Guest Post by Renee Hannon
This post examines CO2 ice core measurements from Antarctica during the Holocene Epoch. The key CO2 dataset for paleoclimate studies is the EPICA Dome C (EDC) data also known as Dome Charlie or Dome Concordia. Dome C is located on the eastern Antarctic Plateau, one of the coldest places on Earth and our planet’s largest frozen desert. The average air temperature is -54.5 degrees C with little to no precipitation throughout the year. CO2 measurements from the unique conditions at Dome C are compared to other Antarctic ice cores.
CO2 Parallels Antarctic Temperature Trends
Antarctic ice core CO2 data is readily available and has been studied extensively (Bauska, 2015, Ahn, 2012, Siegenthaler, 2005 Marcott, 2014 and Rubino, 2019). Bereiter, 2014, established a CO2 composite that is considered the gold standard and used by IPCC AR6 (although AR6 incorrectly references Bereiter as 2015). This CO2 composite shown in Figure 1 is based on three ice cores over the Holocene: Law Dome, EPICA Dome C and WAIS. Most of the Holocene (2,000 to 11,000 years BP) is covered by Dome C. The Late Holocene (0 to 2000 years BP) is covered by Law Dome and the Early Holocene (>11,000 years) is covered by WAIS which is shifted downward by 4 ppm to match Dome C (Bereiter, 2014).
Also shown on Figure 1 are Dome C proxy temperature anomalies derived from oxygen isotopes. The CO2 data correlates well with Antarctic ice core temperature anomalies as noted by the authors above.
Antarctic temperature is not representative of global temperatures or Arctic temperatures. According to Ahn, 2012, Antarctic data represents a specific set of processes in the climate system such as the transport of heat to and from the Southern Hemisphere via oceanic and atmospheric circulation. Local southern insolation also plays a role (WAIS members, 2013). Although Antarctic temperatures are unique to the Southern Hemisphere, CO2 data from Antarctica is considered representative of past global CO2.
What is surprising is that CO2 from Dome C is used as a key dataset for most of the Holocene and past 11,000 years. The Dome C ice core site with the low snow accumulation rates and extremely low temperatures resulting in one of the lowest temporal resolution records.
Antarctic CO2 Records Show Scatter
All public Antarctic CO2 datasets were plotted in Figure 2 to see if higher resolution CO2 data exists during the Holocene. CO2 was measured using the dry extraction technique that minimizes ice core melt and potential chemical reactions. Surprisingly, there is quite a bit of scatter between the different Antarctic datasets. The maximum is nearly 25 ppm and the average is 7 ppm. The largest amount of scatter appears at the end of deglaciation and on the shoulder of the Early Holocene interglacial from 10,500 to 11,500 years BP.
Ahn, 2004, describes key Holocene CO2 trends based on Siple and Taylor Dome data. After the Younger Dryas (YD) event, CO2 rose during the deglaciation reaching up to 284 ppm at the beginning of the Holocene (11,500 years BP). During the mid-Holocene, CO2 concentrations decrease to a local minimum of 260 ppm around 8,000 years BP and then increase to 285 ppm in the late Holocene.
The past 2000 years is documented by detailed sampling done in studies by Ahn, 2012, and Rubino, 2019 and others. In summary, CO2 data show a small bump up to 287 ppm during the Medieval Warm Period (MWP) around 800 BP (1150 AD), a decrease of CO2 during the Little Ice Age (LIA) to almost 270 ppm and the recent modern rapid rise. Ahn, 2014, also conducted a detailed sampling program using Siple Dome over the 8000-year CO2 Antarctic minimum in pursuit of the distinct 8,200-year cooling event that is observed in both Greenland ice core oxygen isotope and methane records. This rapid decrease is absent in Antarctic methane and oxygen isotope data, and instead shows a more gradual decrease labeled the Antarctic CO2 minimum in Figure 2. The Antarctic temperature and CO2 minimum are likely associated with the Arctic 8,200-year event and not coincident.
