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Atmospheric CO2 and Global Warming

Telling excerpts from this valuable paper by Z. Jaworowski, T.V. Segalstad, and V. Hisdal, publ. Norsk Polarinstitutt 1992

Contemporary Measurements | Depletion of CO2 in surface snow

SUMMARY

The projections of man-made climate change through burning of fossil carbon fuels (coal, gas, oil) to CO2 gas are based mainly on interpretations of measured CO2 concentrations in the atmosphere and in glacier ice. These measurements and interpretations are subject to serious uncertainties. Dominant factors in the Earth's surface CO2 cycle are the ocean, in addition to mineral equilibria. Due to their vast buffer capacity, they stabilize the geochemical equilibrium of CO2 gas between the hydro-, atmo-, litho- and biosphere. Radiocarbon (14C) studies indicate that the turnover time of dissolved organic carbon in the upper ocean is a few decades. This suggests that CO2 produced by burning the Earth's whole fossil carbon fuel reservoir would be dissolved in the ocean before reaching the double concentration of its current atmospheric level.

The 19th century measurements of CO2 in the atmosphere were carried out with an error of up to 100%. A value of 290 ppmv (parts per million, by volume) was chosen as an average for the 19th century atmosphere, by rejecting "not representative" measured values which differed more than 10% from the "general average for the time". This introduced a subjective factor in the estimates of the pre-industrial level of CO2 in the atmosphere.

The Mauna Loa (Hawaii) observatory has been regarded an ideal site for global CO2 monitoring. However, it is located near the top of an active volcano, which has, on average, one eruption every three and a half years. There are permanent CO2 emissions from a rift zone situated only 4 km from the observatory, and the largest active volcanic crater in the world is only 27 km from the observatory. These special site characteristics have made "editing" of the results an established procedure, which may introduce a subjective bias in the estimates of the "true" values. A similar procedure is used at other CO2-observatories. There are also problems connected to the instrumental methods for measurements of atmospheric CO2.

The CO2 concentrations in air bubbles trapped in glacier ice are often interpreted as previous atmospheric concentrations, assuming that the composition of the air in the bubbles remained unchanged. This was based on another assumption: liquid does not exist in ice below a mean annual temperature of about -24EC, and no changes due to diffusion may be expected. However, it was recently found that liquid can be present in Antarctic ice at temperatures as low as -73EC. Numerous studies indicate that, due to various chemical and physical processes, the CO2 content in ice can be largely enriched or depleted in comparison with the original atmospheric level. In the air inclusions from pre-industrial ice the CO2 concentrations were found to range between 135 and 500 ppmv. Methods using dry extraction of CO2 from crushed ice release only about half of this gas present in the ice. CO2 in air inclusions can penetrate the ice by diffusion or dissolution into the liquid present at the ice grain boundaries, at a rate different from rates of other gases in the air. A problem for the determination of CO2 levels in gas inclusions is the formation of solid CO2 clathrates (hydrates). Other gases in air also form clathrates, but at different temperatures and pressures. This leads to important changes in the composition of the inclusion air at different core depths and indicates that glacier ice cannot be regarded as a steady state matrix suitable for observation of long-term atmospheric trends. Thus, the results of CO2 determinations in air inclusions in ice cannot be accepted as representing the original atmospheric composition.

Another difficulty in this respect is a speculative assumption that air is 90 to 2800 years younger than the ice in which it is trapped. Without this assumption the CO2 concentration in air recovered from 19th century ice is the same as now. Atmospheric N2 / O2 / Ar ratios in trapped air are not preserved. Instead the ratios agree with those from aqueous solubility data. 85Kr and 39Ar measurements indicate that 36 to 100% of gas from the ice cores are contaminated by ambient air. Paleo-temperature calculations based on light stable isotope ratios (D/H and 18O/16O) in ice have large uncertainties. After the discovery of liquids between ice crystals in the deeply frozen Antarctic ice, considerable isotopic exchange and fractionation should be expected in the ice, making calculated paleo-temperatures meaningless if phase changes occurred in the presence of a mobile fluid phase.

Attempts have been made to calculate the paleoatmospheric CO2 content from C/ C carbon 13 12 stable-isotope ratios in tree rings. It is concluded here that the CO2 content in the atmosphere calculated from such carbon isotope analyses cannot be considered a valid tool in paleoclimatology, nor can it be used as evidence of changing atmospheric CO2 levels. The so-called increasing "greenhouse effect" signal, i.e. anthropogenic increase of the global air temperature, which was claimed to have been observed during the last decades, is not confirmed by recent studies of long temperature series.

