Greenland Ice CO2 – Chemical Reactions or Natural Variability?

Guest Post By: Renee Hannon

Introduction
This post examines whether CO₂ measurements in Greenland ice cores demonstrate natural variability as an alternative hypothesis to in-situ chemical reactions. Twenty years ago, scientists theorized Greenland ice core CO₂ data were unreliable because CO₂ trapped in air bubbles had potentially been altered by in-situ chemical reactions. This theory was put forward to explain why Greenland CO₂ data showed higher variability and higher concentrations when compared with Antarctic ice core CO₂ measurements located in the opposite polar region about 11,000 miles away. The theory of chemical alteration has gone unchallenged during the last twenty years. Since then, CO₂ data from Greenland ice cores was dismissed and only CO₂ data from Antarctic ice cores is currently used as the “gold standard” database. As a result, Greenland CO₂ datasets are not used in climate science studies to understand Northern and Southern hemispheres interactions and the sensitivity of greenhouse gases under various climatic conditions.

Greenland Ice CO₂ Chemical Reaction Theory
From earlier studies it was apparent that CO₂ concentrations of trapped air in bubbles from Greenland ice core data were different than CO₂ concentrations from Antarctic ice cores (Anklin, 1995, Stauffer, 1985). CO₂ concentrations in Greenland ice cores are generally 20 ppm higher and show higher standard deviations (6-10 ppm versus 2-3 ppm) than in Antarctic ice cores during the recent Holocene interglacial period. Greenland CO₂ concentrations from ice cores (GRIP, NGRIP, DYE3) agree well with each other and all show similar disagreements from Antarctica. See my previous post for a more thorough discussion of Greenland CO₂ over the past 50,000 years here.

Detailed laboratory measurements to understand CO₂ differences between the poles were performed on annual layers of Greenland ice cores, using the Antarctic Byrd ice core as the baseline (Tschumi, 2000). The focus of these studies was to determine potential chemical reactions that may cause variable and surplus CO₂ measurements in the Greenland ice cores. CO₂ was analyzed with an infrared laser absorption spectrometer. Some results of the study are shown in Figure 1.

Greenland CO₂ “surplus” in the study is defined where CO₂ in the GRIP ice samples are higher than CO₂ in the Antarctic Byrd ice core. In addition to being high, CO₂ surplus in the samples show short-term variations from 15-25 ppm within annual layers younger than 2700 years BP. In older sections of the Greenland ice core, larger CO₂ variability ranging from 40-80 ppm were measured.

Figure 1. Greenland CO2 and H₂O₂ measurements on GRIP ice cores. Data from Tschumi, 2000. Years are Before Present (BP) from 1950. Horizontal red dashed line is Antarctic Byrd CO2 values for the approximate same timeframe.

Tschumi concludes that a “generally positive” correlation exists between CO₂ surplus and carbonates or the Ca⁺ proxy used to explain high CO₂ in Greenland ice cores. Correlation coefficients ranged from 0.24 to 0.55. Therefore, production of CO₂ from an acid-carbonate reaction is reasonable when CO₂ peak values correlate with a maximum of the carbonate. More interestingly, there is a strong negative correlation between CO₂ peak values and hydrogen peroxide (H₂O₂) with correlation coefficients from -0.65 to -0.93. This strong negative correlation led Tschumi to introduce another reaction; the oxidation of organic compounds to produce enriched CO₂ values in Greenland samples.

In Greenland ice cores which experience high snow accumulation, annual timing of different species is highly resolved, clearly detected at all depths and used for annual layer counting (Rasmussen, 2006). Tschumi acknowledges that acidity, carbonates and calcium show short-term annual variations in ice samples. However, he states that oxidants like HCHO (formaldehyde) and H₂O₂ were believed to show no seasonal variations below the firn-ice transition, although his data shows strong annual variations. Annual amplitude can be dampened by diffusion, especially for gases. Studies show that most of the smoothing of H₂O₂ occurs in the top 10 meters of the ice sheet and there is no significant reduction in the H₂O₂ amplitude after 25 years (Anklin, 1997). Fuhrer, 1993 and Rasmussen, 2006, both show short-term annual variations of calcium and H₂O₂ at depths of 1400 m in the Summit GRIP and NGRIP ice cores. Rasmussen also shows that calcium has fewer peaks and lower resolution than H₂O₂ in ice cores, similar to what is seen in Tschumi’s data.

