Radiocarbon - a unique tracer of global carbon cycle dynamics

Climate on earth strongly depends on the radiative balance of its atmosphere, and, thus, on the abundance of the radiatively active greenhouse gases. Largely due to human activities since the Industrial Revolution, the atmospheric burden of many greenhouse gases has increased dramatically. Direct measurements during the last decades and analysis of ancient air trapped in ice from polar regions allow to quantify the change of these trace gas concentrations in the atmosphere. From a presumably "undisturbed" pre-industrial situation several hundred years ago until today, the CO2 mixing ratio increased by almost 30%. In the last decades this increase was nearly exponential, leading to a global mean CO2 mixing ratio of almost 370 ppm by the turn of the millenium. The atmospheric abundance of CO2 the main greenhouse gas containing carbon, is strongly controlled by exchange with the organic and inorganic carbon reservoirs. The world oceans are definitely the most important carbon reservoir, with a buffering capacity for atmospheric CO2 largest on time scales of centuries and longer. In contrast, the buffering capacity of the terrestrial biosphere is largest on shorter time scales from decades to centuries. Although today equally important, the role of the terrestrial biosphere as a sink of anthropogenic CO2 emissions is still poorly understood. Any prediction of future climate strongly relies on an accurate knowledge of the greenhouse gas concentrations in the present day atmosphere, and of their development in the future. This implies the need to quantitatively understand their natural geophysical and biochemical cycles including the important perturbations by man's impact. In attempting to disentangle the complexity of these cycles, 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

The tropospheric 14CO2 level in mid latitudes of the Northern Hemisphere (1959-2003)

A comprehensive tropospheric 14CO2 data set of quasi-continuous observations covering the time span from 1959 to 2003 is presented. Samples were collected at three European mountain sites at height levels of 1205m (Schauinsland), 1800m (Vermunt) and 3450 m a.s.l. (Jungfraujoch), and analysed in the Heidelberg Radiocarbon Laboratory. The data set from Jungfraujoch (1986-2003) is considered to represent the free tropospheric background level at mid latitudes of the Northern Hemisphere as it compares well with recent (yet unpublished) measurements made at the marine baseline station Mace Head (west coast of Ireland). The Vermunt and Schauinsland records are significantly influenced by regional European fossil fuel CO2 emissions. The respective Delta14CO2 depletions, on an annual mean basis, are, however, only less than 5 permil compared to Jungfraujoch. The Vermunt and Schauinsland sites are considered to represent well the mean continental European troposphere

Revision of the stratospheric bomb 14CO2 inventory

About 4900 values of 14CO2 activity have been measured on stratospheric air samples collected between 1953 and 1975 when the major nuclear weapon tests injected large amounts of 14C into the atmosphere. However, the validity of these data published in the Health and Safety Laboratory reports where repeatedly criticized and their relevance is thus usually denied in model studies tracing the global carbon cycle with bomb 14CO2. To oppose this criticism, we perform here a comprehensive analysis of the measurements and calculate stratospheric bomb 14CO2 inventories for the period in question. We find out that the recognized weakness of the survey do not justify a general discrimination against the 14CO2 observations. Our 14CO2 inventories determined using numerical methods to interpolate the observations widely confirm more "hand-made" results from a former study from Telegadas (1971) except in the northern poleward stratosphere. We are also able to clear away the reasons commonly advanced to call into question the stratospheric bomb 14CO2 inventories by up to 20%. These findings rehabilitate the most extensive data set of stratospheric 14CO2 observations and establish them, together with our corresponding bomb 14CO2 inventories, as a valuable observational constraint which should be seriously accounted for in global carbon cycle models and in other studies relying on an accurate simulation of air mass transport in the atmosphere

Refining of atmospheric transport model entries by the globally observed passive tracer distributions of 85krypton and sulfur hexafluoride (SF6)

Our high precision data base of the global distribution of SF6 in the troposphere [Maiss et al., 1996] is used in a two-dimensional atmospheric transport model (2D-HD model) to study the behaviour of this new tracer in comparison to the classical global atmospheric transport tracer 85Krypton. The 2D-HD model grid has been deduced from the 3D Hamburg TM2 model with the same resolution in the vertical and meridional direction, and was designed to run on any standard personal computer. The same vertical convection scheme and wind field as in the TM2 model, reduced to two dimensions, were used in the calculations. In addition, the horizontal diffusion parameter of the model was fine-tuned by matching the model estimated mean meridional 85Krypton distribution with observations over the Atlantic ocean. For simulating global tropospheric SF6 concentrations, an almost linearly increasing SF6 source strength was applied since 1970. The latitudinal distribution of the SF6 source was assumed to be similar to the global electrical power production. Excellent agreement between SF6 model results and observations is achieved with the 85Krypton-tuned 2D-HD transport model with respect to the global meridional concentration distribution and particularly in mid to high northern latitudes. In the southern hemisphere, at the German Antarctic Neumayer station, a significant seasonal cycle of SF6 has been observed which is reproduced by the model, however with a smaller ampitude. This finding may point to possible shortcomings of the model's transport scheme when simulating the seasonality of stratosphere-troposphere exchange in high southern latitudes

Global increase of SF6 observed in the atmosphere

High precision long-term observations of the trace gas sulphur hexafluoride (SF6) in background air at Neumayer station, Antarctica (1986-1991), and at Izana observatory, Tenerife (1991-1992), are presented. Since the very first measurements in 1970 (0.03pptv), the purely anthropogenic greenhouse gas SF6 has increased by two orders of magnitude to a global mean value of 2.8pptv in 1992. The observations can best be fitted by a quadratic curve with a recent increase rate of 8.3%/yr. A significant north-south gradient of 0.29pptv is observed. From this gradient an interhemispheric exchange time of 1.4 years is derived. A modeled atmospheric budget history agrees reasonably well with estimates of global SF6 production rates and leads to an extrapolated SF6 concentration of about 20pptv for the year 2030

