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108 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.7.1 Global long-term observations of Radiocarbon in atmospheric CO2, revisited Participating scientist Ingeborg Levin, the Radiocarbon Laboratory, Bernd Kromer Abstract A global network of high-precision atmospheric 14 CO2 observations is maintained, providing an independent constraint for carbon cycle modelling. Apart from this innovative application important in research of the anthropogenic CO2 problematic also various modern dating techniques make use of our unique 14 C records [Mayorga et al. , 2005; Spalding et al. , 2005]. Figure 2.60: Observed ∆ 14 C in atmospheric CO2 and tree rings Background Initiated by the late Karl Otto Münnich, Heidelberg was among the first laboratories involved in precise long-term observations of bomb 14 C perturbations in the global carbon system. Related atmospheric 14 CO2 monitoring extended globally and carried into the present time constitutes a world-wide unique exercise [Levin & Hesshaimer, 2000]. This long-term and sometimes painful effort of the Carbon Cycle Group at IUP now offers a wealth of intriguing applications which range from environmental issues like studying global carbon cycle dynamics to life science and forensic aspects. Methods and results Continuous bi-weekly integrated CO2 samples are collected at eight globally distributed background stations as well as in Germany in the Black Forest (Schauinsland) and in Heidelberg (Schönherr, article 2.7.4, this issue) and are analysed at high precision for their 14 C activity by conventional radioactive counting (Kromer, section 6.1, this issue). The long term trend of ∆ 14 C in CO2 in the Northern Hemisphere troposphere from 1959 until 2005 is displayed in Figure 2.60 [Levin & Kromer, 2004]. Due to atmospheric nuclear weapon testing in the 1950s and early 1960s the atmospheric 14 CO2 level increased by about a factor of two (∆ 14 C≈1000�) compared to the natural reference level. The present steady decline which is mainly driven by the exchange with oceanic CO2 and by input of 14 C-free fossil fuel CO2 into the atmosphere are underlain relatively weak perturbations. These secondary variations reflect interesting processes, for example the very regular seasonal cycle of 14 CO2 at mid latitude northern hemispheric sites today (inlay of Figure 2.60) are mainly caused by stratospheretroposphere exchange and by the seasonal release of bomb 14 C stored in the biosphere for the last 50 years now re-entering the atmosphere (Naegler, article 2.7.2, this issue). This recent net source of bomb 14 C to the atmosphere is also reflected in a tropical 14 CO2 maximum while a relative minimum is observed in mid-to-high southern latitude which is caused by disequilibrium fluxes between ocean surface water and the atmosphere [Levin & Hesshaimer, 2000]. Outlook/Future work The small but significant signals observed in atmospheric 14 CO2 are used in combination with the GRACE model (Naegler, article 2.7.2, this issue) and more sophisticated global 3-dimensional models to put independent constraints on carbon exchange on earth. Main publication: Levin & Kromer [2004] Funding Funding: CarboEurope-IP, DFG (Atmospheric Radiocarbon).
2.7. CARBON CYCLE GROUP 109 2.7.2 Simulating Bomb Radiocarbon: Implications for the Global Carbon Cycle Participating scientist Tobias Naegler, Vago Hesshaimer, Ingeborg Levin, Philippe Ciais (LSCE, Gif-Sur-Yvette, France), Keith Rodgers (LODYC, Paris, France) Abstract With the help of the two-dimensional global (radio-)carbon cycle model GRACE, we estimated the total production of bomb 14 C and simulated bomb 14 C inventories in the main carbon cycle reservoirs. Furthermore, we exploited the constraints that observed bomb 14 C inventories impose on stratosphere-troposphere and air-sea gas exchange. Figure 2.61: Observed (symbols) and simulated (lines) bomb 14 C inventories in the troposphere, stratosphere, biosphere and ocean. The black line denotes the total simulated accumulated bomb 14 C production, the black symbols the ”observation-based” total inventory. Background Bomb radiocarbon has been produced in atmospheric nuclear bomb tests. It is injected mainly into the stratosphere. Oxydised to 14 CO2, it takes part in the global carbon cycle. The production of bomb radiocarbon peaks sharply in the late 1950s and early 1960s. Due to this pulse-like injection, bomb radiocarbon acts like a one-time release of a dye-tracer into the stratosphere. The analysis of the subsequent distribution of bomb 14 C in the earth system allows to constrain exchange times between the main carbon reservoirs stratosphere, troposphere, biosphere and ocean, e.g. to quantify stratosphere-troposphere exchange (STE), the atmospheric interhemispheric exchange time, air-sea gas exchange and turnover times of biospheric carbon pools. Methods and results We simulated the global bomb radiocarbon cycle with the Global RAdioCarbon Exploration model GRACE, consisting of a 22-box zonally averaged twodimensional atmosphere coupled to a well-mixed 4-reservoir biosphere. Radiocarbon exchange between atmosphere and the ocean was calculated according to observed tropospheric and ocean surface ∆ 14 C . The model comprises all relevant sources of radiocarbon (natural, bomb and industrial production). The seasonal air mass exchange between model boxes was calibrated with stratospheric and tropospheric 14 CO2 observations as well as tropospheric observations of SF6 and the 10 Be/ 7 Be ratio. The simulated air mass transport across the tropopause then gives an estimate of the effective STE necessary to reproduce the observed tracer distributions. The total radiocarbon production by atmospheric nuclear bomb tests was determined with the help of the model and available stratospheric and tropospheric radiocarbon observations. The model simulated the distribution of bomb radiocarbon among the carbon reservoirs stratosphere, troposphere, biosphere, and ocean. Simulated bomb radiocarbon inventories turned out to be in very good agreement with all available stratospheric and tropospheric radiocarbon observations as well as with the latest estimates of the ocean bomb radiocarbon inventories during the GEOSECS and WOCE surveys [Peacock, 2004; Key et al. , 2004]. For the very first time, the GRACE model is thus capable of closing the bomb radiocarbon budget in the global carbon system. Using the available latest atmospheric and oceanic (radio-) carbon data, we developed a new, improved parameterisation of air-sea gas exchange which is easily adaptable to new estimates of the ocean bomb 14 C inventory and the global wind speeds, two significant sources of uncertainty in this approach. Furthermore, we estimated an observation-based biospheric bomb 14 C inventory which can be used to constrain the setup and parameterisation of biosphere models. Outlook/Future work We will analyse in detail bomb 14 C constraints on biospheric parameters. Furthermore, the GRACE model will be improved by adding an ocean module, allowing carbon isotope simulations in the coupled atmosphere-ocean-biosphere system. Main publication: Naegler [2005] Funding This PhD thesis was a close collaboration with the LSCE, Gif-sur-Yvette, France and has been funded by the EU projects AEROCARB and CarboEurope-IP and by the DFG.
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Contents 1 Introduction/Overview of
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CONTENTS 7 3.2.1 Glaciological pilo
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Introduction In Heidelberg, environ
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Overview Summary Atmospheric resear
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2.3. RADIATIVE TRANSFER 45 2.3 Radi
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2.3. RADIATIVE TRANSFER 53 Rothman,
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Forschungsstelle “Radiometrie”
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7.2. GREY PUBLICATIONS 215 Grey Pub
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7.3. PHD THESES 217 PhD Theses Baye
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7.5. INVITED TALKS 221 Invited Talk
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