Papers on anthropogenic carbon dioxide observations

This is a list of papers on carbon dioxide sources of atmospheric carbon dioxide concentration. Emphasis is on the papers that study the cause for the decadal increasing trend of carbon dioxide concentration in the atmosphere. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

UPDATES October 15, 2021: Levin et al. (1995) added. September 22, 2014: Lopez et al. (2013) and Lehman et al. (2013) added. April 26, 2012: Miller et al. (2012) and two papers of Graven et al. (2012) added. December 31, 2011: Turnbull et al. (2011) added. April 1, 2011: Schmidt et al. (1996), Zondervan & Meijer (1996) and Levin (1987) added. March 14, 2011: Freyer (1979) added. August 9, 2010: van der Laan et al. (2010) added. June 1, 2010: Levin et al. (2003) added. February 26, 2010: Ghosh & Brand (2003) and Revelle & Suess (1957) added. January 28, 2010: Francey & Farquhar (1982), Keeling & Shertz (1992), Bender et al. (1995), Suess (1955), Stuiver & Quay (1981), and Levin & Hesshaimer (2000) added. January 23, 2010: Damon et al. (1978) and Tans et al. (1979) added.

Allocation of Terrestrial Carbon Sources Using 14CO2: Methods, Measurement, and Modeling – Lehman et al. (2013) “The radiocarbon content of whole air provides a theoretically ideal and now observationally proven tracer for recently added fossil-fuel-derived CO2 in the atmosphere (Cff). Over large industrialized land areas, determination of Cff also constrains the change in CO2 due to uptake and release by the terrestrial biosphere. Here, we review the development of a Δ14CO2 measurement program and its implementation within the US portion of the NOAA Global Monitoring Division’s air sampling network. The Δ14CO2 measurement repeatability is evaluated based on surveillance cylinders of whole air and equates to a Cff detection limit of ≤0.9 ppm from measurement uncertainties alone. We also attempt to quantify additional sources of uncertainty arising from non-fossil terms in the atmospheric 14CO2 budget and from uncertainties in the composition of “background” air against which Cff enhancements occur. As an example of how we apply the measurements, we present estimates of the boundary layer enhancements of Cff and Cbio using observations obtained from vertical airborne sampling profiles off of the northeastern US. We also present an updated time series of measurements from NOAA GMD’s Niwot Ridge site at 3475 m asl in Colorado in order to characterize recent Δ14CO2 variability in the well-mixed free troposphere.” Scott J Lehman, John B Miller, Chad Wolak, John Southon, Pieter P Tans, Stephen A Montzka, Colm Sweeney, Arlyn Andrews, Brian LaFranchi, Thomas P Guilderson, Jocelyn C Turnbull, Radiocarbon, Vol 55, No 2–3 (2013), DOI: 10.2458/azu_js_rc.55.16392. [Full text]

CO, NO_x and ¹³CO₂ as tracers for fossil fuel CO₂: results from a pilot study in Paris during winter 2010 – Lopez et al. (2013) “Measurements of the mole fraction of the CO₂ and its isotopes were performed in Paris during the MEGAPOLI winter campaign (January–February 2010). Radiocarbon (¹⁴CO₂) measurements were used to identify the relative contributions of 77% CO₂ from fossil fuel consumption (CO₂ff from liquid and gas combustion) and 23% from biospheric CO₂ (CO₂ from the use of biofuels and from human and plant respiration: CO₂bio). These percentages correspond to average mole fractions of 26.4 ppm and 8.2 ppm for CO₂ff and CO₂bio, respectively. The ¹³CO₂ analysis indicated that gas and liquid fuel contributed 70% and 30%, respectively, of the CO₂ emission from fossil fuel use. Continuous measurements of CO and NO_x and the ratios CO/CO₂ff and NO_x/CO₂ff derived from radiocarbon measurements during four days make it possible to estimate the fossil fuel CO₂ contribution over the entire campaign. The ratios CO/CO2ff and NO_x/CO2ff are functions of air mass origin and exhibited daily ranges of 7.9 to 14.5 ppb ppm^-1 and 1.1 to 4.3 ppb ppm^-1, respectively. These ratios are consistent with different emission inventories given the uncertainties of the different approaches. By using both tracers to derive the fossil fuel CO₂, we observed similar diurnal cycles with two maxima during rush hour traffic.” Lopez, M., Schmidt, M., Delmotte, M., Colomb, A., Gros, V., Janssen, C., Lehman, S. J., Mondelain, D., Perrussel, O., Ramonet, M., Xueref-Remy, I., and Bousquet, P.: CO, NOx and 13CO2 as tracers for fossil fuel CO2: results from a pilot study in Paris during winter 2010, Atmos. Chem. Phys., 13, 7343-7358, doi:10.5194/acp-13-7343-2013, 2013. [Full text]

