Greenland Ice Core CO2 Concentrations Deserve Reconsideration

Guest post by Renee Hannon

Introduction
Ice cores datasets are important tools when reconstructing Earth’s paleoclimate. Antarctic ice core data are routinely used as proxies for past CO₂ concentrations. This is because twenty years ago scientists theorized Greenland ice core CO₂ data was unreliable since CO₂ trapped in air bubbles had potentially been altered by in-situ chemical reactions. As a result, Greenland CO₂ datasets are not used in scientific studies to understand Northern and Southern hemispheres interactions and sensitivity of greenhouse gases under various climatic conditions.

This theory was put forward because Greenland CO₂ data were more variable and different than Antarctic CO₂ measurements located in the opposite polar region about 11,000 miles away. This article re-examines Greenland ice cores to see if they do indeed contain useful CO₂ data. The theory of in-situ chemical reactions to explain a surplus and deficit of CO₂, relative to Antarctic data, will be shown to be tenuous. The Greenland CO₂ data demonstrates a response to the Medieval Warm Period, Little Ice Age, Dansgaard-Oeschger and other past climate change events. This response to past climate changes offers an improved explanation for why Greenland and Antarctic CO₂ measurements differ. Further, Greenland CO₂ measurements show rapid increases of 100 ppm during warm events in relatively short periods of time.

Atmospheric CO₂ is More Variable in Northern Latitudes

Figure 1, from NOAA, shows atmospheric CO₂ concentrations measured from the continuous monitoring program at four key baseline stations spanning from the South Pole to Barrow, Alaska. CO₂ has risen from about 330 ppm to over 400 ppm since 1975 and is increasing at approximately 1-2+ ppm/year. Many scientists believe that rapidly increasing CO₂ is mostly due to fossil fuel emissions.

Although the increasing trends from these four baseline stations appear similar, the Northern Hemispheric (NH) atmospheric CO₂ concentrations are increasing slightly faster than the Southern Hemisphere (SH). Longer-term trends from all latitudes are de-seasonalized and used for calculations of the inter-hemispheric CO₂ gradient and trends. During pre-industrial times the NH CO₂ mean annual concentration was estimated to be 1-2 ppm higher than the SH (Stauffer, 2000). Currently, the annual mean CO₂ concentration is about 5-6 ppm higher in the NH than the SH. De-seasonalized trends also show that SH CO₂ lags the NH CO₂ by about 2 years. For example, the annual CO₂ reading at Barrow, Alaska broke 400 ppm in May 2014 whereas the annual South Pole hit 400 ppm May 2016. However, note the 1st monthly average at Barrow hit 400 ppm in April 2012 which is four years earlier than the South Pole.

Although all observation stations show that CO₂ is rising, there are annual amplitude cycles reflecting seasonal differences that vary by latitude (N-S) superimposed on the overall rising longer-term trend. Figure 2 shows a graph comparing the past two years of CO₂ data for Barrow, Alaska and South Pole (SPO) observatories. On the right-hand side, are global CO₂ visualizations from NASA which incorporate CO₂ measurements from the Orbiting Carbon Observatory spacecraft.

In the NH, atmospheric CO₂ rises during the winter months and falls during the summer showing strong evidence of a natural biospheric signal (Barlow et. al, 2105). In NH springs and summers, CO₂ concentrations decrease rapidly during a period of two months due to the growth of plants and adsorption of CO₂. During autumn/fall, CO₂ is released by respiration and increases. During the NH winters, there is a more stable period when the highest CO₂ readings are observed. This dormant period lasts 6-7 months each year when there is less terrestrial plant growth.

Barlow, et. al calculated an increase in the NH CO₂ amplitude of 0.09 ppm/yr. on detrended data. For example, NH CO₂ amplitudes have increased from 14 ppm in 1975 to 18 ppm in 2019. The amplitude increase is associated with the enhanced vegetation greenness partly due to elevated warming as discussed by Yue et.al. Barlow et. al suggests the changes in CO₂ uptake and release is evidence that NH vegetation may be progressively capturing more carbon during northern spring and summer as global CO₂ levels increase.