Lastly, Tschumi and Anklin make no mention of atmospheric annual CO₂ variability at Mauna Loa, Hawaii and Point Barrow, Alaska, versus the lack of annual variability at the South Pole. This difference persists even though the Hawaiian and Alaskan observatories have been measuring atmospheric CO₂ since 1958 and 1975, respectively. Their omission of annual CO₂ variability as a plausible explanation for ice cores from Greenland showing variable CO₂ measurements is puzzling.

Present day Arctic CO₂ shows More Variability than Antarctic
Investigating present day atmospheric CO₂ concentrations in the Arctic compared to the Antarctic provides a reasonable analog. Figure 2 shows the past few years of CO₂ measurements for Barrow and South Pole (SPO) observatory stations for comparison.

Presently in the Arctic, as shown by Barrow data, short-term annual CO₂ amplitude cycles range by almost 20 ppm. CO₂ rises during the winter months after terrestrial respiration and falls during the summer during photosynthesis showing strong evidence of a natural biospheric signal (Haverd, 2020). These same annual trends occur in many Northern Hemisphere observatories such as Barrow BRW, Summit SUM, Finland PAL, Norway ZEP and Russia TIK (NOAA, 2020). Additionally, CO₂ annual amplitudes in the northern latitudes have increased by 35% over the past 50 years (Graven, 2013). For example, CO₂ amplitudes have increased from 14 ppm in 1970 to almost 20 ppm in 2020. Changes in CO₂ uptake and release is evidence that the Northern Hemisphere terrestrial component is progressively taking up more carbon during spring and summer as CO₂ levels increase (Haverd, 2020).

Figure 2. Left graph shows Barrow CO₂ values in blue and South Pole SPO CO₂ values in gray over two years. Upper right plot shows Short-Wavelength InfraRed (SWIR) CO₂ column-averaged mixing ratio data projected on a global map for April and lower right plot for August 2018.

In contrast, a very weak fluctuation of opposite polarity is seen in South Pole (SPO) CO₂ measurements shown by the gray dark line in Figure 2. This same weak amplitude occurs in Antarctic Syowa Station SYO, Halley Station HBA, Palmer Station PSA, Drake Passage DRP, and Ushuaia, Argentina USH observatories. The CO₂ short-term annual amplitudes in the Southern Hemisphere are barely noticeable and only fluctuate by 1-2 ppm per yearly cycle over the past 50 years. It seems reasonable that the Antarctic, which is surrounded by oceans, would see minimal terrestrial influence on annual CO₂ variability.

The Mauna Loa MLO observatory has annual cycles that are in-between Barrow and South Pole with CO₂ amplitudes varying 5-6 ppm per year. The amplitude at MLO has only increased by 15% over the past 50 years, while the amplitude at Barrow has increased by twice that much. There is no distinguishable amplitude change over time south of 35 deg N (Graven, 2013).

The global maps in Figure 2 are from GOSAT, a satellite designed specifically for monitoring greenhouse gases from space. The maps show monthly overviews of the global distribution of CO₂ and demonstrate the seasonal oscillation which is restricted to the northern latitudes. During the summer months, CO₂ is lower in the Arctic and high latitudes compared to the tropics as shown by the August SWIR CO₂ global concentrations. During the winter dormant period, CO₂ increases by almost 20 ppm across most of the high latitudes dominated by land masses with maximums in the spring as shown by the April SWIR CO₂ global map.

