Guest contribution by Renee Hannon
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introduction
This article examines the CO2 data collected from ice cores in the Antarctic and compares the CO2 measurements in both the Ice Age and the Gas Age. The age of the gas trapped in the ice varies dramatically in Antarctica and depends on the rates of accumulation. To compensate for this age difference, peer-reviewed studies use a simple method to move CO2 measurements from the core ice age to a more recent CO2 gas age.
The CO2 hockey stick
The CO2 hockey stick is a familiar act. Figure 1 shows CO2 data from ice core bubbles, CO2 data from firn and atmospheric instrumental CO2 measurements from Cape Grim. Atmospheric data is only available for the past 150 years. Therefore, firn and ice core data are used to extend the CO2 dataset further into the past. Ice records with high accumulation such as DE08 often do not go back to the past and do not even cover the Little Ice Age (LIA). It’s amazing how all the very different CO2 datasets overlap quite well with a few exceptions.
Figure 1: CO2 concentrations corrected for the gas age. The inset is blown up between 1900 and 2020 AD and shows a flat carbon dot. Different volumes and the approximate location of the ice bubble zone are highlighted in the boxes. The CO2 data for Law Dome (DE08, DSS) comes from Rubino 2019; WAIS is from Bauska 2015; Siple is from Neftel 1994; and EDML is from Siegenthaler, 2005.
Aside from the eye-catching hockey stick, there are a few other notable observations. In the case of CO2 measurements from 1900 and older, there is more scatter between the various ice core records. One reason for the higher scatter is that the WAIS CO2 data are systematically 3-4 ppm higher than the CO2 data from the Law Dome ice core (Ahn, 2012). Scientists cannot explain this deviation and often only subtract 4 ppm from this data set (Bereiter, 2014).
A CO2 flat point and stabilization of 310-312 ppm from 1940-1960 can be seen from the Law Dome data (MacFarling, 2006). The smoothing due to gas diffusion in the firn and inclusion in bubbles reduces the CO2 variation, so the actual atmospheric variation is likely greater than the Law Dome ice core record. Unfortunately, the CO2 flattening ended shortly before atmospheric records began in Mauna Loa.
A CO2 bulge occurs in all ice core records from around AD 1000 to AD 1600 and lasts for over 600 years. This increase in CO2 also probably had a larger atmospheric signature than is preserved in ice cores. The CO2 bulge ends with the onset of the LIA around AD 1600, where CO2 decreases in all ice core records. A unique drop in CO2 in the Law Dome DSS data occurs around AD 1610 near the beginning of the LIA and may be due to its higher resolution (Rubino, 2019). This dip is not seen in any other ice record and contributes to the dispersion of the CO2 data. DSS also has other CO2 lows at 1780 AD towards the end of the LIA and at 1278 and 1350 AD in the middle of the CO2 bulge. Rubino points out that understanding these ice-recorded amplitude fluctuations and the actual size of the original atmospheric signatures before the snow is smoothed is a critical piece of the CO2 puzzle.
The CO2 shift
As discussed in my previous WUWT article here, atmospheric gases are modified during the firn transition to ice and bubble trapping. There are two major modifications that depend on the rate of snow accumulation and temperature. First, the CO2 variability is smoothed out by atmospheric mixing and diffusion with firn CO2 concentrations. Second, the gas is believed to be younger than the age of the ice when it finally becomes trapped in bubbles. (Battle, 2011; Trudinger, 2002, Blunier, 2000). Once trapped in the bubbles, the gas is believed to age with the ice. This age difference is known as the Ice Gas Age Delta. The delta ranges from about 30 years in the Law Dome to 835 years in the EDML ice core with less accumulation. Very low accumulation sites like Dome C and Vostok have a large delta of thousands of years.
Figure 2 shows CO2 measurements from the actual age of the ice in which it is trapped for five ice cores in Antarctica and before adjustments by applying ice gas age deltas as shown in Figure 1. Atmospheric data from Cape Grim and Firn data are shown on the plot for comparison. The delta difference in years between the Younger Gas Age and the Older Ice Age is determined. The top of the ice, which roughly corresponds to the base of the bubble zone, is also shown. This representation is a profile that is seldom found or discussed in the published literature.