The Barrow and South Pole observatories show that CO₂ amplitudes in the NH are significantly larger than in the SH. A very weak amplitude of opposite polarity is seen in the SPO CO₂ measurements shown by the dark gray line in figure 2. The SH CO₂ amplitudes are significantly lower at only 1-2 ppm per annual cycle. The amplitude differences result in CO₂ being 12-15 ppm higher in the NH than in the SH during northern winter months almost 60% of the year. This is shown in the NASA global visualizations during the dormant period. Dettinger and Ghil, 1998, suggest the smaller SH amplitudes reflect less seasonal variability due to a much-reduced terrestrial influence on CO₂ concentrations. They also conclude that South Pole CO₂ variations are affected mostly by marine influences such as marine upwelling and release of CO₂.

CO₂ Data from Greenland Ice Cores Do NOT agree with Antarctic

CO₂ concentrations of trapped air in ice bubbles in Greenland and Antarctic ice cores were examined to evaluate differences between the SH and NH paleoclimate atmospheric CO₂.  Antarctic ice core CO₂ data is readily available and used as the key dataset for CO₂ trends during interglacial and glacial periods for the SH. Surprisingly, Antarctic CO₂ data are also used for NH paleoclimate CO₂ trends.

Finding Greenland ice core CO₂ data is extremely difficult especially in any useful format. It seems to be written out of history. There are four ice cores in Greenland; GISP2, GRIP, Camp Century, and Dye 3 with mention of atmospheric CO₂ gas measurements. There is only scant data available in digital formats.

Several mid 1990’s articles have published some of the Greenland CO₂ ice core data. Anklin et. al. shows Greenland GRIP and Dye 3 CO₂ profiles from 5,000 to 40,000 years BP. Digital data from this study is available for GRIP CO₂ concentrations in core depths. Smith et. al have published on CO₂ concentrations of trapped air from the GISP2 ice core also available in core depths. Neftel et. al. published on CO₂ concentrations from the Camp Century ice core compared to the Antarctic Byrd ice core. CO₂ concentrations were as high as 400 ppm about 1100-1200 years ago using a dry extraction technique analyzed by laser spectrometer. Unfortunately, I am unable to located Camp Century and Dye 3 ice core CO₂ data in digital format.

In 1995 Barnola et. al had recent Holocene interglacial ice core samples from both Greenland GRIP and Antarctic Siple Dome ice cores analyzed in two different laboratories, Grenoble and University of Bern. Digital data is not available; however, tables of the data are included in their publication. The results are plotted in Figure 3a. The black curve is smoothed CO₂ data using Antarctica ice cores. The symbols represent Greenland GRIP CO₂ from the laboratory measurements (Gren and Bern). Barnola found there is good agreement between the lab measurements on different cores in the same hemisphere. However, the measured CO₂ values between Greenland and Antarctic did not agree. This discrepancy of up to 20 ppm was more than could be explained by the inter-hemispheric gradient of atmospheric CO₂ concentrations.

Figure 3b shows the CO₂ ppm difference between the lab measurements on the Greenland lab samples versus Antarctic samples. The present day inter-hemispheric gradient is also highlighted. Interestingly, CO₂ values are in good agreement between Greenland and Antarctica from about 1600 AD to 2000 AD. However, Greenland CO₂ values ranged up to 20 ppmv higher from 1600 to 900 years AD. The approximate time of the Medieval Warm Period (MWP) and Little Ice Age (LIA) are noted on the graphs.

Smith’s 1997 evaluation of CO₂ in Greenland ice cores focused on the older portion, on stadials and interstadials of the Dansgaard-Oeschger (D-O) events during the glacial period. Results showed even higher CO₂ variability than during the Holocene interglacial period. The warm interstadials increased on average by 50-90 ppm over a short period of 100 to 200 years. Detailed sampling over one 4-cm ice section showed three samples of CO₂ higher than 400 ppm within a warm interstadial.

