The CO2 Shift; Ice Age to Gas Age

Guest Post by Renee Hannon

Introduction
This post examines CO₂ data collected from Antarctic ice cores and compares CO₂ measurements in both ice age and gas age. The age of trapped gas in ice varies dramatically across the Antarctic and is dependent on accumulation rates. To compensate for this age difference, peer reviewed studies use a simple method of shifting CO₂ measurements from the core ice age to match a younger CO₂ gas age.

The CO₂ Hockey Stick
The CO₂ hockey stick is a familiar plot. Figure 1 shows CO₂ data from ice core bubbles, CO₂ data from the firn, as well as Cape Grim atmospheric instrumental CO₂ measurements. Atmospheric data is available for only about the past 150 years. Therefore, firn and ice core data are used to extend the CO₂ record further into the past. High accumulation ice records such as DE08 frequently do not extend as far back into the past nor do they even cover the Little Ice Age (LIA). It is amazing how all the vastly different CO₂ datasets overlap quite nicely with few exceptions.

Other than the eye-popping hockey stick, there are several other observations worth noting. There is more scatter in CO₂ measurements from 1900 and older between the various ice core records. One reason for the higher scatter is WAIS CO₂ data is systematically 3-4 ppm higher than Law Dome ice core CO₂ data (Ahn, 2012). Scientists cannot explain this deviation and frequently just subtract 4 ppm from this dataset (Bereiter, 2014).

A CO₂ flat spot and stabilization of 310-312 ppm from 1940-1960 is apparent on the Law dome data (MacFarling, 2006). Smoothing due to gas diffusion in the firn and enclosure in bubbles reduces CO₂ variation, so the actual atmospheric variation is likely larger than the Law Dome ice core record. Unfortunately, the CO₂ flattening ended just before atmospheric records at Mauna Loa started.

A CO₂ bulge occurs in all ice core records from about 1000 AD to 1600 AD and is over 600 years in duration. Again, this increase in CO₂ likely had a larger atmospheric signature than is preserved in ice cores. The CO₂ bulge ends with the onset of the LIA around 1600 AD where CO₂ declines in all ice core records. A unique CO₂ dip in the Law Dome DSS data occurs at 1610 AD 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 records and adds to the scatter in CO₂ data. DSS also has other CO₂ lows at 1780 AD near the end of the LIA, and at 1278 and 1350 AD in the middle of the CO₂ bulge. Rubino points out that understanding these amplitude variations recorded by ice and the actual size of the original atmospheric signatures before firn smoothing is a critical piece of the CO₂ puzzle.

The CO₂ Shift
As discussed in my previous WUWT post here, atmospheric gases are modified during the firn transition to ice and bubble trapping. There are two key modifications that are dependent on snow accumulation rates and temperature. First, CO₂ variability is smoothed due to atmospheric mixing and diffusion with firn CO₂ concentrations. Secondly, the gas is believed to be younger than the age of the ice when it is eventually trapped within bubbles. (Battle, 2011; Trudinger, 2002, Blunier, 2000). Once trapped within the bubbles, gas is assumed to age with the ice. This age difference is referred to as the ice-gas age delta. The delta ranges from about 30 years in Law Dome to 835 years in the lower accumulation EDML ice core. Very low accumulation sites such as Dome C and Vostok have a large delta of thousands of years.

Figure 2 shows CO₂ measurements from the actual age of ice in which it is trapped for five ice cores in the Antarctic 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 older ice age are noted. The top of ice which roughly corresponds to the base of the bubble zone is also shown. This plot is a profile rarely found or discussed in published literature.

Figure 2 leads to a question about how the delta between ice and gas ages is calculated? When gas measurements in ice or firn are the same as instrumental data it is simply shifted to the age of instrumental data. For example, Law Dome DE08 ice gas data is uniformly shifted by 31 years to match instrumental atmospheric data. Various other methods are used to estimate the delta and ensuing uniform shift. Firn models can calculate the ice-gas age delta for ice cores using density and temperature data and are constrained by using nitrogen-15 data, a proxy for firn thickness (Raynaud, 2005). Another approach uses ice depths in the core that are contemporaneous with ice cores where gas ages are well constrained (Bender, 2005). DSS and Siple are shifted 58 and 83 years, respectively, to match the DE08 data. After all the shifting is done, a big hockey stick of increasing CO₂ concentrations appears around 1900 AD as shown in Figure 1.

The CO₂ ‘Shift Method’ using Siple data was notably highlighted by Jaworowski, 2004. He pointed out that high CO₂ concentrations of 328 ppm occurred in 1890 AD in the Siple ice core which did not match the interpreted CO₂ baseline. The entire Siple CO₂ data was simply shifted by 83 years to match modern instrumental CO₂ measurements at Mauna Loa in 1973. This simple shift method continues to be an accepted technique for “correcting” the younger gas age in ice cores.

The amount of age shifting is interpretive, and scientists use various methods resulting in different shifts for the same dataset. Ice-gas age deltas have uncertainties of 10-15% (Seigenthaler, 2005). So, why is this important? Temperatures from the water isotope composition of the ice are in ice age. Thus, temperatures are always presented in the same age as the ice. Whereas the gas data is corrected from the age of the ice to an interpreted gas age. Any evaluation of lead-lag relationships should consider the 10-15% uncertainty associated with the calculation CO₂ ice-gas age deltas.

CO₂ Hockey Stick Preservation in Ice
When the CO₂ shift or age delta is removed as shown in Figure 2, then the CO₂ variability suppression with lower accumulation sites become readily apparent. Except for DE08, ice core records below the bubble zone show CO₂‘s highest recording is only 312-316 ppm which is almost 100 ppm lower than the current atmospheric reading of 410 ppm (Figure 3a). It is interesting to note these readings of 312-316 ppm are comparable to the DE08 flat spot.

Many authors have documented gas smoothing in the firn layer due to vertical gas diffusion and gradual bubble close-off during the transition from firn to ice (Trudinger, 2002; Spahni, 2003; MacFarling, 2006; Joos and Spahni, 2008; Ahn, 2012; Fourteau, 2019; Rubino, 2019). To compensate for cores from different rate of accumulation sites, a gas age distribution width or smoothing is modeled. For example, high accumulation Law Dome cores have a gas age average of 10-15 years, WAIS gas average is about 30 years, and DML is 65 years. Low accumulation sites such as Dome C and Vostok show gas is averaged or smoothed over hundreds of years. This means a smoothing factor needs to be applied to atmospheric gas measurements when comparing to various ice cores. However, most CO₂ historical graphs simply splice on atmospheric and firn CO₂ to ice CO₂ data without applying any smoothing as shown in Figure 1.

Low accumulation ice cores that experience bigger shifts and larger ice-gas age deltas also preserve lower CO₂ variability and higher smoothing. The relationship between ice-gas age shifting and gas amplitude smoothing in shown in Figure 3b.

Observations
Many variables and data assumptions are used when comparing the rapid rise in atmospheric CO₂ during this century to past ice core data. CO₂ measurements from very different datasets are frequently linked; atmospheric, firn, and ice cores. Atmospheric CO₂ gas is modified in the firn by diffusion and gradual bubble trapping and cannot be directly compared to CO₂ data in ice cores beneath the Bubble Zone. The common method of simply shifting CO₂ ice core age measurements combined with not applying the appropriate atmospheric attenuation results in the amplified CO₂ hockey stick.

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

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