Browsing by Author "Sturges, WT"
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- ItemAtmospheric CO2 and d13C-CO2 reconstruction of the little ice age from antarctic ice cores(Copernicus Publications, 2015-04-12) Rubino, M; Etheridge, DM; Trudinger, CM; Allison, CE; Rayner, PJ; Mulvaney, R; Steele, LP; Langenfelds, RL; Sturges, WT; Curran, MAJ; Smith, AMThe decrease of atmospheric CO2 concentration recorded in Antarctic ice around 1600 AD is one of the most significant atmospheric changes to have occurred during the last millennia, before the onset of the industrial period.Together with the temperature decrease, the CO2 drop has been used to derive the sensitivity of carbon stores to climate. However, the cause of it is still under debate because models are not yet able to reproduce either its magnitude, or its timing. Here we present new measurements of the CO2 concentration decrease recorded in an ice core from a medium accumulation rate site in Antarctica (DML). We show that the new record is compatible(differences <2 ppm) with the CO2 record from the high accumulation rate DSS site on Law Dome (East Antarctica), when the different age distributions are taken into account. We have also measured the d13C-CO2 change in DML ice, filling a gap around 1600 AD in the DSS d13C record. We use a double deconvolution of the CO2 and d13C records together to provide quantitative evidence that the CO2 decrease was caused by a change in the net flux to the terrestrial biosphere. Finally, we provide a new interpretation of a published record showing increasing atmospheric carbonyl sulphide during the CO2 decrease, suggesting that cooler LIA climate affected terrestrial biospheric fluxes. Altogether our findings support the hypothesis that reduced soil heterotrophic respiration is likely to have given the most significant contribution to the LIA CO2 decrease implying a positive CO2-climate feedback. © 2015, Authors.
- ItemCorrigendum to "Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland'' published in Atmos. Chem. Phys., 12, 4259–-4277, 2012(Copernicus Publications, 2014-04-09) Buizert, C; Martinerie, P; Petrenko, VV; Severinghaus, JP; Trudinger, CM; Witrant, E; Rosen, JL; Orsi, AJ; Rubino, M; Etheridge, DM; Steele, LP; Hogan, C; Laube, JC; Sturges, WT; Levchenko, VA; Smith, AM; Levin, I; Conway, TJ; Dlugokencky, EJ; Lang, PM; Kawamura, K; Jenk, TM; White, JWC; Sowers, T; Schwander, J; Blunier, TIt was kindly pointed out to us by M. Battle that Eq. (2) on p. 4263 contains a typo, and should instead be [X]corr(z) = [X]meas(z) ΔMδgrav(z)/1000 + 1 , (2) where [X]corr ([X]meas) is the gravity-corrected (measured) mixing ratio of gas species X, 1M = MX − Mair is the molar mass difference between gas X and air, and grav(z) is the gravitational fractionation per unit mass difference at depth z. All calculations in the study were done correctly, following Eq. (2) as given here. Furthermore, the present-day 1age value for NEEM is incorrect in the original manuscript, and underestimates Δage by 6 years. The correct value is 188+3 −9 yr. In our original, incorrect calculation we used the ice age in years before 2000 CE (b2k), while we should have used the ice age relative to the surface ice age. In the updated 1age calculation we use the ice age found by annual layer counting of the shallow NEEM 2011 S1 core (Sigl et al., 2013). The NEEM chronology published in Rasmussen et al. (2013) uses the correct, updated Δage estimate. Both errors addressed in this corrigendum affect neither the discussion nor the main conclusions of the original publication. © Author(s) 2014.
- ItemGas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland(Copernicus Publications, 2012-05-14) Buizert, C; Martinerie, P; Petrenko, VV; Severinghaus, JP; Trudinger, CM; Witrant, E; Rosen, JL; Orsi, AJ; Rubino, M; Etheridge, DM; Steele, LP; Hogan, C; Laube, JC; Sturges, WT; Levchenko, VA; Smith, AM; Levin, I; Conway, TJ; Dlugokencky, EJ; Lang, PM; Kawamura, K; Jenk, TM; White, JWC; Sowers, T; Schwander, J; Blunier, TAir was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1σ Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (Δage) at the firn-ice transition is calculated to be 182+3−9 yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records. © Author(s) 2012.