The CO2 Composite Displays Muted Values
How does the Antarctic global CO2 composite compare to other Antarctic CO2 ice core data? Figure 3 shows the “gold standard” Antarctic CO2 composite highlighted by the red line along with all other Antarctic CO2 records represented by dots. The widely used CO2 composite for the Holocene underrepresents CO2 and has lower CO2 readings than other Antarctic datasets by 2 to 20 ppm.
Vostok is also highlighted by an orange line as it is the other CO2 dataset frequently used for past paleoclimate interglacial comparisons. The Vostok CO2 data is even more muted but does capture long-term underlying trends. The last data point is quite interesting around 2500 years BP, which is the highest Vostok reading of 285 ppm.
Byrd, Siple Dome, and WAIS all show elevated CO2 variations in the early Holocene with differences up to 20 ppm higher than the CO2 composite from 11,500 to 10,500 years BP. CO2 elevated levels are 10 ppm higher than the composite until the CO2 minimum around 8000 years BP. From 8000 years until about 2000 years BP, the scatter is minimal and less than 5 ppm. Increasing differences begin to occur again during the past 2000 years.
Ahn, 2004, also noted similar intervals where the CO2 concentrations in Siple Dome ice are higher than in the Vostok, Taylor Dome and Dome C cores. Multiple causes for the discrepancies are discussed ranging from age scale uncertainty, chemical reactions due to carbonates and organic material, melt layers and fractures. Ahn concluded the cause of these elevated concentrations is not known with certainty.
Figure 3 shows elevated CO2 as the climate transitioned out of the cold Younger-Dryas and this shoulder may represent a period of climate instability which results in more scatter in the Antarctic ice core data. These unstable CO2 conditions lasted for about 1000 years. Relatively stable CO2 climatic conditions occur after the CO2 minimum around 8000 years BP where CO2 scatter between ice cores is minimal. The scatter in CO2 over the past 1000 years suggests a return to less stable climate conditions.
Dome Fiju wet extraction CO2 data points from Kawamura, 2007 are added to Figure 3. They follow general trends but also show higher CO2 by up to 10 ppm, mostly during the early Holocene shoulder excursion and more recently during the RWP and MWP. Kawamura found the higher CO2 concentrations are not related to Ca2+ concentrations which are the indicator for carbonate concentration and potential chemical reaction. He suggested the dry extraction method might not always be suitable for analyzing CO2 concentrations of ice samples containing clathrate hydrates. He also states the wet extracted CO2 data should be regarded as an upper limit of estimation of atmospheric CO2 in Antarctic cores instead of completely ignoring the data.
Temporal Resolution Matters
There are two very different processes that impact CO2 data resolution. One is related to gas smoothing during snow densification and the other is simply sample spacing. There does not appear to be a section in the newly released IPCC AR6 that discusses either of these data resolution issues. Although I have not yet read all 3,949 pages of the document.
Gas smoothing due to the firn to snow transition is site dependent and related to snow accumulation and temperature. Low snow accumulation sites include Dome C and Vostok. High snow accumulation sites include Law Dome, Siple Dome, and WAIS. As atmospheric CO2 passes from firn to ice, it is altered due to gas mixing processes (Ahn, 2012; Rubino, 2019). The gas is younger than the ice age when it is trapped in the bubbles, so scientists calculate an ice-gas age delta. This age delta is very large in low accumulation sites such as in Dome C and Vostok with the delta exceeding thousands of years as shown in Table 1.
Vertical gas diffusion and gradual bubble close-off during the transition from firn to ice also result in gas that is mixed over multiple years. A gas age width or smoothing is calculated or modeled. Frequently, the gas width is estimated to be on the order of 10% of the delta age value. Low accumulation sites in East Antarctic such as Dome C and Vostok show that gas is averaged or smoothed over hundreds of years. This smoothing effect removes high frequency variations from the ice core record.
