Greenland Ice Core CO2 during the past 1,000 years

Guest Post by Renee Hannon

Introduction

This post compares CO₂ ice core measurements from Greenland to those from Antarctica over the last millennium. Paleoclimate studies typically use only Antarctic ice cores to evaluate past CO₂ fluctuations. This is because the entire Greenland CO₂ datasets were deemed unreliable due to chemical reactions with impurities in the ice and therefore have not been used in studies since the late 1990’s. This post will demonstrate that CO₂ data from Greenland ice cores have scientific value and respond to key paleoclimate events such as the Little Ice Age and Medieval Warm Period.

Antarctic Ice Core CO₂ Trends

Antarctic ice CO₂ data is readily available and has been studied extensively (Bauska, 2015, Ahn, 2012, Siegenthaler, 2005 and Rubino, 2019). Most of the focus of recent studies has been on high snow accumulation sites which tend to have higher resolution and less smoothing of the trapped gas age in ice bubbles due to the firn to ice transition. Gas age width and resolution ranges from 10 years in Law Dome ice cores to 65 years in Dronning Maud Land DML. Figure 1 shows CO₂ data from Antarctic high-resolution ice cores over the past millennium.

Ahn et al, 2012, compiled CO₂ records from the West Antarctic Ice Sheet (WAIS) and compared them to other key datasets such as Dronning Maud Land (DML), and Law Dome. Their study recognizes and discusses elevated CO₂ during the Medieval Warm Period (MWP) at 1000 AD, decrease of CO₂ around 1600 AD during the Little Ice Age (LIA) and the subsequent rapid increase beginning around 1850 AD.

More recently, Rubino 2019, presented revised ice core gas records for Law Dome. His robust study evaluates multiple gases such as CO₂, CH₄, N₂O as well as carbon isotopes. He has a good discussion on the LIA where the ice record shows CO₂ decreasing as delta ¹³C increases which favors reduced soil or terrestrial respiration in response to cooling. Rubino also discusses the CO₂ decrease of 10 ppm in the Law Dome record around 1610 AD which is not present in any other records. This rapid decrease demonstrates the higher resolution of Law Dome which is the highest Antarctic snow accumulation site.

Figure 2 compares the Antarctic CO₂ trends. Surprisingly, there is quite a bit of difference ranging from 0-11 ppm. Ahn, 2012 noted that WAIS is generally 2-4 ppm higher than Law Dome CO₂ data. WAIS CO₂ is also systematically higher than DML CO₂ by up to 6 ppm, with an average of 4 ppm higher. This shift occurs with both CO₂ and its carbon isotopes but not with CH₄, methane. The reason for the shift is not well understood.

The high-resolution Law Dome data can identify CO₂ rises and dips of less than 30 years. WAIS is up to 10 ppm higher during the Law Dome dip at 1610 AD; due to lower resolution as noted by Rubino, 2019. Law Dome also shows modest CO₂ rises during the MWP, but not as high as WAIS. Interestingly, Law Dome is 2-4 ppm higher than DML pre-1600 AD except for a few dips.

DML CO₂ has the lowest snow accumulation and is the lowest resolution Antarctic dataset shown. The resolution is about 65 years with a more muted representation of atmospheric CO₂ history, explaining the systematic lower CO₂ readings. DML does show key CO₂ events such as a decrease during the LIA, subtle increase during the MWP, and a rapid increase beginning around 1850 AD. DML and Law Dome CO₂ trends converge during the LIA and recent rapid rise.

The Siple D47 D57 data shown as the dashed line in Figure 2 was not utilized in either Ahn or Rubino’s studies due to uncertainty in age dating and/or imprecise experimental methods. Clearly it is the odd dataset, particularly pre-1400 AD. There is an 11-ppm difference in CO₂ from Law Dome at 1200 AD. The oddities of the D47 D57 measurements are important to note because this is the Antarctic dataset used to compare and discredit Greenland CO₂ data (Barnola, 1995).