Linking emissions of fossil fuel CO₂ and other anthropogenic trace gases using atmospheric ¹⁴CO₂ – Miller et al. (2012) “Atmospheric CO₂ gradients are usually dominated by the signal from net terrestrial biological fluxes, despite the fact that fossil fuel combustion fluxes are larger in the annual mean. Here, we use a six year long series of ¹⁴CO₂ and CO₂ measurements obtained from vertical profiles at two northeast U.S. aircraft sampling sites to partition lower troposphere CO₂ enhancements (and depletions) into terrestrial biological and fossil fuel components (C_bio and C_ff). Mean C_ff is 1.5 ppm, and 2.4 ppm when we consider only planetary boundary layer samples. However, we find that the contribution of C_bio to CO₂ enhancements is large throughout the year, and averages 60% in winter. Paired observations of C_ff and the lower troposphere enhancements (Δ_gas) of 22 other anthropogenic gases (CH₄, CO, halo- and hydrocarbons and others) measured in the same samples are used to determine apparent emission ratios for each gas. We then scale these ratios by the well known U.S. fossil fuel CO₂ emissions to provide observationally based estimates of national emissions for each gas and compare these to “bottom up” estimates from inventories. Correlations of Δ_gas with C_ff for almost all gases are statistically significant with median r² for winter, summer and the entire year of 0.59, 0.45, and 0.42, respectively. Many gases exhibit statistically significant winter:summer differences in ratios that indicate seasonality of emissions or chemical destruction. The variability of ratios in a given season is not readily attributable to meteorological or geographic variables and instead most likely reflects real, short-term spatiotemporal variability of emissions.” Miller, J. B., et al. (2012), Linking emissions of fossil fuel CO2 and other anthropogenic trace gases using atmospheric 14CO2, J. Geophys. Res., 117, D08302, doi:10.1029/2011JD017048.

Observations of radiocarbon in CO₂ at La Jolla, California, USA 1992–2007: Analysis of the long-term trend – Graven et al. (2012) “High precision measurements of Δ¹⁴C were performed on CO₂ sampled at La Jolla, California, USA over 1992–2007. A decreasing trend in Δ¹⁴C was observed, which averaged −5.5 ‰ yr⁻¹ yet showed significant interannual variability. Contributions to the trend in global tropospheric Δ¹⁴C by exchanges with the ocean, terrestrial biosphere and stratosphere, by natural and anthropogenic ¹⁴C production and by ¹⁴C-free fossil fuel CO₂ emissions were estimated using simple models. Dilution by fossil fuel emissions made the strongest contribution to the Δ¹⁴C trend while oceanic ¹⁴C uptake showed the most significant change between 1992 and 2007, weakening by 70%. Relatively steady positive influences from the stratosphere, terrestrial biosphere and ¹⁴C production moderated the decreasing trend. The most prominent excursion from the average trend occurred when Δ¹⁴C decreased rapidly in 2000. The rapid decline in Δ¹⁴C was concurrent with a rapid decline in atmospheric O₂, suggesting a possible cause may be the anomalous ventilation of deep ¹⁴C-poor water in the North Pacific Ocean. We additionally find the presence of a 28-month period of oscillation in the Δ¹⁴C record at La Jolla.” Graven, H. D., T. P. Guilderson, and R. F. Keeling (2012), Observations of radiocarbon in CO₂ at La Jolla, California, USA 1992–2007: Analysis of the long-term trend, J. Geophys. Res., 117, D02302, doi:10.1029/2011JD016533.

Observations of radiocarbon in CO₂ at seven global sampling sites in the Scripps flask network: Analysis of spatial gradients and seasonal cycles – Graven et al. (2012) “High precision measurements of Δ¹⁴C were conducted for monthly samples of CO₂ from seven global stations over 2- to 16-year periods ending in 2007. Mean Δ¹⁴C over 2005–07 in the Northern Hemisphere was 5 ‰ lower than Δ¹⁴C in the Southern Hemisphere, similar to recent observations from I. Levin. This is a significant shift from 1988–89 when Δ¹⁴C in the Northern Hemisphere was slightly higher than the South. The influence of fossil fuel CO₂ emission and transport was simulated for each of the observation sites by the TM3 atmospheric transport model and compared to other models that participated in the Transcom 3 Experiment. The simulated interhemispheric gradient caused by fossil fuel CO₂ emissions was nearly the same in both 1988–89 and 2005–07, due to compensating effects from rising emissions and decreasing sensitivity of Δ¹⁴C to fossil fuel CO₂. The observed 5 ‰ shift must therefore have been caused by non-fossil influences, most likely due to changes in the air-sea ¹⁴C flux in the Southern Ocean. Seasonal cycles with higher Δ¹⁴C in summer or fall were evident at most stations, with largest amplitudes observed at Point Barrow (71°N) and La Jolla (32°N). Fossil fuel emissions do not account for the seasonal cycles of Δ¹⁴C in either hemisphere, indicating strong contributions from non-fossil influences, most likely from stratosphere-troposphere exchange.” Graven, H. D., T. P. Guilderson, and R. F. Keeling (2012), Observations of radiocarbon in CO₂ at seven global sampling sites in the Scripps flask network: Analysis of spatial gradients and seasonal cycles, J. Geophys. Res., 117, D02303, doi:10.1029/2011JD016535.