But there’s more: Greenland CO₂ measurements are also lower than Antarctic CO₂ values

CO₂ concentration records from Greenland ice cores are generally higher than those from Antarctic ice cores for the same time interval. However, there are some data which show lower concentrations.  Anklin et. al. found values in the GRIP ice core that were too low compared with Antarctic records. Smith and others (1997) also found values that were too low in some samples from the cold stadial phases during the last glacial period.

In summary, the conclusions from published studies on CO₂ concentrations of trapped air in ice bubbles from Greenland ice core data are surprisingly similar:

1. CO₂ concentrations in Greenland ice cores (GRIP, GISP2, Camp Century, Dye 3) are generally 20 ppm higher than Antarctic during the Holocene interglacial period younger than about 8000 years before present (BP). For older samples during the glacial period interstadials/stadials, CO₂ is higher by over 50 ppm. An inter-hemisphere difference of 20-50 ppm is unrealistic and higher than present day.
2. CO₂ concentrations in Greenland ice cores show more variability than Antarctic ice core CO₂ data. In addition to having higher CO₂ values they also had lower CO₂ values than the Antarctic data.
3. BUT Greenland CO₂ concentrations from ice cores agree well with each other and all show similar variances from Antarctic.

Condemnation of Greenland Ice Core CO₂ Data

Group think – Jury’s out – One Verdict

The Greenland CO₂ values are too high, too low, show more variability and most importantly do not agree with Antarctic CO₂ data. Thus, something must be wrong with the Greenland ice core CO₂ data. Scientists attempted to explain the potential surplus as well as depletion of Greenland CO₂ values. Many technical articles and research in the mid to late 1990’s were based on a hypothesis that acid-carbonate chemical reactions in the Greenland ice bubbles created a surplus of CO₂.

“The high degree of variability associated with Greenland CO₂ measurements may be related to CO₂ liberation from carbonates due to the dissolution by acid species in ice.” Anklin et. al, 1997 and other papers like Delmas, 1993, Barnola, et. al 1995; Smith et. al, 1997; Tschumi et. al, 2000.

Some doubts about this chemical reaction were raised because the carbonate content of ice is difficult to measure directly and so the carbonate content is estimated indirectly from the Ca²⁺ concentrations. Tschumi and Stauffer concluded, after completing a detailed lab study on Greenland cores, that the acid-carbonate reaction can explain only about 20% of the CO₂ surplus and they suggested oxidation of organic compounds may also be responsible. Therefore, the theory to explain surplus CO₂ evolved to become the result of a combination from two different chemical reactions. Additionally, they were unable to find any clear evidence to explain CO₂ depletion in the Greenland ice cores. Smith et.al. also acknowledges it was unclear how reactants could be mobile in ice where diffusion is extremely slow assuming the reactions occurred after the air bubbles in ice are formed.

Surprisingly, the acid-carbonate hypothesis was accepted as valid despite the fact carbonate content in ice is difficult to directly measure, the CO₂ surplus cannot be attributed to a specific chemical reaction mechanism, nor is there clear evidence for depletion of CO₂ by a chemical reaction, and these chemical reactions occurred after bubble closure. This acceptance meant the discrepancy between Greenland and Antarctic ice core data was explained. Consequently, the CO₂ data extracted from air bubbles in Greenland ice core data was deemed useless.

Re-examination of Greenland CO₂ Measurements

One positive outcome of the Greenland CO₂ variability denial is that several detailed, high density sampling studies were conducted. Figure 4 examines CO₂ measurements from the Greenland GISP2 ice cores from two stadials around 45,000 years and 62,000 years BP by Smith, et. al. Note age and depth is on the vertical axis and Ca, electrical current, CO₂ values, δ¹⁸O, and layer thickness are plotted on the horizontal axis.

The stadials correspond to a thinner annual layer suggesting lower accumulation rates and more negative δ¹⁸O isotope signatures suggesting colder temperatures. The stadials contain the lowest concentrations of CO₂, 200-240 ppm and lowest conductivity. Both stadials contain high amounts of Ca interpreted as related to dust accumulation (McGee, 2010).

