It's Science Baby! And, It Shows Us Much of the CO2 Discussion Misses the Point!

Guest Post from Renee Hannon at the CO2 Coalition.

The CO2 Coalition has just come out with a rather remarkable study titled “Measurement of C Concentrations Through Time Using Ice Core and Stomatal Proxy Data.” Don't be scared by intimidated by the academic wording; it’s well worth reading and relatively easy to understand. Read or at least peruse the document and yiou'll find it offers some extremely insightful observations regarding all the talk (hype really) about CO2 levels.

To entice you to take a look I am offering below some of the earlier background from the study and its conclusions:

INTRODUCTION

CO2 records from air bubbles in Antarctic ice cores are regarded as the gold standard for paleo- atmospheric global CO2 concentrations during past interglacial and glacial periods over the last 800,000 years. Antarctic ice-core data are openly available from the National Centers for Environmental Information (NCEI) and have been studied extensively.

The trapped gas in ice is the most direct measurement of CO2 concentrations for past atmospheric records, providing a continuous global CO2 baseline. CO2 measurements are repeatable and have low analytical errors of 1-2 ppmv per sample. CO2 measurements from ice cores show that CO2 ranged from 180 ppmv during glacial periods to as much as 300 ppmv during interglacials.

CO2 concentrations from plant stomata on the other hand are indirect proxy measurements consisting of discontinuous records over the Holocene and deglaciation period. The highly variable CO2 concentrations have high uncertainties in measurements as well as calibration models.

Plant stomata studies are mostly from the Northern Hemisphere where local conditions can strongly influence the resulting CO2 proxy estimates. The uncertainties and shortcomings associated with plant stomata CO2 reconstructions outweigh using this dataset as a valid quantitative indicator for paleo-atmospheric global CO2 concentrations.

SCIENTIFIC DISCUSSION

ICE CORE DATA

Cores from Antarctic glacial ice provide past temperature proxies from oxygen isotopes as well as gas composition from small air bubbles trapped within the ice. As snow accumulates and compacts over time, it is transformed into firn, a granular intermediate stage between snow and ice.

The firn eventually becomes dense enough that open pores begin to close, forming bubbles that trap atmospheric gases. Bender (1997) is a good overview of the firn to ice processes. This section examines how the CO2 concentrations measured in ice bubbles have varied over the past and the effects of compaction on the temporal resolution of these data.

Figure 1 shows key Antarctic ice cores and global CO2 concentrations over the past 800,000 years. In general, CO2 concentrations rise to nearly 300 ppmv during warm interglacial periods and decrease to as low as 180 ppmv during the cold glacial periods. The interglacial periods are noted using the marine isotope stage (MIS).

There are numerous ice cores that cover our current interglacial (MIS1) known as the Holocene and the preceding glacial period, with only a few that extend over the previous interglacial period referred to as MIS 5, and only one that extends the entire 800,000 years, the EPICA Dome C ice core.

Notable differences exist between ice cores taken in high accumulation/warmer temperature sites versus low accumulation/colder temperature sites. The deeper, older ice cores tend to occur in low-snow accumulation sites in Eastern Antarctica, and include EPICA Dome C, Vostok, and Dome Fuji.

These cores tend to have a prolonged entrapment of gas within ice bubbles and more extensive gas mixing of CO2 with the atmosphere, resulting in lower temporal resolution. The shallower ice cores are drilled in high-snow accumulation sites and in the peripheral areas of the ice field. High-accumulation sites such as Law Dome and WAIS have rapid burial, less gas diffusion within the firn, and better temporal resolution, but shorter records.

Years Before Present (1950)

Figure 1: a. Global CO2 concentrations from ice cores after Bereiter, 2014, plotted over time. b. Key ice cores showing approximate cored interval length. Colors indicate CO2 temporal resolution: red is low (>100 years), gray is intermediate (50-100 years), and green is high (10-50 years).

Ice Age to Gas Age Time Shift

Two key adjustments occur to atmospheric gases during the firn transition to ice prior to being trapped in ice bubbles. First, atmospheric CO2 concentrations are smoothed over time due to atmospheric mixing and gas diffusion within the firn. Second, the gas is believed to be younger than the age of the ice when it is eventually trapped within bubbles (Trudinger, 2002; Schwander, 1984). 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 31 years in Law Dome to 835 years in the lower snow accumulation EDML ice core. Very low snow accumulation sites such as Dome C and Vostok have a delta of thousands of years. In addition, the ice-gas age delta varies between interglacial and glacial periods due to varying snow accumulation rates.