- ItemLow atmospheric CO2 levels during the Little Ice Age due to cooling-induced terrestrial uptake(Springer Nature, 2016-07-25) Rubino, M; Etheridge, DM; Trudinger, CM; Allison, CE; Rayner, PJ; Enting, I; Mulvaney, R; Steele, LP; Langenfelds, RL; Sturges, WT; Curran, MAJ; Smith, AMLow atmospheric carbon dioxide (CO2) concentration1 during the Little Ice Age has been used to derive the global carbon cycle sensitivity to temperature2. Recent evidence3 confirms earlier indications4 that the low CO2 was caused by increased terrestrial carbon storage. It remains unknown whether the terrestrial biosphere responded to temperature variations, or there was vegetation re-growth on abandoned farmland5. Here we present a global numerical simulation of atmospheric carbonyl sulfide concentrations in the pre-industrial period. Carbonyl sulfide concentration is linked to changes in gross primary production6 and shows a positive anomaly7 during the Little Ice Age. We show that a decrease in gross primary production and a larger decrease in ecosystem respiration is the most likely explanation for the decrease in atmospheric CO2 and increase in atmospheric carbonyl sulfide concentrations. Therefore, temperature change, not vegetation re-growth, was the main cause of the increased terrestrial carbon storage. We address the inconsistency between ice-core CO2 records from different sites8 measuring CO2 and δ13CO2 in ice from Dronning Maud Land (Antarctica). Our interpretation allows us to derive the temperature sensitivity of pre-industrial CO2 fluxes for the terrestrial biosphere (γL = −10 to −90 Pg C K−1), implying a positive climate feedback and providing a benchmark to reduce model uncertainties. © 2016, Nature Publishing Group.
- ItemA record of carbonyl sulfide from Antarctic ice over the last 1000 years(Geochemical Society, 2013-01-01) Allin, SJ; Sturges, WT; Laube, JC; Etheridge, DM; Rubino, M; Trudinger, CM; Curran, MAJ; Smith, AM; Mulvaney, RCarbonyl sulfide (COS) is a trace gas, present in the troposphere, and also in the stratosphere, where it contributes to the stratospheric sulfate aerosol layer. It has both natural and anthropogenic sources. Natural processes include uptake by plants, while oceans, wetlands, volcanism and biomass burning all contribute to natural COS emissions. We have measured COS in Antarctic ice cores from Dronning Maud Land, drilled in 1998, the DE08 core drilled at Law Dome in 1987, and the DSS0506 core drilled in 2006. Ice samples with COS gas ages between about 1050 AD and the early 20th centrury have been examined. A large volume ice crusher at the CSIRO Marine and Atmospheric Research laboratory was used to extract air from bubbles occluded in the ice cores. These air samples were analysed for CO2, CH4, CO and 13CO2 at CSIRO, and then for COS and several halocarbons at the University of East Anglia on a high sensitivity gas chromatograph/tri-sector mass spectrometer system. Initial results indicate that good sample integrity can be achieved. Measurements from the DML samples indicate low and uniform abundances across the last few hundred years, and at concentrations significantly below those in the modernday atmosphere. Measurements in more recent ice from DE08 show the start of increasing concentrations in the early 1900s, confirming earlier evidence that the global atmospheric abundance of COS has increased as a result of industrial activity during the 20th century.
- ItemTerrestrial uptake due to cooling responsible for low atmospheric CO2 during the Little Ice Age(Antarctic Climate and Ecosystems Cooperative Research Centre, 2016-03-07) Rubino, M; Etheridge, DM; Trudinger, CM; Allison, CE; Rayner, PJ; Enting, I; Mulvaney, R; Steele, LP; Langenfelds, RL; Sturges, WT; Curran, MAJ; Smith, AMModels of future carbon cycle-climate changes predict a large range in atmospheric CO2, mainly because of uncertainties in the response of the land carbon cycle to the future temperature increase. The Little Ice Age (LIA, 1500-1750 AD) CO2 decrease is the most significant pre-industrial atmospheric change over the last millennia and has been used to derive the climate sensitivity of the global carbon cycle (δ). While a recent study confirms that pre-industrial CO2 variations were caused by changes in land carbon stores, there are open questions about the size of the atmospheric LIA CO2 decrease reconstructed from ice cores, and about what caused the land to sequester CO2. To quantify the size of the LIA CO2 decrease, we have produced new CO2 measurements from DML ice, that support the DSS LIA CO2 decrease as a real atmospheric feature. To partition the contribution of ocean and land, we have measured the δ 13C-CO2, showing that the cause of the CO2 drop was uptake by the terrestrial biosphere. To identify whether the land uptake was caused by temperature, or by a decline in farming due to pandemics, we have simulated the effect of a temperature perturbation on atmospheric Carbonyl Sulfide (COS). In agreement with the previously published positive COS anomaly, our results indicate that Global Primary Productivity (GPP) decreased during the LIA, ruling out the early anthropogenic land use change hypothesis as the dominant cause of increased terrestrial carbon storage. This allows us to obtain a new, more coherent estimation of δ in the range -10/-60 Pg of C K-1.