Atmospheric observations of carbon monoxide and fossil fuel CO2 emissions from East Asia – Turnbull et al. (2011) “Flask samples from two sites in East Asia, Tae-Ahn Peninsula, Korea (TAP), and Shangdianzi, China (SDZ), were measured for trace gases including CO₂, CO and fossil fuel CO₂ (CO₂ff, derived from Δ¹⁴CO₂ observations). The five-year TAP record shows high CO₂ff when local air comes from the Korean Peninsula. Most samples, however, reflect air masses from Northeastern China with lower CO₂ff. Our small set of SDZ samples from winter 2009/2010 have strongly elevated CO₂ff. Biospheric CO₂ contributes substantially to total CO₂ variability at both sites, even in winter when non-fossil CO₂ sources (including photosynthesis, respiration, biomass burning and biofuel use) contribute 20–30% of the total CO₂ enhancement. Carbon monoxide (CO) correlates strongly with CO₂ff. The SDZ and TAP far-field (China influenced) samples have CO: CO₂ff ratios (R_CO:CO2ff) of 47 ± 2 and 44 ± 3 ppb/ppm respectively, consistent with recent bottom-up inventory estimates and other observational studies. Locally influenced TAP samples fall into two distinct data sets, ascribed to air sourced from South Korea and North Korea. The South Korea samples have low R_CO:CO2ff of 13 ± 3 ppb/ppm, slightly higher than bottom-up inventories, but consistent with emission ratios for other developed nations. We compare our CO₂ff observations with modeled CO₂ff using the FLEXPART Lagrangian particle dispersion model convolved with a bottom-up CO₂ff emission inventories. The modeled annual mean CO₂ff mole fractions are consistent with our observations when the model inventory includes the reported 63% increase in Chinese emissions from 2004 to 2010, whereas a model version which holds Chinese emissions flat is unable to replicate the observations.” Turnbull, J. C., P. P. Tans, S. J. Lehman, D. Baker, T. J. Conway, Y. S. Chung, J. Gregg, J. B. Miller, J. R. Southon, and L.-X. Zhou (2011), J. Geophys. Res., 116, D24306, doi:10.1029/2011JD016691.

Observation-based estimates of fossil fuel-derived CO₂ emissions in the Netherlands using Δ¹⁴C, CO and ²²²Radon – van der Laan et al. (2010) “Surface emissions of CO₂ from fossil fuel combustion (ΦFFCO₂) are estimated for the Netherlands for the period of May 2006-June 2009 using ambient atmospheric observations taken at station Lutjewad in the Netherlands (6° 21′ E, 53° 24′ N, 1 m. a.s.l.). Measurements of Δ¹⁴C on two-weekly integrations of CO₂ and CO mixing ratios are combined to construct a quasi-continuous proxy record (FFCO₂*) from which surface fluxes (ΦFFCO₂*) are determined using the ²²²Rn flux method. The trajectories of the air masses are analysed to determine emissions which are representative for the Netherlands. We compared our observationally based estimates to the national inventories and we evaluated our methodology using the regional atmospheric transport model REMO. Based on three years of observations we find annual mean ΦFFCO₂* emissions of (4.7 ± 1.6) kt km−2 a−1 which is in very good agreement with the Dutch inventories of (4.5 ± 0.2) kt km−2 a−1 (average of 2006–2008).” S. Van Der Laan, U. Karstens, R.E.M. Neubert, I.T. Van Der Laan-Luijkx, H.A.J. Meijer, Tellus B, Volume 62, Issue 5, pages 389–402, November 2010, DOI: 10.1111/j.1600-0889.2010.00493.x. [Full text]

Variations of anthropogenic CO₂ in urban area deduced by radiocarbon concentration in modern tree rings – Rakowski et al. (2008) “Radiocarbon concentration in the atmosphere is significantly lower in areas where man-made emissions of carbon dioxide occur. This phenomenon is known as Suess effect, and is caused by the contamination of clean air with non-radioactive carbon from fossil fuel combustion. The effect is more strongly observed in industrial and densely populated urban areas. Measurements of carbon isotope concentrations in a study area can be compared to those from areas of clear air in order to estimate the amount of carbon dioxide emission from fossil fuel combustion by using a simple mathematical model. This can be calculated using the simple mathematical model. The result of the mathematical model followed in this study suggests that the use of annual rings of trees to obtain the secular variations of ¹⁴C concentration of atmospheric CO₂ can be useful and efficient for environmental monitoring and modeling of the carbon distribution in local scale.”