Figure 2 shows CO2 concentrations versus the age of ice in which it is trapped for five ice cores in the Antarctic before adjustments. Atmospheric data from Cape Grim and firn data are included on the plot for comparison. Noted are the delta differences in years between the (younger) gas age and (older) ice age as well as the age of the ice at the boundary between firn and ice (which roughly corresponds to the base of the bubble trapping zone). This plot is a profile rarely found in published literature.

Figure 2: CO2 concentrations measured in ice age. The numbers show the average ice-gas age difference in years. The dashed line is the top of ice and approximate position of the base of the bubble zone. CO2 data for Law Dome (DE08, DSS) is from Rubino 2019; WAIS is from Bauska 2015; Siple is from Neftel 1994; and EDML is from Siegenthaler, 2005.

There are several methods to calculate the delta between ice age and gas age. Stratigraphic markers such as volcanic markers and distinct gas variations are used to determine the ice-gas age delta and for multi-core synchronization (Buizert, 2021). When gas measurements in ice or firn show the same dramatic increase as modern records, they are 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. DSS and Siple are shifted 58 and 83 years, respectively, to match the DE08 data. Various other methods are used to estimate the delta and resulting shift.

Firn models 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 (Buizert, 2021).

Another approach uses ice depths in the core that are contemporaneous with ice cores where gas ages are well constrained (Bender, 1997). Ice-gas age deltas have uncertainties of 10-15%, especially in low accumulation sites where the ice-gas age deltas are exceptionally large (Seigenthaler, 2005).

Gas age is the age of CO2 which is reported in most public datasets and graphs. Different CO2 datasets overlap quite nicely, with few exceptions, when corrected to the appropriate gas age (Figure 3). Within multiple ice cores, these gas shifts appear reasonable as distinct CO2 highs, such as the CO2 bulge, and lows, such as the Little Ice Age (LIA). Law Dome ice and firn CO2 measurements also closely match the modern CO2 rise measured in southern hemisphere monitoring stations.

Atmospheric/Firn

Bubble ZoneCO2 Measurements in Gas Age

Figure 3: CO2 concentrations corrected to appropriate gas age. Boxes on the right side highlight different data medium and approximate location of the ice bubble zone. CO2 data references noted in Figure 2.…

…PAST CO2 ICE CORE AND STOMATA CONCLUSIONS

CO2 concentrations from bubbles in ice cores are considered the key paleo-atmospheric global CO2 dataset, and rightfully so. The data consists of continuous records over 800,000 years, instrumental measurement error is low at around 1 ppmv, and the repeatability of multiple ice core datasets show CO2 concentrations within 4 ppmv of each other. Additionally, the location of ice cores in Antarctica reduces potential local CO2 modification due to seasonal fluctuations, which can range up to 25 ppmv in the Northern hemisphere.

CO2 concentrations from plant stomata on the other hand are indirect proxy measurements consisting of discontinuous records over the Holocene and deglaciation period. The highly variable CO2 concentrations have high uncertainties in both stomata density measurements as well as the CO2-SI models.

Plant stomata studies are mostly from the Northern Hemisphere where local conditions can strongly influence the resulting CO2 proxy estimates.

The uncertainties and shortcomings associated with plant stomata CO2 reconstructions outweigh the use of this dataset as a valid quantitative indicator for paleo-atmospheric global CO2 concentrations. The main value of such studies is to test and calibrate methodologies for application to earlier time periods for which ice core data is unavailable.

So, the bottom line is simply this: most CO2 discussions are based on unreliable data and the real science indicates CO2 levels have always fluctuated with ice ages and the after-effects. The hype is simply so much bunk.

Renee Hannon is a geologist with 40 years’ experience in the energy sector at Arctic locations and is a member of the CO2 Coalition, Arlington, Virginia. She has expertise in utilizing subsurface datasets to understand paleo depositional settings, tectonic evolution and source rocks.

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