The warm interstadials bounding the stadials also have unique characteristics. There is a sharp boundary where the stadial is terminated by a younger abrupt warming. Ca disappears quickly, δ¹⁸O isotopes rapidly become less negative, and CO₂ increases by 50-100 ppm in a period of 50-100 years (Smith, 1997). The transition from the older preceding interstadial to the cooler stadial is more gradual. This is also reflected in more variable CO₂ values and more variable conductivity or electrical current.

The chemical production of CO₂ is speculated to occur with higher acidity in the ice core, which is measured by higher electrical conductivity, H⁺ (Smith et. al). In the younger stadial, conductivity is very low throughout the interval except at the shallowest portion less than 2358 meters. However, CO₂ begins to increase at 2,377 meters more in-line with warmer δ¹⁸O isotope values.

An alternative hypothesis from these high-resolution data is that the CO₂ concentrations, while more variable, do show a qualitative correlation with the ice core properties of thickness, electric current signatures, oxygen isotopes and calcium content for each unique layer and are not chemically altered. Both stadial and interstadials in the study show well-behaved patterns and similar characteristics.

Figure 5 shows the high sample density of Greenland GISP2 CO₂ data, low sample density GRIP CO₂ data, and Antarctic Byrd CO₂ data in relation to Greenland temperature anomalies from oxygen isotopes. Age synchronization between Greenland and Antarctic ice cores was achieved by atmospheric CH₄ by Ahn and Brook, 2008.

Yet again, Greenland ice cores shows CO₂ concentrations that tend to mimic Greenland temperature anomalies of stadial/interstadial cooling and warming periods. Large, rapid increases of CO₂ occur during the rapid abrupt warming of interstadial events. As temperatures increase by 6 degrees C over a short period of 50-100 years shown at interstadial 12, the Greenland GISP2 CO₂ values in blue also increase rapidly to from 200 ppm to 280 ppm. The Greenland GRIP CO₂ shown in green was only randomly sampled over the D-O events but shows higher values in the interstadials when sampled of 280-300 ppm and lower values in the stadials of 220 ppm.

Note the Antarctic Byrd CO₂ values in gray show a minimal response of slightly increasing CO₂ in the long duration interstadials and show no increase in the short interstadials. In the longer interstadials 8 and 12, the Antarctic CO₂ values do slightly rise by 10 ppm. In short interstadial 13, Greenland GISP2 CO₂ rises rapidly to 260 ppm whereas the Antarctic CO₂ values stay low at 205 ppm. Interstadial 11 shows Greenland GRIP CO₂ values up to 300 ppm and again the Antarctic CO₂ values remain around 205 ppm. Antarctic CO₂ values do not show any response to interstadials 9, 10, 11 or 13.

During the cold stadials, Greenland CO₂ is more similar to or slightly lower than Antarctic CO₂ and averages around 190 to 200 ppm.

Antarctic CO₂ Ignores Past Cold Events

Surplus CO₂ can be produced by chemical reactions in theory and the necessary measured compounds (Ca, H⁺) are present in Greenland ice cores. However, Tschumi states that depleted CO₂ cannot be explained by chemical reactions. Let’s examine the times when Greenland CO₂ values are lower than Antarctic. When Greenland CO₂ values drop below Antarctic values, the timing corresponds to well-known Greenland cold climate events like the Younger Dryas (YD) and Holocene interglacial 8.2 kyr event. Figure 6 compares Greenland GRIP and GISP2 CO₂ to Antarctic Byrd CO₂ data. Times when Greenland CO₂ values are lower than Antarctic values are shaded in blue. Times when Greenland CO₂ values are higher are shaded in pink.

It is obvious that Antarctic Byrd CO₂ (gray line) shows no response to either the Y/D or 8.2 kyr cold events. Also obvious is the Greenland GISP2 and GRIP ice core CO₂ data tell a different story. During the Holocene interglacial 8.2 kyr cold event, Greenland CO₂ values drop from 270 to 210 within about 500 years and are 50 ppm lower than Antarctic CO₂ values. Greenland CO₂ values also show an abrupt rise after the 8.2 kyr event of 80 ppm within 200 years.