High resolution atmospheric monitoring of urban carbon dioxide sources – Pataki et al. (2006) “We used a tunable diode laser absorption spectrometer (TDL) to measure CO₂ mixing ratios and carbon isotope composition of CO₂ in order to estimate the contribution of gasoline versus natural gas combustion to atmospheric CO₂ in Salt Lake City. The results showed a pronounced diurnal pattern: the proportional contribution of natural gas combustion varied from 30–40% of total anthropogenic CO₂ during evening rush hour to 60–70% at pre-dawn. In addition, over a warming period of several days, the proportional contribution of natural gas combustion decreased with air temperature, likely related to decreased residential heating. These results show for the first time that atmospheric measurements may be used to infer patterns of energy and fuel usage on hourly to daily time scales.” [Full text]

Controlling for anthropogenically induced atmospheric variation in stable carbon isotope studies – Long et al. (2005) “Recent elevation of atmospheric CO₂ concentration, related primarily to fossil fuel combustion, has reduced atmospheric CO₂ δ¹³C (¹³C/¹²C), and this change in isotopic baseline has, in turn, reduced plant and animal tissue δ¹³C of terrestrial and aquatic organisms. Such depletion in CO₂ δ¹³C and its effects on tissue δ¹³C may introduce bias into δ¹³C investigations, and if this variation is not controlled, may confound interpretation of results obtained from tissue samples collected over a temporal span. … …we estimated a correction factor that controls for atmospheric change…”

Diurnal variability of δ¹³C and δ¹⁸O of atmospheric CO₂ in the urban atmosphere of Kraków, Poland – Zimnoch et al. (2004) “This article presents the results of measurements of the isotopic composition and concentration of atmospheric carbon dioxide, performed on air samples from Kraków (Southern Poland) in different seasons of the year. … The calculations show that during the summer and early autumn the dominant contribution to local CO₂ peaks is the biosphere, making up to 20% of atmospheric CO₂ during the nocturnal temperature inversion in the lower troposphere. During early spring and winter, anthropogenic emissions are the main local source.” [Full text]

Stable isotope ratio mass spectrometry in global climate change research – Ghosh & Brand (2003) “Stable isotope ratios of the life science elements carbon, hydrogen, oxygen and nitrogen vary slightly, but significantly in major compartments of the earth. Owing mainly to antropogenic activities including land use change and fossil fuel burning, the ¹³C/¹²C ratio of CO2 in the atmosphere has changed over the last 200 years by 1.5 parts per thousand (from about 0.0111073 to 0.0110906). In between interglacial warm periods and glacial maxima, the ¹⁸O/¹⁶O ratio of precipitation in Greenland has changed by as much as 5 parts per thousand (0.001935–0.001925). While seeming small, such changes are detectable reliably with specialised mass spectrometric techniques. The small changes reflect natural fractionation processes that have left their signature in natural archives.” [Full text]

A novel approach for independent budgeting of fossil fuel CO₂ over Europe by ¹⁴CO₂ observations – Levin et al. (2003) “Long-term atmospheric 14CO₂ observations are used to quantify fossil fuel-derived CO₂ concentrations at a regional polluted site, and at a continental mountain station in southwest Germany. Fossil fuel CO₂ emission rates for the relevant catchment areas are obtained by applying the Radon-Tracer-Method. They compare well with statistical emissions inventories but reveal a larger seasonality than earlier assumed, thus contributing significantly to the observed CO₂ seasonal cycle over Europe. Based on the present approach, emissions reductions on the order of 5–10% are detectable for catchment areas of several hundred kilometres radius, as anticipated within a five-years commitment period of the Kyoto Protocol. Still, no significant change of fossil fuel CO₂ emissions is observed at the two sites over the last 16 years.” [Full text]

Seasonal cycle of carbon dioxide and its isotopic composition in an urban atmosphere: Anthropogenic and biogenic effects – Pataki et al. (2003) “Atmospheric CO₂ mixing ratios and carbon and oxygen isotope composition were measured at 18 m above the ground in Salt Lake City, Utah, United States, for a one-year period. … The isotope-tracer technique used shows promise for quantifying the impacts of urban processes on the isotopic composition of the atmosphere and partitioning urban CO₂ sources into their component parts.” [Full text]

Stable carbon isotope constraints on mixing and mass balance of CO₂ in an urban atmosphere: Dallas metropolitan area, Texas, USA – Clark-Thorne & Yapp (2003) “The concentrations and δ¹³C values of atmospheric CO₂ were measured in 150 air samples collected at 8 sites in the Dallas metropolitan area over the period August 1998 to December 1999. … …but the overall pattern suggests that, as temperature decreases, the proportion of anthropogenic CO₂ derived from combustion of natural gas increases. This increase appears to reflect increased use of natural gas for home heating, etc., in cooler weather. Therefore, seasonally changing patterns of fossil fuel use are detectable in the atmospheric CO₂ of this urban environment.”