Figure 2: CO2 concentrations in the Ice Age. The numbers show the age difference between ice gas and years. The dashed line is the top of the ice and the approximate location of the base of the bubble zone. CO2 data references given in Figure 1.
Figure 2 leads to the question of how the delta between ice and gas ages is calculated. If gas measurements in ice or firn are identical to instrument data, they are simply shifted to the age of the instrument data. For example, Law Dome DE08 ice gas data is evenly shifted 31 years to match the instrumental atmospheric data. Various other methods are used to estimate the delta and the resulting smooth displacement. Firn models can calculate the ice gas age delta for ice cores based on density and temperature data and are restricted using nitrogen-15 data, a proxy for the firn thickness (Raynaud, 2005). Another approach uses ice depths in the core that occur simultaneously with ice cores, where gas aging is severely limited (Bender, 2005). DSS and Siple are shifted 58 and 83 years, respectively, to match the DE08 dates. After all shifts are complete, a large hockey stick appears around AD 1900 with increasing levels of CO2, as shown in Figure 1.
The CO2 shift method using siple data was highlighted by Jaworowski, 2004. He pointed out that in 1890 AD, high CO2 concentrations of 328 ppm occurred in the siple ice core, which did not agree with the interpreted CO2 baseline. All of the siple CO2 data has simply been shifted 83 years to match Mauna Loa’s modern instrumental CO2 measurements in 1973. This simple displacement method continues to be an accepted technique for “correcting” the younger gas age in ice cores.
The extent of the age shift is interpretive, and scientists use different methods that result in different shifts for the same data set. Ice gas age deltas show uncertainties of 10-15% (Seigenthaler, 2005). Why is that important? The temperatures from the water isotopic composition of the ice are in the ice age. Thus, the temperatures are always given at the same age as the ice. While the gas data is corrected from the age of the ice to an interpreted gas age. Whenever the lead-lag relationships are assessed, the uncertainty of 10 to 15% associated with the calculation of the CO2 ice gas age delta should be taken into account.
CO2 hockey stick preservation in the ice
If the CO2 shift or age delta is removed, as shown in Figure 2, the suppression of CO2 variability with spots with less accumulation becomes easy to see. With the exception of DE08, ice core records below the bubble zone show that the highest CO2 record is only 312-316 ppm, which is almost 100 ppm below the current atmospheric value of 410 ppm (Figure 3a). It is interesting to note that these values from 312 to 316 ppm are comparable to the DE08 flat point.
Many authors have documented gas smoothing in the firn layer due to vertical gas diffusion and gradual bubble closure during the transition from firn to ice (Trudinger, 2002; Spahni, 2003; MacFarling, 2006; Joos and Spahni, 2008; Ahn, 2012); Fourteau, 2019; Rubino, 2019). In order to compensate for nuclei from different accumulation points, a gas age distribution width or smoothing is modeled. For example, high accumulation law dome cores have an average gas age of 10-15 years, a WAIS gas average of about 30 years, and a DML of 65 years. Low accumulation sites like Dome C and Vostok show that the gas is averaged or smoothed over hundreds of years. This means that a smoothing factor must be applied to atmospheric gas measurements compared to various ice cores. Most historical CO2 graphs, however, simply combine atmospheric and firn CO2 with ice CO2 data without applying any smoothing, as shown in Figure 1.
Figure 3: a) Initial CO2 concentration in each ice core below the bubble zone. b) Graph showing the relationship between the ice gas age delta and the gas smoothing in years. Data from Ahn, 2012; Trudinger, 2002, and Seigenthaler, 2005.
Ice cores with low accumulation, which experience larger displacements and larger deltas of the Ice Gas Age, also retain less CO2 variability and greater smoothing. The relationship between the shift in ice gas age and the smoothing of the gas amplitude is shown in Figure 3b.
Observations
Many variables and data assumptions are used to compare the rapid increase in atmospheric CO2 this century with previous ice core data. CO2 measurements from very different data sets are often linked. Atmospheric, firn and ice cores. Atmospheric CO2 gas is modified in the firn by diffusion and gradual trapping of bubbles and cannot be compared directly to CO2 data in ice cores below the bubble zone. The usual method of simply shifting the age measurements of the CO2 ice core in combination with not applying the appropriate atmospheric damping results in the reinforced CO2 hockey stick.
Acknowledgments: Special thanks to Donald Ince and Andy May for reviewing and editing this article.
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