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CO₂ hockey stick?

The idea of one single natural CO₂ level is largely based on the results of ice core drillings in Antarctica. They indicate a very stable level at around 280 ppm. It is however very unlikely that the measured concentration in the bubbles in the ice is an accurate representation of the concentration at the time these bubbles were formed.

The results of ice core measurements in Antarctica are an important argument in the belief that there is a single stable natural CO₂ level for the Earth at around 280 ppm. Ice core measurements involve analyzing air bubbles trapped in ice cores to determine past atmospheric CO2 concentrations. Scientists drill ice cores from places like Antarctica and Greenland, where layers of snow have accumulated over thousands of years and turned into ice. These ice cores can provide information on climate variables, including CO2 levels, going back up to 800,000 years.

The fundamental idea is the assumption that the gases in these air bubbles give an accurate representation of the atmospheric conditions at the time these bubbles were closed. Scientists reconstruct past atmospheric CO2 concentrations by measuring the concentration in the bubbles.

The CO₂ hockey stick
Figure 1: The constructed hockey stick graph based on a combination of ice core data (until 1960) and measurements on Mauna Loa (from 1960). Source: Scripps Institution of Oceanography

These proxies suggest that the natural equilibrium concentration has been stable for a very long time: between 260 and 280 ppmv in the last 10,000 years and between 180 and 290 ppmv in the past 800,000 years (Bereiter, B. et al. (2015). This is however at odds with our conclusion that that the natural atmospheric CO₂ level depends on the greenness of the Earth. It is improbable that there have been no fluctuations in the Earth’s GPP over the past 800,000 years, or that the GPP has ever been as high as it is today. See the example we gave in the article about the green CO₂ level, where we conclude that the concentration 10,000 years ago was at least as high was as it is today.

Bubbles in the ice layers

The ice cap as in Antarctica is formed by accumulated layers of snow. Each layer of snow is different in chemistry and texture, summer snow is different from winter snow, and so on. Over time, the buried snow compresses under the weight of the snow above it and forms ice. The layers of snow in the polar regions initially contain a lot of air. Under the weight of the higher layers, more and more air disappears from the snow. Some of the air is trapped in minuscule air bubbles. This process is shown in the following figure for ice cores in Law Dome (Antarctica).

Schematic diagram of the formation of air bubbles in the ice at Law Dome
Figure 2: Schematic diagram of the formation of air bubbles in the ice at Law Dome. Source: Middleton, D. (2017).

There are several stages to this process. In the mixing zone, there is still a large amount of exchange with air in the atmosphere via the upper layers. This layer (called 'firn') is an intermediate form of snow and ice. Due to the increasing weight, the pores of the air bubbles gradually become more closed in the deeper layers. The sealing zone (or Lock-in-Zone) is a transitional phase in which the pores in the air bubbles become increasingly closed. Only in the so-called sealed ice are the air bubbles isolated from the atmosphere. The enclosed air bubbles form the basis for determining the CO₂ concentration.

The idea is that the composition of the air in the bubbles provides information about the concentrations from the time the air bubbles were formed. There are however numerous problems involved in this reconstruction of the CO₂ levels, which have been extensively discussed in the papers of Jaworowski. The two most fundamental issues are the dissolvement of CO₂ in melting water and the low resolution of the ice core data.

CO₂ dissolved in melting water

The all determining assumption for ice core proxies is that the air bubbles in the ice form a closed system without any interaction with the environment. But it is very unlikely that the CO₂ level in the bubbles is still the same as at the moment they were formed. Every summer melting water penetrates in the firn. CO₂ dissolves very easily in the water in the many years before the air bubbles in the ice are fully closed, especially at low temperatures and high pressure. CO₂ is 50 times more soluble in water than nitrogen and 200 times more soluble than oxygen, while liquid water can exist in ice down to -73 °C. Consequently, the absolute value of the measured concentration represents only a fraction of the original value (Jaworowski 1992).

An indication that ice core measurements are biased is the observation that the concentration decreases with depth of the air bubbles in the ice. See Figure 3. The measured carbon dioxide concentration is lower in deeper layers than in higher layers. This lower level has been attributed to assumed lower historical CO₂ levels, but it could also very well be related to the dissolution of CO₂ in meltwater.

Lower CO2 concentrations in deeper layers
Figure 3: The left graph shows CO₂ concentrations from different ice cores as a function of depth. In the right graph, the CO₂ data are normalized to the 1996 value. Data from Battle, 2011 and Rubino, 2019. Source: Hannon, R. (2021).