Evidence for preindustrial variations in the marine surface water carbonate system from coralline sponges – Böhm et al. (2002) “Carbon isotope records from coralline sponges clearly reflect the industrial ¹²C increase in atmospheric CO₂ with a precision that permits quantitative interpretations. … All δ¹³C records (appendix A) show the full extent of the industrial decline (Figure 3) caused by the anthropogenic addition of ¹²C-enriched CO₂ to the atmosphere. … The industrial decline in δ¹³C started in the first half of the 19th century after a short period of stable values around 1800 A.D.” [Full text]

Radiocarbon – a unique tracer of global carbon cycle dynamics – Levin & Hesshaimer (2000) “Radiocarbon observations have played a crucial role as an experimental tool enlightening the spatial and temporal variability of carbon sources and sinks. Studies of the “undisturbed” natural carbon cycle profit from the radioactive decay of ¹⁴C in using it as a dating tracer, e.g. to determine the turnover time of soil organic matter or to study internal mixing rates of the global oceans. Moreover, the anthropogenic disturbance of ¹⁴C through atmospheric bomb tests has served as an invaluable tracer to get insight into the global carbon cycle on the decadal time scale. … It has been erroneously argued that the observed atmospheric CO₂ increase since the middle of the 19th century may be due to an ongoing natural perturbation of gross fluxes between the atmosphere, biosphere, and oceans. That the increase is in fact a predominantly anthropogenic disturbance, caused by accelerated release of CO₂ from burning of fossil fuels, has been elegantly demonstrated through ¹⁴C analyses of tree rings from the last two centuries (Stuiver and Quay 1981; Suess 1955; Tans et al. 1979).” [Full text]

A 1000-year high precision record of δ¹³C in atmospheric CO₂ – Francey et al. (1999) “We present measurements of the stable carbon isotope ratio in air extracted from Antarctic ice core and firn samples. … Here, we start by confirming the trend in the Cape Grim in situ δ¹³C record from 1982 to 1996, and extend it back to 1978 using the Cape Grim Air Archive. … An almost continuous atmospheric history of δ¹³C over 1000 years results, exhibiting significant decadal-to-century scale variability unlike that from earlier proxy records. The decrease in δ¹³C from 1860 to 1960 involves a series of steps confirming enhanced sensitivity of δ¹³C to decadal timescale-forcing, compared to the CO₂ record.”

Isotopic characterisation of CO₂ sources during regional pollution events using isotopic and radiocarbon analysis – Zondervan & Meijer (1996) “At the station Kollumerwaard (The Netherlands), for monitoring tracers in the troposphere, air is sampled in 16 containers for off-line ¹³C, ¹⁸O and ¹⁴C isotopic analysis of CO₂. The timing of the sampling is chosen such that CO₂ variations correlating with pollutants like CO and CH₄ are optimally covered. The ¹⁴C measurements enable us to discriminate between biospheric and fossil fuel contributions to atmospheric CO₂. The analysis of one series sampled on 23 November 1994 resolves the increased CO₂ mixing ratio into a purely biospheric component with a δ¹³C of (− 22.2 ± 1.5)‰, and a fossil component of up to 35 ppm with a δ¹³C of (− 34.1 ± 1.6)‰. Another series, recorded on 2 and 3 February 1995, shows a nearby emission of fossil CO₂, methane and carbon monoxide, most likely due to the flaring of natural gas. Both events clearly indicate the importance of natural gas consumption in or in the vicinity of Holland. These experimental values can be compared with estimates of CO₂ emissions from combustion of fossil fuels and the corresponding δ¹³C values. The results for 18O show the pronounced difference in behaviour between the O and C isotopes in atmospheric CO₂, due to the fast isotopic exchange processes with (plant, soil or ocean) water. As a side result, the method produces the ratio CO: fossil CO₂, a direct measure for combustion quality on a regional scale.” Albert Zondervan, Harro A. J. Meijer, Tellus B, Volume 48, Issue 4, pages 601–612, September 1996, DOI: 10.1034/j.1600-0889.1996.00013.x. [Full text]

Isotopic characterisation of anthropogenic CO₂ emissions using isotopic and radiocarbon analysis – Meijer et al. (1996) “At the station Kollumerwaard (Netherlands), for monitoring tracers in the troposphere, air is sampled in sixteen containers for off-line ¹³C, ¹⁸O and ¹⁴C isotopic analysis of CO₂. The timing of the sampling is chosen such that CO₂ variations correlating with pollutants like CO and CH₄ are optimally covered. The ¹⁴C measurements enable us to discriminate between biospheric and fossil fuel contributions to background atmosphere CO₂. Results during the first year of operation show that the δ¹³C values for the anthropogenic CO₂ are significantly more negative than generally assumed (values ranging from -30 to -58 ‰ VPDB), which clearly indicates the importance of natural gas consumption in the Netherlands. We compare these experimental values with results from a detailed study of CO₂ emission estimates from combustion of fossil fuels and the corresponding δ¹³C values. As an important side result, the method produces reliable values for the regionally averaged ratio CO : fossil CO₂ (results ranging from 0.5 to 1%), a direct measure for combustion quality.” H. A. J. Meijer, H. M. Smid, E. Perez and M. G. Keizer, Physics and Chemistry of The Earth, Volume 21, Issues 5-6, October-December 1996, Pages 483-487, doi:10.1016/S0079-1946(97)81146-9. [Full text]