Greenland Ice Core CO₂ Trend

CO₂ measurements from Greenland ice cores are believed to be unreliable due to in situ production of CO₂ by carbonate-acid reactions and oxidation of organic compounds (Anklin 1995, Barnola 1995, and Tschumi 2000). This premise was put forward because Greenland CO₂ data differed from Antarctic CO₂ ice core data. CO₂ concentrations in Greenland ranged up to 20 ppm higher than Antarctic CO₂ although the records are in good agreement for about the last 300 years. Greenland CO₂ had more variability with standard deviations of 6-10 ppm compared to 2-3 ppm in Antarctic ice cores. Also, of note, Greenland CO₂ concentrations from ice cores (Summit, GISP2, GRIP, Dye3) agree well with each other and all show similar disagreements from Antarctic. See my previous post for a more thorough discussion of Greenland CO₂ data here.

Let’s take a closer look at Greenland ice core CO₂ data during the past 1000 years. Barnola, 1995, analyzed ice core samples from Greenland Summit at two different laboratories, Grenoble and University of Bern. Summit has high snow accumulation rates, and the smoothing is only about 15 years. Digital data is not available; however, tables of the data are included in their publication. Sample spacing is erratic and ranges from 5-80 years with an average of 30 years. Carbon isotope data from Greenland ice cores are not publicly available.

Barnola, 1995, states the agreement between the Greenland Summit CO₂ measurements from the two laboratories is very good with the mean difference being about 2 ppm. It is common to see 3 ppm discrepancies in CO₂ between different laboratories according to Rubino, 2019. Due to the good agreement, Greenland CO₂ samples at the same depth were averaged between laboratories. The data was then resampled and smoothed over 60 years. The results are plotted in Figure 3.

Greenland ice CO₂ decreases to 280 ppm during the LIA and shows a rapid CO₂ increase starting about 1850. Greenland CO₂ data shows two earlier increases: a better defined Medieval Warm Period and another distinct rise around 1550 AD. CO₂ data reaches nearly 300 ppm on individual data points during the MWP.

CO₂ Inter-Hemispheric Differences

One of the reasons Greenland CO₂ measurements in ice were questioned was due to a large inter-hemispheric difference (IHD) of 20 ppm when compared to Antarctic data. The modern inter-hemispheric gradient of atmospheric CO₂ concentrations after being de-seasonalized range from 1-6 ppm measured during the short window of the past 45 years.

The CO₂ trends between Greenland and Antarctic data are shown in the top graph in Figure 4. Law Dome is not plotted due to its higher resolution as WAIS and Siple D47 57 mostly bracket the range of Antarctic CO₂ data.

Greenland CO₂ was originally compared to Antarctic Siple D47 D57 and demonstrated differences up to 20 ppm, which is unreasonable according to Barnola, 1995. Indeed, the bottom graph in Figure 4 shows that the difference between Siple D47 57 and Greenland (gold line) is up to 20 ppm briefly around 1200 AD. For the most part, the IHD is less than 5 ppm from 1300 AD to the LIA, with an exception at 1550 AD. Greenland and Antarctic differences are practically zero during the LIA to present day. The IHD tends to become higher during the MWP and around 1550 AD. As previously mentioned, the Siple D47 D57 ice core is an odd outlier Antarctic CO₂ dataset.

If the Greenland CO₂ trend is compared to Antarctic WAIS ice core, then the interhemispheric difference is always less than 10 ppm shown in blue in Figure 4. And it’s less than 5 ppm during 70% of the past 1000 years like the present atmospheric CO₂ differences (the heavy gray line). It should be emphasized that Antarctic datasets have up to 10 ppm difference between them alone. Thus, comparing Antarctic and Greenland datasets and discovering a difference of 5-10 ppm appears to be within the range of reasonable values considering their geographic distance.

The highest polar inter-hemispheric difference and the highest difference between the Antarctic CO₂ datasets both occur during the MWP. The D47 D57 shows the greatest divergence from the Antarctic datasets but it is not used in recent publications such as Rubino, 2019 or Ahn, 2012. Unfortunately, the Greenland CO₂ data was originally compared to this outlier Antarctic D47 D57 dataset and deemed unacceptable.