In the Arctic, according to model calculations, this warming should be most pronounced. However, cooling rather than warming has been recorded in this region during the last two decades. Glacier balance studies provide evidence for a recent decrease in glacier retreat, and for an increased accumulation over the polar ice caps, corresponding to a sea level lowering of about 1 mm per year.

3.1 CONTEMPORARY MEASUREMENTS (MAUNA LOA AND SCANDINAVIAN)

An important component of the "greenhouse" warming hypothesis is the analysis of CO2 concentrations in the atmosphere. The first large scale measurements were started in 1955 in Scandinavia (Bischof, 1960). Since 1958 systematic monitoring of CO2 has been made at the Mauna Loa Observatory in Hawaii (Bacastow at al., 1985) and later at several other stations (Boden et al., 1990). One should note that nondispersive infrared techniques now used at Mauna Loa and other stations are not direct chemical measurements. The results may be influenced by the presence of other "greenhouse" gases in air samples with absorption bands overlapping those of CO2. This is suggested by results from 19 Scandinavian stations (Bischof, 1960) in which a sudden increase in CO2 concentration was observed after a chemical method was replaced by the infrared (IR) technique in 1959. Other IR-absorbing gases than CO2 have continuously increased their abundances in the global atmosphere. This could have given continuously increasing and too high "CO2 readings" at Mauna Loa and other stations using the infrared technique. Independent non-instrumental chemical analyses of the reference gases and flask samples of the atmosphere have not been seen reported, and should certainly be required.

The annual mean concentrations reported from the Mauna Loa observatory increased from 315.55 ppm in March 1958 to 351.45 in January 1989 (Pales and Keeling, 1965; Keeling et al., 1989; Thoning et al., 1989; Boden et al., 1990). The Mauna Loa data have been regarded as representative for the global concentration of CO2 in the atmosphere. This seems to be rather doubtful, due to the fact that the site is exposed to vast local natural emissions of CO2, and also CO2 from man-made sources.

The published results of the Mauna Loa measurements indicate that the atmospheric CO2 load has systematically increased about 10% during the past 30 years. Together with concentrations of CO2 found in air bubbles trapped in glacier ice, these results have often been used as a proof that the atmospheric CO2 level has increased by 25% since about 1850 (e.g. Schneider, 1989; IPCC, 1990). The predictions that the atmospheric CO2 level will double around the year 2030 are based on extrapolating the combined results of glacier and air measurements, and on the assumption that the 25% increase is solely due to man-made sources. Combining the glacier data with atmospheric measurements into one smooth curve was made possible only by assuming that the air entrapped in the ice is 95 years younger than the age of ice in which the air was entrapped. However, this assumption was found to be incorrect (Jaworowski et al., 1992; see also discussion in 5.2). Without this assumption the CO2 concentration in air recovered from the 19th century ice is the same as that at about 1980 in the Mauna Loa record.

The measurements in Scandinavia were carried out by a chemical method, different from that used at Mauna Loa. Therefore, it is especially interesting to compare them. For 19 stations in Scandinavia the total annual mean CO2 concentrations were 326 ppmv in 1955, 321 ppmv in 1956, 323 ppmv in 1957, 315 ppmv in 1958, and 331 ppmv in 1959 (Bischof, 1960). The first Mauna Loa annual mean -17- for 1959 was 315.83 ppmv, 316.75 ppmv for 1960, 317.49 for 1961, 318.30 for 1962, and 318.83 for 1963. There was an apparently decreasing trend in Scandinavia during the first four years before introducing the infrared technique, with a marked rise after its introduction, and a steadily increasing trend at Mauna Loa where only the infrared technique was used. No increasing trend in the CO2 air concentrations between 1957 and 1961, measured by the infrared technique, was observed in Scandinavia at altitudes of 1000 to 3000 meters (Bischof, 1962). The decreasing trend in Scandinavia could hardly be due to errors in the analysis, which had an accuracy not much different from that of the technique used at that time at Mauna Loa. The cause of the inconsistency of the Scandinavian and Mauna Loa data remains unclear.

As the Mauna Loa data are extensively used as representative for the average global air concentration of CO2, we discuss here the accuracy of the Mauna Loa measurements, to illustrate the difficulties involved in estimating levels of CO2 in the atmosphere.