The ¹³C Suess Effect in the World Surface Oceans and Its Implications for Oceanic Uptake of CO₂: Analysis of Observations at Bermuda – Bacastow et al. (1996) “Surface ocean water δ¹³C measurements near Bermuda are examined in an attempt to find the annual decrease caused by the addition of anthropogenic CO₂ to the atmosphere. … Results are, in general, consistent with the low side of the Intergovernmental Panel on Climate Control estimation of 2.0 ± 0.8 GtC yr⁻¹.” [Full text]

Carbon dioxide and methane in continental Europe: a climatology, and ²²²Radon-based emission estimates – Schmidt et al. (1996) “4-year records of gas chromatographic carbon dioxide and methane observations from the continental mountain station Schauinsland in the Black Forest (Germany) are presented. These data are supplemented by continuous atmospheric ²²²Radon observations. The raw data of CO₂ concentration show a large seasonal cycle of about 16 ppm with monthly mean wintertime enhancements up to 10 ppm higher and summer minima up to 5 ppm lower than the maritime background level in this latitude. These offsets are caused by regional and continental scale CO₂ sources and sinks. The mean CH₄ concentration at Schauinsland is 31 ppb higher than over the Atlantic ocean, due to the European continent acting as a net source of atmospheric CH₄ throughout the year. No significant seasonal cycle of methane has been observed. The long term CO₂ and CH₄ increase rates at Schauinsland are found to be similar to background stations in the northern hemisphere, namely 1.5 ppm CO₂ yr^-1 and 8 ppb CH₄ yr^-1. On the time scale of hours and days, the wintertime concentrations of all three trace gases are highly correlated, the mean ratio of CH₄/CO₂ is 7.8 ± 1.0 ppb/ ppm. The wintertime monthly mean concentration offsets relative to the maritime background level show a CH₄/CO₂ ratio of 6.5 ± 1.1 ppb/ ppm, thus, not significantly different from the short term ratio. Using the wintertime regressions of CO₂ and ²²²Radon respectively CH₄ and ²²²Radon we estimate winter time CO₂ flux densities of 10.4 ± 4.3 mmol CO₂ m^-2 h^-1 (from monthly mean offsets) and 6.4 ± 2.5 mmol CO₂ m^-2 h^-1 (from short term fluctuations) and winter time methane flux densities of 0.066 ± 0.034 mmol CH₄ m^-2 h^-1 (from monthly mean offsets) and 0.057 ± 0.022 mmole CH₄ m^-2 h^-1 (from short-term fluctuations). These flux estimates are in close agreement to CO₂ respectively CH₄ emission inventories reported for Germany from statistical data.” Martina Schmidt, Rolf Graul, Hartmut Sartorius, Ingeborg Levin, Tellus B, Volume 48, Issue 4, pages 457–473, September 1996, DOI: 10.1034/j.1600-0889.1994.t01-2-00002.x-i1.

Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980 – Keeling et al. (1995) “Observations of atmospheric CO₂ concentrations at Mauna Loa, Hawaii, and at the South Pole over the past four decades show an approximate proportionality between the rising atmospheric concentrations and industrial CO₂ emissions. This proportionality, which is most apparent during the first 20 years of the records, was disturbed in the 1980s by a disproportionately high rate of rise of atmospheric CO₂, followed after 1988 by a pronounced slowing down of the growth rate. To probe the causes of these changes, we examine here the changes expected from the variations in the rates of industrial CO₂ emissions over this time, and also from influences of climate such as El Niño events. We use the ¹³C/¹²C ratio of atmospheric CO₂ to distinguish the effects of interannual variations in biospheric and oceanic sources and sinks of carbon. We propose that the recent disproportionate rise and fall in CO¹³ growth rate were caused mainly by interannual variations in global air temperature (which altered both the terrestrial biospheric and the oceanic carbon sinks), and possibly also by precipitation. We suggest that the anomalous climate-induced rise in CO¹³ was partially masked by a slowing down in the growth rate of fossil-fuel combustion, and that the latter then exaggerated the subsequent climate-induced fall.” [Full text]

Variability in The O₂/N₂ Ratio of Southern Hemisphere Air, 1991-1994: Implications for the Carbon Cycle – Bender et al. (1995) “We present a record of variations in the O₂/N₂ ratio of air at 41° S latitude from 1991–1994 based on the mass spectrometric analysis of flask samples from Cape Grim, Tasmania, and Baring Head, New Zealand. Results for Cape Grim for the period from June 1991 to February 1992 are in good agreement with previously published data of Keeling and Shertz [1992]. … The O₂/N₂ ratio of air decreased at the rate of 12±4 per meg/yr (0.012 ‰/yr) between winter 1991 and winter 1993. This value is considerably less than the O₂ consumption rate associated with fossil fuel burning (about 20 per meg/yr), suggesting that the land biosphere was an O₂ source and an important CO₂ sink during this period. Alternatively, the oceans may have been a transient O₂ sink during 1991–1993, most likely caused by an enhanced rate of thermocline ventilation with respect to the steady-state value.”