Ice core measurements from before 1985

In the mid-1950s, people started to determine carbon dioxide concentrations in snow and ice in glaciers and in Greenland and Antarctica. It is striking to see that almost all studies in the period up to about 1985 showed much higher concentrations of CO₂ than later studies. In the following screenshot, Jaworowski (1992) summarized several of these studies. As can be seen, there is a large spread in the values found (from 100 to 7400 ppm) and the highest values are far removed from the later observations. After 1985, only lower pre-industrial CO2 levels were reported and used as evidence for a recent anthropogenic CO2 increase (Jaworowski et al. 1992). All higher measured values were disregarded as outliers

The CO₂ hockey stick
Figure 4: Ice core measurements from before 1985 show a much larger spread in the measured concentrations (from 100 to 7400 ppm). Source: Jaworowski et al. (1992).

Low resolution

Ice core reconstructions also give a very flattened representation, in which only slow changes are visible. In the data recorded over the past 800,000 years, a single observation in an ice layer represents a period of 10s to many 100s of years, up to 5000 years (on average 730 years). Short fluctuations, even with much higher concentrations, are therefore not visible. Averaging smooths out variation. Unlike the ice core records, direct measurements taken between 1800 and 1960 and proxies from plant stomata show much greater values and more variations, which is more in line with what could be expected from our findings (Kouwenberg, L. (2003); Beck, E.-G. (2021).

Ice core proxies provide a highly smoothed view
Figure 5: Ice core data provide a very flattened, low-resolution view. Combining datasets with very different resolutions in one chart is misleading. Original map source: NASA 2023.

Presenting data with very different resolutions in one chart as in Figure 5, is misleading and scientifically unacceptable. It suggests that the present high level is unprecedented, but if we take the same time scale, i.e. the average of the last 730 years, there is no deviation from the past records whatsoever.

Although ice core measurements are incredibly valuable for providing trend information on CO₂ levels, temperature and other parameters, it is likely that the absolute measured CO₂ values are much lower than the original concentrations, due to the dissolvement of CO₂ from the capsulated air in firn and ice into water (Jaworowski 2007).

CO₂ measurements

Atmospheric measurements on Mauna Loa started in 1958, but many measurements of the CO₂ concentration in the atmosphere are also available before that date. Between 1812 and 1961, more than 90,000 direct measurements were made at many locations around the world, which are available in a large number of scientific publications. The dataset compiled by Ernst Beck in 2022, which is based on roughly 400 well-documented studies, shows more variation and higher CO₂ values.

The CO₂ measurements since 1826
Figure 6: The trend of CO₂ concentration in the atmosphere, determined by numerous scientific measurements, along with data from Mauna Loa dating back to 1960. The gray area represents the margin of error. Source: Beck, E.-G. (2021).

The graph indicates a distinct peak around 1940, with carbon dioxide concentrations reaching up to 390 ppmv, which is close to current levels. This peak seems to align closely with the trends in Earth's (sea surface) temperature during that time. Below, the same graph is presented, now incorporating the temperature trend. Harde, H. (2023) even showed that the fluctuations in the CO₂ concentration can be fully explained by the extra emissions from land (soil respiration) and oceans as a result of the changes in temperature.

The CO₂ measurements since 1826 in combination with the sea surface temperature
Figure 7: The trend of atmospheric CO₂ concentration, as observed by Beck and Mauna Loa, alongside sea surface temperature, shows a correlation factor of 0.668. Notably, the CO₂ data lags one year behind the temperature data. Source: Beck, E.-G. (2021).

It is remarkable that this dataset is so underutilized today. The vast majority of these measurements have been disregarded by the IPCC. Only measurements showing low CO₂ concentrations were selected; all higher values were ignored. Normally proxies like ice core records are tested against real measurements, but in this case it is the other way around. Measurements with higher concentrations that do not fit the picture of the stable low concentration based on the proxies are therefore disregarded.

CO₂-proxies (from before 1826)

Prior to 1826, our understanding relies solely on proxy data. Notably, proxies other than Antarctic ice core data are often overlooked. This includes data derived from plant stomata (which are the openings in fossilized leaves and needles indicating CO₂ levels) and ice core samples from other regions, especially Greenland.

The dataset exemplifies Kouwenberg, L. (2003) research on plant stomata, displaying significantly more variation and higher values than those found in Antarctic ice core proxies. See Figure 8. Many more investigations on stomata in plant leaves and needles all over the world show similar results.

CO₂ measurements based on plant stomata.
Figure 8: The atmospheric CO₂ concentration trend, as indicated by plant stomata, is represented alongside the dotted line which denotes the proxies from Antarctica's ice cores. Source: Kouwenberg, L. (2003).




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