Radiocarbon evidence for a smaller oceanic carbon dioxide sink than previously believed

Radiocarbon produced naturally in the upper atmosphere or artificially during nuclear weapons testing is the main tracer used to validate models of oceanic carbon cycling, in particular the exchange of carbon dioxide with the atmosphere and the mixing parameters within the ocean itself. Here we test the overall consistency of exchange fluxes between all relevant compartments in a simple model of the global carbon cycle, using measurements of the long-term tropospheric CO2 concentration and radiocarbon composition, the bomb 14C inventory in the stratosphere and a compilation of bomb detonation dates and strengths. We find that to balance the budget, we must invoke an extra source to account for 25% of the generally accepted uptake of bomb 14C by the oceans. The strength of this source decreases from 1970 onwards, with a characteristic timescale similar to that of the ocean uptake. Significant radiocarbon transport from the remote high stratosphere and significantly reduced uptake of bomb 14C by the biosphere can both be ruled out by observational constraints. We therefore conclude that the global oceanic bomb 14C inventory should be revised downwards. A smaller oceanic bomb 14C inventory also implies a smaller oceanic radiocarbon penetration depth, which in turn implies that the oceans take up 25% less anthropogenic CO2 than had previously been believed

Long-term observations of atmospheric CO2 and carbon isotopes at continental sites in Germany

A network for regional atmospheric CO2 observations had already been established in Germany by 1972, consisting of 5 stations with basically different characteristics: Westerland, a coastal station at the North Sea, 2 regional stations, Waldhof and Deuselbach, as well as 2 mountain stations, Brotjacklriegel at the eastern border of Germany and Schauinsland in the Black Forest. In addition to CO2 concentration observations, from 1977 onwards quasi-continuous 13CO2 and 14CO2 measurements were performed on samples from the Schauinsland site, and for the short period 1985-1988, 14CO2 measurements were also made on Westerland samples. CO2 data selection based on wind velocity allows for an estimate of the representative continental CO2 level over Europe. The peak-to-peak amplitude of the seasonal cycles and the German sites is shifted if compared to maritime background sites with the concentration maxima occuring already between beginning of February and beginning of April, the minima in August. 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 delta13C of the seasonal source CO2 at Schauinsland is calculated from unselected delta13C and CO2 data to be -25.1 permil. 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 sesonality 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

The Schauinsland CO2 record: 30 years of continental observations and their implications for the variability of the European CO2 budget

Since 1972, the German Environment Agency (UBA) has been measuring continuously CO2 concentration at Schauinsland station (southwest Germany, 1205 m asl). Because of its vicinity to biogenic and anthropogenic sources and sinks, the Schauinsland CO2 record shows considerably variability. In order to remove these disturbances and derive the large-scale representative "background" CO2 levels for the respective area (southwest Germany) we perform rigorous data selection based on wind speed and time of day. During the past 30 years, the selected CO2 mixing rations increased by 1.47 ppm per year, following the mean trend in midlatitudes of the Northern Hemisphere. The average seasonal cycle (peak-to-peak) amplitude has decreased slightly from 13.8+/-0.6 ppm in the first decade (1972-1981) to 12.8 +/- 0.7 ppm in the last two decades (1982-2001). This is opposite to other northern latitude sites and is attributed to the decrease of fossil fuel CO2 emissions in the catchment area (southwest Germany and France) and its respective change in the seasonal variation. Except for May and June, monthly mean CO2 mixing ratios at Schauinsland are higher by up to 8ppm if compared to marine boundary layer air, mainly as a consequence of fossil fuel CO2 emissions in Europe. The CO2 measurements when combined with continuous 222Rn observations at the same site allow an estimate of the net CO2 flux in the catchment area of Schauinsland: mean seasonal fluxes compare very well with estimates from a process-oriented biosphere model (SIB-2) as well as from an inverse modelling approach (Peylin et al, 2000). Annual CO2 fluxes vary by more than a factor of 2, although atnthropogenic fossil fuel CO2 emissions show interannual variations of only about 10%. The major part of the variability must therefore be associated to interannual changes of biospheric uptake and release, which are on the order of the total fossil fuel emissions in the same area. This has to be taken into account when reliably quantifying and verifying the long-term carbon balance and emission reduction targets in the European Union

Carbon dioxide and methane in continental Europe: a climatology, and 222Radon-based emission estimates

4-year records of gas chromatographic carbon dioxide and methane observations from the continental mountain station Schauninsland 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 16ppm with monthly mean wintertime enhancements up to 10ppm 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 31ppb 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 cylce 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 and 8 ppb CH4/yr. 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+/-1.0ppb/ppm. The wintertime monthly mean concentrations 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/m2/hr (from monthly mean offsets) and 6.5+/-2.5 mmol CO2 /m2/hr (from short term fluctuations) and winter time methane flux densities of 0.066+/-0.034 mmol CH4 /m2/hr (from monthly mean offsets) and 0.057 +/-0.022 mmole CH4/m2/hr (from short term fluctuations). These flux estimates are in close agreement to CO2 respectively CH4 emission inventories reported for Germany from statistical data

A novel approach for independent budgeting of fossil fuel CO2 over Europe by 14CO2 observations

Long-term atmospheric 14CO2 observations are deployed to quantify fossil fuel derived CO2 concentrations at a regional polluted site, and at a continental mountain station in south-west Germany. Fossil fuel CO2 emission rates for the relevant catchment areas are obtained by applying the Radon-Tracer-Method. They are shown to compare well with statistical emissions inventories but reveal a larger seasonality than assumed earlier, 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