Greenland Ice CO₂ Variability Compares well with Present-day Arctic Cycles
Short-term CO₂ variability in Greenland ice cores matches reasonably well to present day atmospheric CO₂ annual cycles seen in the Arctic observatories. Figure 3 compares an annual CO₂ cycle at the Barrow observatory to CO₂ variations in the Greenland ice cores. The duration and variability, or amplitude, are similar. The amplitude is about 15 ppm for the 1100-year BP sample and up to 25 ppm for the older 2700-year BP sample. The Barrow observatory currently shows amplitude in the 18-20 ppm range presently and as low as 14 ppm during 1970.

Figure 3. GRIP ice core CO₂ cycles compared to Barrow CO2 cycles for approximately 2 annual layers. Ages are in years BP from 1950.

Anklin, 1995 asserts that Greenland ice sections show short term CO₂ variations on the order of 10-20 ppm in annual layers which cannot represent atmospheric variation. However, as discussed, Arctic observatories clearly measure summer and winter CO₂ fluctuations of that magnitude over the past 50 years (Graven, 2013).

In contrast, CO₂ in Antarctic ice cores only show variations of 1-2 ppm annually (Anklin, 1995). This is expected because southern latitude observatories show that CO₂ has a very weak, if any, annual amplitude variation due to the lack of extensive terrestrial regions. Additionally, annual layers in Antarctic ice cores are barely distinguishable, if at all, due to less annual precipitation and more closely packed layers (Rasmussen, 2006). Therefore, CO₂ should show more variability in Greenland ice cores than Antarctic ice cores as observed in present-day CO₂ observatories.

Arctic H₂O₂ Variations are Anticorrelated with CO₂
Hydrogen peroxide is recognized as an atmospheric oxidant and was extensively studied in the 1980s for its role in acid rain (Sakugawa, et. al, 1990). H₂O₂ is associated with water vapor content in the air and solar radiation. Its formation, decomposition, and deposition processes are not clearly understood. Studies do confirm that atmospheric H₂O₂ concentrations show distinct seasonal variations of higher concentrations in the summer and lower in the winter.

Hydrogen peroxide is also present in high concentrations in both Arctic and Antarctic polar snow and high snow accumulation ice cores (Sigg, 1988 and Beer, 1991). Like atmospheric variability, hydrogen peroxide shows a strong seasonality with high concentrations in summer snow layers and low values in winter snow shown in Figure 4.

Figure 4. The annual variation of H₂0₂ compared to δ¹⁸O and CO₂. H₂0₂ data from Beer, 1990. The δ¹⁸O value is used as a proxy for the temperature.

Interestingly, present day atmospheric CO₂ in the Arctic shows a strong anticorrelation to hydrogen peroxide as shown in Figure 4b. As H₂O₂ rapidly increases in summer, CO₂ rapidly decreases due to terrestrial photosynthesis as described earlier. During the winter, CO₂ is at its high while H₂O₂ is low.

Tschumi also noted that many Greenland ice core samples demonstrate remarkable anticorrelations between CO₂ and hydrogen peroxide shown in Figure 1. Tschumi assumed the reduction of the oxidant H₂O₂ was a chemical reaction with an organic compound to create surplus CO₂ in the ice core sample. Recall, Tschumi’s research was premised upon finding a chemical reaction to explain the variability and “surplus” CO₂ in the Greenland ice cores.

Both atmospheric hydrogen peroxide and CO₂ demonstrate large annual variations in the Arctic today. Additionally, a strong anticorrelation between CO₂ and hydrogen peroxide is observed both present day as well as in ice cores. A much simpler explanation of the ice core anticorrelation between these two gases is natural annual variability rather than a chemical reaction.

Holocene Greenland Ice CO₂ Variability
Ranges and standard deviations for CO₂ variations in Greenland ice cores in addition to present day observatories are shown in the table below. The data are separated into three groups. Group 1 are annual observatory ranges from Barrow, Mauna Loa, and South Pole highlighted in blue for present day comparison. Group 2 are Greenland ice core data between 850 to 2700 years highlighted in green. Greenland ice samples between 7000 to 8300 years are Group 3, highlighted in orange.