The YD event was a cold period during the recent Holocene glacial to interglacial transition about 12,000 years ago. It was preceded by and interrupted the Bolling Allerod (B/A) interstadial. The YD event was barely recognized in Antarctic ice core temperatures, only 1 degree C colder. However, in Greenland ice cores the temperatures plummeted by minus 10 degrees for hundreds of years shown by the Greenland temperature anomaly above in red (Figure 6).

The B/A interstadial and YD cold event demonstrate the qualitative correlation of Greenland CO₂ values to Greenland temperature fluctuations. Greenland CO₂ responds to the warmer B/A interstadial with an intermittent rise that is 20-30 ppm higher than the gradual Antarctic CO₂ increase. Greenland CO₂ peaks at 290 ppm and then decreases to 235 ppm during the cold YD. Contrarily, the muted Antarctic CO₂ data shows a gradual rise from 250 to 270 ppm ignoring both the B/A and YD and simply responds to the gradual Holocene deglaciation. This is not surprising because Antarctic ice core temperatures derived from δ¹⁸O isotopes also show no, to only minor, temperature fluctuations during these events (not shown).

Past literature studies on Greenland Younger Dryas and the 8.2 kyr event used only Antarctic CO₂ data resulting in the following observations:

• Ahn and Brook, 2013 observed small 1-2 ppm increases of Antarctic CO₂ and imply that the sensitivity of atmosphere CO₂ to the Northern hemisphere cooling of the 8.2 kyr event was limited. Conversely, Greenland CO₂ data shows a dramatic 80 ppm reduction within 200 years during this cold event.  
• Lui et. al. 2012, concludes that Greenland climate during the cold YD should be substantially warmer because the increase seen in Antarctic atmospheric CO₂ should be associated with an increase in surface temperature especially at high latitudes. Raynaud et. al. 2000 states that the long-term glacial Holocene increase in CO₂ was not interrupted during the YD. Raynaud was surprised by this result. Marchal et. al, 1998 states that CO₂ records from the Antarctic ice core shows that CO₂ remained constant during the Younger Dryas cold climate event. He states this suggests the North Atlantic ocean has a minor influence on CO₂.  
• Kohler, et. al, studied the B/A using CO₂ data from the Antarctic Dome C ice core which shows a CO₂ increase of about 10 ppm. Their models showed that atmospheric CO₂ should have increased by 20-35 ppm which is a factor of 2-3.5 greater than the CO₂ data showed. As a matter of fact, Greenland CO₂ does exactly that during the B/A by increasing 20-30 ppm perhaps suggesting the data is not chemically altered.

Alternative Hypotheses for Greenland Ice Core CO₂ “Bad” Behavior

What if Greenland ice core CO₂ data is not chemically altered and is just as accurate as the Antarctic ice core CO₂ data? The Greenland ice core isotopes do express more variable CO₂ fluctuations than Antarctic ice cores, but the variability appears to be synchronous with Greenland’s larger rapid temperature variations. And all the Greenland ice core CO₂ data generally agrees with each other.

Seasonal bias may exist in the Greenland ice cores.

Greenland ice cores may preferentially record more northern winter CO₂ readings than summer CO₂ variability. Seasonality with preferential preserved winter readings can easily explain the 18-20 ppm differences observed during the recent Holocene Medieval Warm period. Recent atmospheric CO₂ measurements between NH and SH observatories show up to 15 ppm differences for 6-7 months during the northern winter dormant season and 60% of the year. During the other 40% of the year, NH CO₂ is transitional either increasing or decreasing due to vegetation photosynthesis or respiration and is highly variable.

Greenland CO₂ Variability is Synchronous with Greenland Temperatures.
CO₂ values from Greenland GISP2 and GRIP ice cores qualitatively correlate with their δ¹⁸O isotopes temperature proxies as shown in the figures above. Dye 3 and Camp Century CO₂ data not presented here shows similar responses to Greenland temperatures (Anklin et. al and Neftel et. al.). This is obvious during Greenland abrupt climate changes such as the D-O events, even in short interstadials, and during the B/A. Greenland CO₂ also decreases corresponding to Greenland cold events such as the YD and 8.2 kyr.