Interestingly, all Antarctic and Greenland CO₂ measurements tend to converge during the colder LIA period and overlie almost perfectly during the subsequent rapid increase.

Greenland Ice CO₂ Rises Mimic Methane

Greenland and Antarctic CO₂ trends plotted alongside methane from ice cores over the past millennium are shown in Figure 5. Methane from various ice cores overlie nicely. Methane shows a distinct separation between the polar regions with an IHD of 24 to 58 ppb (Rubino, 2019). This is lower than the current atmospheric methane difference of 100 ppb between the South Pole and Barrow observatories.

In general, methane shows similar trends over the past millennium as CO₂. Methane decreases during the LIA with a subsequent rapid increase. There is a distinct rise in methane around 1550 AD that is not captured well by Antarctic ice CO₂, especially the DML data. Interestingly, the Greenland CO₂ does show the 1550 AD increase. Greenland CO₂ also shows character during the MWP that mimics the methane trends, again not seen in the Antarctic CO₂ data.

Antarctic CO₂ shows more scatter than methane in the various ice cores over the past millennium. As discussed above, Antarctic WAIS is 2-4 ppm higher than Law Dome and 3-6 ppm higher than DML. Additionally, the lower resolution Antarctic DML is 2-4 ppm lower than Law Dome during the MWP and 1550 event. CO₂ from all datasets tend to converge during the LIA cold period and subsequent rapid rise.

Figure 6 illustrates some of these CO₂ differences for three key events: MWP, 1550, and LIA. All datasets recognize these events; however, the magnitude between the events varies. The left graph shows that Greenland and WAIS have higher average CO₂ concentrations during all events. Greenland and WAIS were normalized on the LIA which required a shift of 3 ppm shown on the right graph. Greenland shows the largest magnitude, or CO₂ amplitude variation, between the cooler LIA and MWP of over 11 ppm. As expected, DML shows the lowest difference of only 2 ppm between the LIA and MWP due to its lower resolution. Law Dome shows a slightly higher difference of 5 ppm compared to WAIS of 4 ppm.

The reasons for the CO₂ scatter and different underlying trends are not well understood and chemical reactions within the core are frequently cited. The scatter in both Greenland and Antarctic tends to occur with elevated CO₂ during warmer times.

Another potential explanation is the modification of CO₂ during the firn to ice transition. Ahn, 2012, states that WAIS CO₂ probably experience additional smoothing processes not captured by firn air models. Their enhanced firn air model still underestimates WAIS CO₂ smoothing by 37 percent (19 years versus 30 years). Temporal resolution is certainly a factor in the systematically reduced CO₂ measurements in the DML ice core.

Scatter in CO₂ may be also be attributed to inter-core variability. Rubino notes that it is not uncommon to see inter-core variability of 3-4 ppm in Antarctic core data. Greenland’s present day atmospheric data shows higher standard deviations from 4-5 ppm with 15-20 ppm seasonal swings than Antarctic atmospheric data. It should not be a surprise that inter-core CO₂ variability is higher in Greenland ice cores than Antarctic cores.

Conclusions

Scientists have classified Greenland ice core CO₂ measurements as contaminated and mostly ignore these data. These data are a high-resolution polar endmember that can provide additional information to complement Antarctic CO₂ datasets. Higher CO₂ events are expressed better in Greenland than Antarctic cores. For example, Greenland CO₂ data captures CO₂ increases almost up to 300 ppm in individual data points during the MWP that correlate well with methane rises. Greenland CO₂ data show increases during 1550 AD like methane rises. Perhaps, Greenland data is suggesting that CO₂ increases during past warm periods are larger than documented by the muted Antarctic CO₂ data. Greenland CO₂ data is trying to tell scientists the Arctic side of the paleoclimate story. But many scientists have chosen not to listen.

“Errors using inadequate data are much less than those using no data at all” – By Charles Babbage, Inventor and Mathematician

Acknowledgements: Special thanks to Donald Ince and Andy May for reviewing and editing this article.

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