Recent observatory data shows significant latitudinal differences between annual CO₂ amplitudes and standard deviations. The Barrow Arctic observatory shows high annual amplitude variations with standard deviations of 5-6 ppm. South Pole CO₂ data shows very small annual amplitude variations of 1-2 ppm with a small standard deviation and Mauna Loa is in between.

In ice cores, Anklin states standard deviations of CO₂ range from 6 to 10 ppm for Greenland ice which is much higher than standard deviations of CO₂ in Antarctic ice cores of 2 to 3 ppm. The CO₂ deviations in Barrow and the South Pole observatories are like the deviations Anklin observes between Greenland and Antarctic ice core data. In summary, the Greenland ice core CO₂ ranges and standard deviations for years younger than 2700 years agree well with Barrow observatory data.

Table 1: CO₂ ranges and standard deviations from observation stations and Greenland ice core data.

Figure 5. CO2 data from Greenland ice cores from NOAA 2020 Barrow observatory, GRIP Anklin 1995, Tschumi 2000, and Dye 3 Neftel 1985. Ranges in blue and means in red. Potential outliers in gray. Only Barrow CO2 ranges are shown for present day. Before present is < 1950.

The graph in Figure 5 shows the ranges of Greenland ice core CO₂ concentrations and the means over the past 8500 years for the detailed annual layer samples. Data from an annual cycle in 1975 and 2000 AD are included from the Barrow observatory station for comparison. Again, amplitudes or ranges during the past 2700 years are comparable to present day except for one data point around 850 years that is an outlier. Greenland CO₂ calculated means during this time are from 280 to 300 ppm which are only slightly higher than Antarctic ice CO₂ means of 277 to 282 ppm, respectively. The Greenland means compared to Antarctic means are close to the present-day interhemispheric CO₂ gradient of 2-5 ppm. Clearly, Greenland CO₂ data appears viable and simply demonstrates CO₂ is variable in the Arctic as seen in present annual atmospheric conditions.

If any data is suspect, it would be the older Greenland CO₂ data from 6500-8700 years which is highly variable showing ranges of 40-90 ppm. These CO₂ data are sampled during the Arctic climate optimum where arctic temperatures were approximately 2 to 4 degrees C warmer than today. During this time, it is speculated there was no permanent sea ice in the Arctic and Arctic biomes were shifted northward, where Arctic tundra was replaced with cool conifer forests. Large amounts of open water during the summer and presence of temperate and significant boreal regions may have led to large increases in CO₂ annual fluctuations in northern latitudes. These highly variable CO₂ data may represent very different climate conditions than we are experiencing present day and provide valuable information on past CO₂ fluctuations.

Natural Variability of Greenland Ice CO₂ is a Viable Explanation
Atmospheric CO₂ annual variability that are unique to northern latitudes can explain why Greenland CO₂ measurements in ice cores behave differently than in Antarctic ice cores. The Arctic and Antarctic polar regions have different natural biospheres. These polar regions exhibit different short-term behavior of atmospheric CO₂; large annual 20 ppm fluctuations in the Arctic from terrestrial influence and minor 1-2 ppm fluctuations in Antarctic. Additionally, the Arctic has higher snow accumulation rates than the Antarctic allowing for preservation of thicker annual ice layers.

A case for Greenland CO₂ data being just as accurate as the Antarctic ice core CO₂ has been offered. This explanation uses modern analog data from observatories rather than the 20-year old theory of in-situ chemical reactions. A theory requiring a complex process of in-situ chemical reactions to explain Greenland CO₂ variability seems unnecessary. Incorporating Greenland ice core CO₂ data would significantly alter scientific studies that utilize only the lazy Antarctic CO₂ profile to make interpretations concerning Global climate change. If Greenland ice core CO₂ data is even qualitatively correct, then it needs to be re-examined and incorporated into polar and interhemispheric greenhouse gas interpretations. Dismissing an entire polar region dataset because it doesn’t match Antarctic ice cores and explaining away the Greenland data with complex chemical reactions ignores data from modern observatories.

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

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