During abrupt climatic events, Greenland and Antarctica CO₂ values can diverge significantly during warm interstadials with Greenland CO₂ values being much higher by 75+ ppm. The interstadial warm patterns in Greenland ice cores are also amplified from Antarctic in many aspects such as temperature, dust content, methane excursions, and possibly CO₂. During the Holocene 8.2 kyr event which was an abrupt cooling event, Greenland CO₂ values plummeted 50+ ppm while Antarctic CO₂ measurements did not recognize this event.

Greenland Ice Cores have High Gas Resolution due to High Accumulation Rates.

There are significant gas resolution differences between Antarctic and Greenland due to differences in surface temperature and snow accumulation rates. This is discussed by Ahn and Brooks, 2012 and by Middleton, 2017, 2019. Gas age samples can be younger by hundreds and up to a thousand years due to diffusion in Antarctic ice cores which have accumulation rates as low as 3 mm/yr. In Greenland where accumulations rates are much higher, gas age samples have a resolution as high as tens of years up to hundreds of years.

Middleton discusses CO₂ gas sample resolutions and gas age distributions for Antarctic ice cores due to gas diffusion before bubble close off. He shows the impact of smoothing filters to match the resolution differences. Instrumental annual CO₂ data should be averaged over 100+ years to compare to past Holocene Antarctic ice core CO₂ values. Of course, observatories have only recorded 40 to 60 years of CO₂ data. For example, Mauna Loa CO₂ annual mean averaged over 60 years of data is 354 ppm compared to the reported global annual mean of 407 ppm for 2018.

CO₂ Increases in Greenland Ice Cores like Methane.
It is well documented that rapid increases in methane, CH₄, concentrations are synchronous with past warming events in Greenland and are more extreme than in Antarctic ice cores. Antarctic and Greenland ice core methane records both a rise during warm interstadials and a fall during stadials but with different concentrations (Blunier and Brook). They suggest significant methane increases in Greenland during past warm periods are related to increased swamp and organic releases during melting periods. During warm periods, greening of the Arctic occurs when exposed terrestrial real estate expands significantly. Photosynthesis and respiratory processes should also be in full force. If swamp and terrestrial vegetation are becoming more exposed and prolific during past warming and “greening” of the Arctic, then past CO₂ should also show larger northern latitude increases like methane does.

Greenland CO₂ Data Could be a Climate Game Changer

While it is possible some of the Greenland CO₂ data could be contaminated, the assumption that ALL the CO₂ data is chemically altered in ALL the Greenland ice cores does not explain why CO₂ is so well behaved with Greenland temperatures or address the observations discussed above. It is also plausible the Greenland ice core CO₂ data has more detailed resolution and higher frequency than the subdued Antarctic ice core CO₂ record.

Figure 7 compares Greenland and Antarctic CO₂ data over the past 50,000 years. CO₂ signals preserved in the Antarctic and Greenland ice cores are significantly different. Greenland CO₂ fluctuations appear synchronized with active Greenland temperature changes just as Antarctic CO₂ data mimics more subdued Antarctic temperatures (not shown). Note the large data gap in digital Greenland CO₂ measurements during most of the Holocene interglacial period.

The Greenland CO₂ responses appear to reflect short-term centennial fluctuations whereas the Antarctic CO₂ fluctuations appear to be responding to longer term millennial changes. These differences may be the result of enhanced terrestrial carbon influences in combination with oceanic releases in the Northern Hemisphere whereas the Antarctic low amplitude CO₂ responses are dominated by global and Southern oceanic processes. Or simply that Antarctic ice core record has insufficient data resolution.

If the Greenland CO₂ data is correct, or even qualitatively correct at best, then it needs to be re-examined and incorporated into polar interhemispheric greenhouse gas /glacial/oceanic interactions and interpretations to establish natural past atmospheric CO₂ variability. Rapidly increasing CO₂ values measured during this Modern Warming may not be unprecedented compared with past natural fluctuations after all.

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

References Cited
(Note – It is very frustrating to find an interesting reference that is paywalled. Many key references are from papers 25+ years old that are still paywalled).

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