected temperatures into the future.

It's amazing how frequently these kinds of mistakes are made, and seemingly how frequently graphs are adapted based on other versions of the graph without consulting the original source data. So mistakes in earlier graphs get propagated to later graphs.

Don Easterbrook

With all of these already covered, Easterbrook's version is perhaps the least interesting, but I suspect he's one of the first to misplot the graph and use the data as if it represents global temperatures.

Easterbrook's Misplot of GISP2 Data

Easterbrook has used this in testimony before government panels where he describes the small warming near the end of the Little Ice Age as current warming, even though it it's clearly plotted wrongly and  confuses local temperatures with global temperatures.

Correlation with CO2

Still others combine the GISP2 ice core with CO2 data to suggest that CO2 and global temperatures are anti correlated, even though the CO2 proxy comes from Antarctica and the temperature proxy comes from Greenland. The correlation between CO2 and temperature using both from the same ice core is very high.

Conclusion

The contrarians above appear to be using this local temperature proxy so that they can claim that temperatures in the Holocene have been much warmer than today. In order to make that case, they can't use any of the global temperature reconstructions, and they can't include an accurate plot of modern temperatures. It's quite possible that Arctic warming has not yet surpassed the warmest years in the Arctic during the Holocene. Orbital forcings contributed to a warmer Arctic during the HTM. But globally, the evidence is pretty clear that the 1.2 C warming above the 1850-1900 mean has pushed global climate from among the coolest temperatures of the Holocene (the end of the LIA) to the warmest temperatures of the last 100,000 years.

References:

[1] Alley, R.B. 2004. GISP2 Ice Core Temperature and Accumulation Data. IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series #2004-013
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/greenland/summit/gisp2/isotopes/gisp2_temp_accum_alley2000.txt

[2] Vinther, B. M., et al. (2009). Holocene thinning of the Greenland ice sheet. Nature, 461, 385. https://doi.org/10.1038/nature08355

[3] https://dotearth.blogs.nytimes.com/2010/02/08/richard-alley-on-old-ice-climate-and-co2/

[4] Wrightstone, Gregory. Inconvenient Facts: The science that Al Gore doesn't want you to know . Mill City Press. Kindle Edition. 

[5] Gareth Renowden. "Confusing Greenland warming vs global warming."

https://skepticalscience.com/10000-years-warmer.htm

[6] Pierre-Henry Deshayes. "Arctic warming three times faster than the planet, report warns.
https://phys.org/news/2021-05-arctic-faster-planet.html

[7] Andy May, "Climate and Civilization for the past 4,000 years." https://andymaypetrophysicist.com/2016/06/22/climate-and-civilization-for-the-past-4000-years/

[8] Andy May, "Comparing the Kobashi and Alley Central Greenland Temperature Reconstructions." Watts Up With That Blog.
https://wattsupwiththat.com/2016/06/25/comparing-the-kobashi-and-alley-central-greenland-temperature-reconstructions/

[9] Kobashi, T., Goto-Azuma, K., Box, J. E., Gao, C.-C., and Nakaegawa, T.: Causes of Greenland temperature variability over the past 4000 yr: implications for northern hemispheric temperature changes, Clim. Past, 9, 2299–2317, https://doi.org/10.5194/cp-9-2299-2013, 2013.

[10] https://www.abd.org.uk/climate-change-legislation-will-ruin-your-future/

[11] D.J. Easterbrook. Chapter 21. "Using Patterns of Recurring Climate Cycles to Predict Future Climate Changes."

http://myweb.wwu.edu/dbunny/pdfs/Evid_Based_Climate_Sci/2016_Ev_Based_Climate_Sci_Chap21_Using_climate_cycle_patterns_to_predict_dje.pdf