The observatory is located at the slope of the active Mauna Loa volcano, which has had on the average one eruption every three and half years since 1832 (Encyclopaedia Britannica, 1974; Simkin et al., 1981). Following an eruption in 1975, the Mauna Loa volcano remained at rest until March 1984, when about 220 million tons of lava covered an area of about 48 km2. Pre-eruption activity had been occurring since about 1980 (Koyanagi and Wright, 1987; Koyanagi et al., 1987). The CO2 content of volcanic gases emitted, associated with various types of lava, was reported by Rubey (1951). The concentration of CO2 in the gases emitted from the Mauna Loa and Kilauea volcanos of Hawaii reaches about 47%. This is more than 50 times higher than in volcanic gases emitted in many other volcanic regions of the world. The reason for this is the alkaline nature of this volcanism, strongly associated with mantle CO2 degassing. The Kilauea volcano alone is releasing about 1 MT CO2 per year, plus 60 - 130 kT SO2 per year (Harris and Anderson, 1983).

The observatory is also exposed to permanent CO2 vents from the volcanic caldera and a rift zone situated only 4 km upslope from the observatory (Pales and Keeling, 1965), and from some distant sources downslope (Keeling et al., 1976). Pales and Keeling (1965), in their description of methodology and the sampling site, did not mention that the world's largest active volcanic mass, Kilauea, with the largest and most active volcanic crater on Earth (5 km long and 2 km wide) is situated only 27 km southeast from the Mauna Loa observatory. Frequent eruptions of this volcano occurred during the 1960s and 1970s. CO2 emission from Kilauea also occurs in non-eruption periods (Decker and Koyanagi, 1983; Decker et al., 1987). Emissions of up to 5000 tons of CO2 per day were recorded from the summit crater of this volcano in non-eruption periods (Gerlach and Taylor, 1990).

More recently, increased activity of Kilauea begun in January 1983 and continued throughout 1984. There were 16 major gas-charged eruptions in 1984, with fountains of lava several hundred meters high, and with an average production of lava of about 10 million tons per episode. A word "vog" (from "volcanic fog") has been coined on the island of Hawaii to define the volcanic haze that has been hanging over the island since Kilauea's latest eruptive phase began in 1983. This "vog" consists of water vapor, CO2, and SO2. The conditions might resemble a mild city smog (Bendure and Friary, 1990). Such conditions should influence the CO2 readings at Mauna Loa Observatory. The -18- question arises how the air at Mauna Loa can give a representative average global atmospheric CO2 level.

To account for the influence of volcanic emissions from the neighboring 10 km long rift zone and caldera at Mauna Loa, Pales and Keeling (1965) calculated an increase in CO2 concentration of 2 ppm for a certain "weather type", which is about three times higher than the observed 0.68 ppm average increase per year. The eruption events of the Mauna Loa and Kilauea volcanoes, or for quiescent emission of CO2 from the gigantic Kilauea crater, were not discussed by these authors. Eleven years later Keeling et al. (1976) mentioned the prolonged period of Kilauea activity which commenced in November 1967 and ended in March 1971. In March 1971 a locked chain gate was erected across the road to the Mauna Loa observatory 0.5 km from the CO2 intakes, to control the automotive traffic. A current tourist guide instructs tourists: "Park in the lot below the weather station [because] the equipment used to measure atmospheric conditions is highly sensitive to exhaust" (Bendure and Friary, 1990). (The chain was not in use and cars were parked immediately under the CO2 intakes when one of the present authors visited the site in March 1992).

5.1.3 DEPLETION OF CO2 IN SURFACE SNOW

An important finding of Raynaud and Delmas (1977) was the observation that in surface firn (up to 1 m depth) at the Pionerskaya and Vostok stations the concentration of CO2 in the interstitial air was 160 to 240 ppm, respectively, whereas at that time in the atmospheric air this concentration was reported to be 310 ppm. This demonstrates that, even in snow that was not subject to longer firnification and firn-ice transition processes, the CO2 content could have been reduced by up to 150 ppm, i.e. about 48% lower than in the ambient air of the same age. This important field experiment was never repeated in the later CO2 studies.

The striking feature of the glacier data used as an evidence for a recent man-made CO2 increase is that all of them are from ice deposited not in the last decades but in the 19th century or earlier. In these studies no information was presented on the recent concentrations of CO2 in firn and ice deposited in the 20th century. The results of CO2 determination in the pre-industrial ice are not compared with the CO2 content in recently deposited snow, firn or ice but with its current levels in the atmosphere. To justify such comparisons an assumption was needed that the entrapment of air in ice is purely a mechanical process, involving no chemical differentiation of gases. However, as appears from the discussion in this report, and as was demonstrated by Jaworowski et al. (1992), this assumption is wrong.

 

31st March 2009

 

 

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