Long-term observations of atmospheric CO2 and carbon isotopes at continental sites in Germany – Levin et al. (1995) “The long-term mean CO2 increase rate in the last 20 years at Westerland and Schauinsland is 1.49 and 1.48 ppmv yr-1, respectively. The mean d13C of the seasonal source CO2 at Schauinsland is calculated from unselected d13C and CO2 data to be -25.1‰. From the 14C observations in unselected CO2, we derive yearly mean fossil fuel contributions at Westerland of 4 ppmv, and at Schauinsland of only 2.5 ppmv. Based on the seasonality of the fossil fuel CO2 component at Schauinsland and on concurrently observed atmospheric 222Radon activities, we derive a seasonal amplitude of the fossil fuel CO2 source which is higher by a factor of 3 compared to emission estimates for Europe.”

Seasonal and interannual variations in atmospheric oxygen and implications for the global carbon cycle – Keeling & Shertz (1992) “Measurements of changes in atmospheric molecular oxygen using a new interferometric technique show that the O₂ content of air varies seasonally in both the Northern and Southern Hemispheres and is decreasing from year to year. The seasonal variations provide a new basis for estimating global rates of biological organic carbon production in the ocean, and the interannual decrease constrains estimates of the rate of anthropogenic CO₂ uptake by the oceans.” [Full text]

Oceanic Uptake of Fossil Fuel CO₂: Carbon-13 Evidence – Quay et al. (1992) “The δ¹³C value of the dissolved inorganic carbon in the surface waters of the Pacific Ocean has decreased by about 0.4 per mil between 1970 and 1990. This decrease has resulted from the uptake of atmospheric CO₂ derived from fossil fuel combustion and deforestation. The net amounts of CO₂ taken up by the oceans and released from the biosphere between 1970 and 1990 have been determined from the changes in three measured values: the concentration of atmospheric CO₂, the δ¹³C of atmospheric CO₂ and the δ¹³C value of dissolved inorganic carbon in the ocean. The calculated average net oceanic CO₂ uptake is 2.1 gigatons of carbon per year. This amount implies that the ocean is the dominant net sink for anthropogenically produced CO₂ and that there has been no significant net CO₂ released from the biosphere during the last 20 years.” [Full text]

Atmospheric CO₂ in continental Europe—an alternative approach to clean air CO₂ data – Levin (1987) “A 5-year record of continuous atmospheric CO2 concentration data from the Schauinsland mountain top station (48°N, 8°E, 1205 m a.s.l.) is analysed for contributions from different continental sources, e.g., fossil fuels and by the natural biosphere. The fossil fuel contribution is determined from monthly averages of parallel carbon isotope data (¹⁴C, (¹³C)) collected from 1977 to 1984. The observed short-term fluctuations are identified as “local contamination” with help of continuous atmospheric ²²²Radon data from the same location. The combination of all tracer data, carbon isotopes and ²²²Radon activity, in addition to CO₂ concentration makes it possible to evaluate a CO₂ concentration record representative for “continental clean air” not influenced locally by natural and anthropogenic sources. Comparison of the processed record with CO₂ data from a marine station (Ocean Weather Station P, 50°N, 145° W) (Wong et al., 1984) shows a significant phase shift at the continental compared to the marine site. This is attributed to the continental biosphere acting as a net sink in early summer (April to June) and as a net source in autumn and winter (October to January).” Ingeborg Levin, Tellus B, Volume 39B, Issue 1-2, pages 21–28, February-April 1987, DOI: 10.1111/j.1600-0889.1987.tb00267.x.

Input of excess CO₂ to the surface ocean based on ¹³C/¹²C ratios in a banded Jamaican sclerosponge – Druffel & Benavides (1986) “Here we present surface ocean ¹³C and ¹⁸O records measured in the skeleton of a living sclerosponge (Ceratoporella nicholsoni), which accretes aragonite in isotopic equilibrium with the surrounding sea water/dissolved inorganic carbon (DIC) system. The ¹³C record reveals a decrease of 0.50 [promille] from 1820 to 1972.”

Ice core record of the ¹³C/¹²C ratio of atmospheric CO₂ in the past two centuries – Friedli et al. (1986) “The release of carbon into the atmosphere due to the activities of humans has caused an increase in concentration as well as a change in the isotopic composition of atmospheric carbon dioxide. CO₂ derived from fossil fuel combustion and from biomass destruction have δ¹³C values of ~-25 [promille] (compared to the atmospheric value of ~-7 [promille]) and are thus depleted in ¹³C. We have measured δ¹³C of CO₂ separated from air trapped in bubbles in ice samples from an ice core taken at Siple Station in Antarctica, in which it has been possible to demonstrate the atmospheric increase of CO₂ (ref. 1) and methane2 with high time resolution. The isotopic results, together with the CO₂ record from the same ice core, yield information on the sources of excess carbon dioxide and provide a data base for testing the consistency of global carbon cycle models.”

