Guest Post By: Renee Hannon
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.
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).
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.
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.
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.
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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August 1, 2020 at 08:16PM