Higher snow accumulation sites Siple Dome, Byrd, WAIS and Law Dome have the smallest ice-gas delta and the highest resolution with smoothing over tens of years. These ice core records show higher CO2 variability in the early Holocene and large deviations from the CO2 composite. Unfortunately, CO2 from Law Dome is only measured over the past 2000 years.
Dome C and Vostok have the lowest temporal resolution. Monnin, 2004, states that the time resolution of Dome C and Vostok records are too low to provide a history of CO2 changes that shows the detailed evolution of the atmospheric CO2 record. These records do not capture the elevated CO2 levels in the early Holocene. The Holocene CO2 composite by Bereiter, 2014, which uses mostly Dome C represents the low end or minimum case for CO2 data and does not represent the average from Antarctic cores. Then shifting WAIS down by 4 ppm to match Dome C in the composite reinforces the CO2 minimum case.
Sample spacing resolution is a problem that is frequently overlooked and/or not addressed. Average sample spacing interval in time for CO2 measurements over the Holocene for each core is shown in Figure 4. It should be noted that very little sample spacing is less than 10 years and most samples are spaced 100 years or further apart.
Several ice cores are sampled across the entire Holocene, but with different sample spacing intervals. The higher temporal resolution Byrd and Siple Dome CO2 data have a sparse sample spacing of 200 to 400 years. Ironically, the lowest temporal resolution records of Dome C have the highest sample frequency of about 100 years over the Holocene. Increased sampling of low snow accumulation sites with low temporal resolution won’t increase the data resolution.
In summary, reduced temporal resolution due to firn densification processes and low sample frequency can explain elevated CO2 levels in the early Holocene not captured by Dome C and Vostok but seen in Siple Dome, Bryd and WAIS.
How does the recently increasing CO2 values compare to altered smoothed CO2 in the ice core records? First, the modern record needs to be smoothed to mimic diffusion within the firn, and then re-sampled. To compare to Dome C, the modern record needs to be smoothed or averaged over 210 years and then resampled every 100 years. To compare to the higher resolution records, the modern record needs to be smooth over about 50 years and then resampled every 100 to 200 years. The modern CO2 increase of 100 ppm over the past 100 years will preserve at most 1 to 2 elevated data points in the past ice record. David Middleton did a nice job of demonstrating modern record smoothing (without resampling) in his WUWT post here.
Interestingly, many scientists frequently flag elevated data as suspicious outliers and reject them from the record because they deviate more than 1-2 standard deviations from a spline trend (Mitchell, 2013). Perhaps these outlier data points in ice cores are indicating higher frequency events but is difficult to know since most are deleted and removed from further study. It is likely the modern CO2 increase, after attenuation due to firn smoothing and sample spacing resolution, would be deleted as an outlier that exceeds the 1-2 standard deviation threshold.
Ice cores from the East Antarctic Plateau, Dome C and Vostok, do not capture the full magnitude of Antarctic CO2 variability. They are the lowest temporal resolution datasets for CO2 measurements and represent the minimum CO2 values as well as reduced variability over the Holocene. Yet they are the key past CO2 records used when comparing to modern CO2 data. Recent CO2 increases appear rapid and radical. If the recent CO2 increase over the past 100 years is attenuated to replicate ice core measurements and then resampled to ice core data intervals it will only represent a mere data point or two within the Holocene ice core record.
Exclusion of high temporal resolution records and sparse sampling results in an incomplete picture of CO2 variability. Dome C data underestimates CO2 values during the early Holocene and perhaps the dynamic behavior of CO2. The need to utilize CO2 data that records short-term variability, as well as longer-term trends, is essential to understand the modern centennial increase. Ice core CO2 records are imperfect data and therefore, the CO2 composite should be an average of all ice core values that includes both a maximum and minimum range.
Lastly, Dome C and Vostok are the few ice core records that cover past interglacial periods. This means that CO2 responses on past interglacial periods are CO2 minimums. And higher frequency centennial CO2 variability is not captured.
Acknowledgements: Special thanks to Donald Ince and Andy May for reviewing and editing this article.
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