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

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

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

Tracing the global carbon cycle with bomb radiocarbon

Bomb radiocarbon (14C) emitted to the atmosphere by nuclear explosion tests has not yet reached equilibrium within the Earth carbon system. In the present thesis this fate of radiocarbon is investigated using coarse-grid models to trace high-precision 14CO2 observations available in the atmosphere since the 1950s. The goal of the study is to progress our quantitative understanding of bomb radiocarbon following the pathways of the global carbon cycle. Inversely, I wanted also to determine new constraints on the atmospheric carbon budget buried in the long-term observations of atmospheric 14CO2. Three relevant findings came out at the different stages of my research. First, the man-made passive tracer SF6 was shown to be a powerful tool for investigating air mass transport in atmospheric transport models. Second, a serious mismatch in the global bomb radiocarbon budget has been detected, suggesting that the oceans take up 25 less anthropogenic CO2 than hitherto believed. Third, tracking both the tropospheric and the stratospheric 14C observations during the period of major bomb 14C activity excursions was found to uncover the cycle of air mass through the stratosphere. The above tools were finally used in a first assessment of the seasonal cycles of recent atmospheric 14 CO2

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

Closing the global radiocarbon budget 1945-2005

The global radiocarbon cycle of the last 60 years was simulated with the Global RAdioCarbon Exploration Model (GRACE). The total radiocarbon production by atmospheric nuclear bomb tests was determined using available stratospheric and tropospheric radiocarbon (14C) observations as constraints. To estimate the range of uncertainty in the explosive force of atmospheric nuclear bomb tests and their respective 14C yield factor, we applied different published bomb test compilations. Furthermore, to account for a possible small bias in the available stratospheric excess radiocarbon observations, we tested the different bomb test compilations with both uncorrected and corrected stratospheric 14C observations. For each of these scenarios of the total bomb 14C burden, the model simulated the distribution of excess radiocarbon among the stratosphere, troposphere, biosphere, and ocean carbon reservoirs. With a global bomb 14C production of 598—632*10^26 atoms (99-105 kmol) 14C between 1945 and 1980, simulated excess radiocarbon inventories are in good agreement with all available stratospheric and tropospheric radiocarbon observations as well as with the latest estimates of the ocean excess radiocarbon inventories during the GEOSECS and WOCE surveys from Peacock (2004) and Key et al. (2004). For the very first time, our model is thus capable of closing the excess radiocarbon budget on the basis of our current knowledge of exchange rates and reservoir sizes in the global carbon system

Modelled natural and excess radiocarbon: Sensitivities to the gas exchange formulation and ocean transport strength

Observation-based surface ocean Δ14C distributions and regional inventories for excess, bomb-produced radiocarbon are compared with results of two ocean models of intermediate complexity. By applying current descriptions of the air-sea gas exchange the models produce similar column inventories for excess 14C among all basins. This result is robust across a wide range of transport parameter settings, but inconsistent with databased inventories. In the absence of evidence of fundamentally different gas exchange mechanisms in the North Atlantic than in the other basins, we infer regional North Atlantic 14C inventories which are considerably smaller than previous estimates. The results further suggest that the gas exchange velocity field should be reduced by (19 ± 16)%, which corresponds to a global mean air-sea gas transfer rate for CO2 in seawater of 17.1 ± 3.3 cm/h-1, to find good agreement of simulated quantities with a range of data-based metrics

Isotopes in pyrogenic carbon: a review

Pyrogenic carbon (PC; also known as biochar, charcoal, black carbon and soot) derived from natural and anthropogenic burning plays a major, but poorly quantified, role in the global carbon cycle. Isotopes provide a fundamental fingerprint of the source of PC and a powerful tracer of interactions between PC and the environment. Radiocarbon and stable carbon isotope techniques have been widely applied to studies of PC in aerosols, soils, sediments and archaeological sequences, with the use of other isotopes currently less developed. This paper reviews the current state of knowledge regarding (i) techniques for isolating PC for isotope analysis and (ii) processes controlling the carbon (<sup>13</sup>C and <sup>14</sup>C), nitrogen, oxygen, hydrogen and sulfur isotope composition of PC during formation and after deposition. It also reviews the current and potential future applications of isotope based studies to better understand the role of PC in the modern environment and to the development of records of past environmental change

The worldwide marine radiocarbon reservoir effect: definitions, mechanisms, and prospects

When a carbon reservoir has a lower radiocarbon content than the atmosphere, this is referred to as a reservoir effect. This is expressed as an offset between the radiocarbon ages of samples from the two reservoirs at a single point in time. The marine reservoir effect (MRE) has been a major concern in the radiocarbon community, as it introduces an additional source of error that is often difficult to accurately quantify. For this reason, researchers are often reluctant to date marine material where they have another option. The influence of this phenomenon makes the study of the MRE important for a broad range of applications. The advent of Accelerator Mass Spectrometry (AMS) has reduced sample size requirements and increased measurement precision, in turn increasing the number of studies seeking to measure marine samples. These studies rely on overcoming the influence of the MRE on marine radiocarbon dates through the worldwide quantification of the local parameter ΔR, that is, the local variation from the global average MRE. Furthermore, the strong dependence on ocean dynamics makes the MRE a useful indicator for changes in oceanic circulation, carbon exchange between reservoirs, and the fate of atmospheric CO2, all of which impact Earth's climate. This article explores data from the Marine Reservoir Database and reviews the place of natural radiocarbon in oceanic records, focusing on key questions (e.g., changes in ocean dynamics) that have been answered by MRE studies and on their application to different subjects