An explanation of ¹³C/¹²C variations in tree rings – Francey & Farquhar (1982) “Variations in the ¹³C/¹²C ratio in trees are examined in the light of a simple expression relating the relative isotope composition of plant material, δ_p¹³ to δ_a¹³, the atmospheric isotope value, c_a, the atmospheric CO₂ concentration and c_i, the internal concentration of CO₂ in leaves. The expression gives good agreement with δ_p¹³ measurements where independent information on c_i exists, such as seasonal growth, growth low in the canopy and in conditions of low humidity. The expression provides possible explanations for two previously unexplained phenomena: the absence of anticipated changes due to fossil fuel-induced changes in δ_a¹³, and regional differences in δ_p¹³ trends.”

Atmospheric ¹⁴C changes resulting from fossil fuel CO₂ release and cosmic ray flux variability – Stuiver & Quay (1981) “A high-precision tree-ring record of the atmospheric ¹⁴C levels between 1820 and 1954 is presented. Good agreement is obtained between measured and model calculated 19th and 20th century atmospheric Δ¹⁴C levels when both fossil fuel CO₂ release and predicted natural variations in ¹⁴C production are taken into account.”

Natural atmospheric ¹⁴C variation and the Suess effect – Tans et al. (1979) “THE dilution of the atmospheric ¹⁴CO₂ concentration by large amounts of fossil-fuel derived CO₂ which do not contain any ¹⁴C is commonly called the Suess effect. Its magnitude can be calculated with the same geochemical models as the global carbon cycle that also predict the future rise of atmospheric CO₂ to be caused by the combustion of fossil fuels. Validation of a CO₂ predictive model with the Suess effect ¹⁴C data is important because these two phenomena have a common cause, and therefore register model responses at roughly the same frequencies. Measurements of the Suess effect yield values between -15, and -25 in ¹⁴C (in 1950), while different model predictions also cover about this range. The requirement that a model correctly reproduces the Suess effect becomes a strong constraint when the accuracy of the measurement is improved to better than 2. ¹⁴C measurements in tree rings to an accuracy of 1.2 are reported here. The results indicate that the natural fluctuations of atmospheric ¹⁴C as yet preclude determination of the Suess effect to the accuracy required by the models.”

On the ¹³C record in tree rings. Part I. ¹³C Variations in northern hemispheric trees during the last 150 years – Freyer (1979) “¹³C data from 26 trees of different northern hemispheric locations are presented covering the time interval of the last 150 years. It has been found that the mean ¹³C data of these trees during the 1850–1920, 1920–1940 and 1960–1975 time intervals decrease with an almost linear slope, while the data in the 1940–1960 time interval are increasing. The total δ¹³C shift since industrialization found from our trees amounts to nearly −29% which is higher than the linear δ¹³C decrease of −1%/100 years due to the biogenic and fossil CO₂ input into the atmosphere, as assumed previously in model calculations of Siegenthaler et al. (1978). The mean data within the given error-probability correspond to those of other authors with the exception of the data for the 1920–1940 time interval. The disturbances in ¹³C data for recording of increasing atmospheric CO₂ levels have been pointed out.” H. D. Freyer, Tellus, Volume 31, Issue 2, pages 124–137, April 1979, DOI: 10.1111/j.2153-3490.1979.tb00889.x.

Recent trends in the ¹³C/¹²C ratio of atmospheric carbon dioxide – Keeling et al. (1979) “The ¹³C/¹²C ratio of atmospheric carbon dioxide has decreased by approximately 0.6 [promille] over 22 yr according to new direct measurements reported here.”

Temporal Fluctuations of Atmospheric ¹⁴C: Causal Factors and Implications – Damon et al. (1978) A review article. “In this review we consider the time variations of the atmospheric concentration of ¹⁴C, a radioisotope induced by cosmic rays and also known as radiocarbon.” [Full text, click PDF-link]

Carbon dioxide exchange between atmosphere and ocean and the – Revelle & Suess (1957) “From a comparison of C¹⁴/C¹² and C¹³/C¹² ratios in wood and in marine material and from a slight decrease of the C¹⁴ concentration in terrestrial plants over the past 50 years it can be concluded that the average lifetime of a CO₂ molecule in the atmosphere before it is dissolved into the sea is of the order of 10 years. This means that most of the CO₂ released by artificial fuel combustion since the beginning of the industrial revolution must have been absorbed by the oceans. The increase of atmospheric CO₂ from this cause is at present small but may become significant during future decades if industrial fuel combustion continues to rise exponentially.” [Full text]

Radiocarbon Concentration in Modern Wood – Suess (1955)