RADIOCARBON IN GLOBAL TROPOSPHERIC CARBON DIOXIDE

ABSTRACT Since the 1950s, observations of radiocarbon (14C) in tropospheric carbon dioxide (CO2) have been conducted in both hemispheres, documenting the so-called nuclear “bomb spike” and its transfer into the oceans and the terrestrial biosphere, the two compartments permanently exchanging carbon with the atmosphere. Results from the Heidelberg global network of Δ14C-CO2 observations are revisited here with respect to the insights and quantitative constraints they provided on these carbon exchange fluxes. The recent development of global and hemispheric trends of Δ14C-CO2 are further discussed in regard to their suitability to continue providing constraints for 14C-free fossil CO2 emission changes on the global and regional scale.

Determination of respiration and photosynthesis fractionation coefficients for atmospheric dioxygen inferred from a vegetation-soil-atmosphere analog of the terrestrial biosphere in closed chambers

The isotopic composition of dioxygen in the atmosphere is a global tracer which depends on the biosphere flux of dioxygen toward and from the atmosphere (photosynthesis and respiration) as well as exchanges with the stratosphere. When measured in fossil air trapped in ice cores, the relative concentration of 16O, 17O and 18O of O2 can be used for several applications such as ice core dating and past global productivity reconstruction. However, there are still uncertainties about the accuracy of these tracers as they depend on the integrated isotopic fractionation of different biological processes of dioxygen production and uptake, for which we currently have very few independent estimates. Here we determined the respiration and photosynthesis fractionation coefficients for atmospheric dioxygen from experiments carried out in a replicated vegetation-soil-atmosphere analog of the terrestrial biosphere in closed chambers with growing Festuca arundinacea. The values for 18O discrimination during soil respiration and dark respiration in leave are equal to −12.3 ± 1.7 ‰ and −19.1 ± 2.4 ‰, respectively. We also found a value for terrestrial photosynthetic fractionation equal to +3.7 ± 1.3 ‰. This last estimate suggests that the contribution of terrestrial productivity in the Dole effect may have been underestimated in previous studies.

Ecological Networks and Fluvial Corridors in Calabria (Southern Italy)

The anthropic pressure on natural systems is the main cause for the present process of biodiversity loss in terrestrial biosphere [1]. Really, the human disturbance on Earth affects the 74.1% of terrestrial and marine habitats, including 22.4% completely modified, 51.7% partially disturbed and just the 25.9% in natural and pristine conditions [2]. At the beginning of third millenium, in the middle of a post-industrial era, named “Anthropocene” [3], mankind is causing the greatest mass extinction of wildlife in terrestrial biosphere [4-6].

Observation Based Budget and Lifetime of Excess Atmospheric Carbon Dioxide

The global budgets of CO2 and of excess CO2 (i.e., above preindustrial) in the biogeosphere are examined by a top-down, observationally constrained approach. Global stocks in the atmosphere, mixed-layer and deep ocean, and labile and obdurate terrestrial biosphere, and fluxes between them are quantified; total uptake of carbon by the terrestrial biosphere is constrained by observations, but apportionment to the two terrestrial compartments is only weakly constrained, requiring examination of sensitivity to this apportionment. Because of near equilibrium between the atmosphere and the mixed-layer ocean and near steady state between the atmosphere and the labile biosphere, these three compartments are tightly coupled. For best-estimate present-day anthropogenic emissions the turnover time of excess carbon in these compartments to the deep ocean and obdurate biosphere is 67 to 158 years. Atmospheric CO2 over the Anthropocene is accurately represented by a five-compartment model with four independent parameters: two universal geophysical quantities and two, specific to CO2, treated as variable. The model also accurately represents atmospheric radiocarbon, particularly the large increase due to atmospheric testing of nuclear weapons and the subsequent decrease. The adjustment time of excess atmospheric CO2, evaluated from the rate of decrease following abrupt cessation of emissions, is 78 to 140 years, consistent with the turnover time, approaching a long-time floor of 15–20 % of the value at the time of cessation. The lifetime of excess CO2 found here, several-fold shorter than estimates from current carbon-cycle models, indicates that cessation of anthropogenic emissions atmospheric would result in substantial recovery of CO2 toward its preindustrial value in less than a century.

Hysteresis of terrestrial carbon cycle to CO2 ramp-up and -down forcing

To prevent excessive global warming, we have faced a situation to reduce net carbon dioxide (CO2) emissions. However, the behavior of Earth’s terrestrial biosphere under negative emissions is highly uncertain. Herein, we show strong hysteresis in the terrestrial carbon cycle in response to CO2 ramp-up and -down forcing. Owing to the strong hysteresis lag, the terrestrial biosphere stores more carbon at the end of simulations than at its initial state, lessening the burden on net-negative emissions. This hysteresis is latitudinally dependent, showing a longer timescale of reversibility in high latitudes. Particularly, carbon in boreal forests can be stored for a long time. However, the hysteresis of the carbon cycle in the pan-Arctic region depends on the presence of permafrost processes. That is, unexpected irreversible carbon emissions may occur in permafrost even after achieving net-zero emissions, indicating the importance of permafrost processes, which is highly uncertain based on our current knowledge.