Abstract: Seasonal and interannual variability in atmospheric carbon dioxide (CO2) concentrations was simulated using fluxes from fossil fuel, ocean and terrestrial biogeochemical models, and a tracer transport model with time-varying winds. The atmospheric CO2 variability resulting from these surface fluxes was compared to observations from 89 GLOBALVIEW monitoring stations. At northern hemisphere stations, the model simulations captured most of the observed seasonal cycle in atmospheric CO2, with the land tracer accounting for the majority of the signal. The ocean tracer was 3–6 months out of phase with the observed cycle at these stations and had a seasonal amplitude only ~10% on average of observed. Model and observed interannual CO2 growth anomalies were only moderately well correlated in the northern hemisphere (R ~ 0.4–0.8), and more poorly correlated in the southern hemisphere (R < 0.6). Land dominated the interannual variability (IAV) in the northern hemisphere, and biomass burning in particular accounted for much of the strong positive CO2 growth anomaly observed during the 1997–1998 El Nio event. The signals in atmospheric CO2 from the terrestrial biosphere extended throughout the southern hemisphere, but oceanic fluxes also exerted a strong influence there, accounting for roughly half of the IAV at many extratropical stations. However, the modeled ocean tracer was generally uncorrelated with observations in either hemisphere from 1979–2004, except during the weak El Nio/post-Pinatubo period of the early 1990s. During that time, model results suggested that the ocean may have accounted for 20–25% of the observed slowdown in the atmospheric CO2 growth rate.

Abstract: Chapter peer-reviewed edited volumes

Abstract: The effective thermal conductivity of snow, keff, is a crucial climatic and environmental variable. Here, we test the intuition that keff is linked to microstructural and mechanical properties by attempting to relate keff to density ρsnow, and to shear strength σ measured with a handheld shear vane. We performed 106 combined measurements of keff, ρsnow and σ in the Alps, Svalbard, Arctic Alaska, and near the North Pole, covering essentially all snow types. We find a good correlation between keff and ρsnow which is not significantly different from that of Sturm et al. (1997). The correlation between keff and a combination of σ and ρsnow is stronger than with density alone. We propose an equation linking keff, (W m-1 K-1) ρsnow (kg m-3) and σ (Pa): keff = 7.114 10-5 ρsnow σ0.333 + 2.367 10-2. This equation places constraints on the calculation of keff, ρsnow and σ in avalanche warning models where σ is a key variable. For our samples, we calculate σ from measured values of keff and ρsnow using our equation and compare the value to that predicted by the French MEPRA avalanche warning model, which uses density and grain type as input data. MEPRA and the prediction of σ based on keff and ρsnow agree within 8%. MEPRA agrees with observations within 11%. Calculating σ from density only yields values 55% lower than measured, showing the interest of using additional data to predict σ.

Abstract: We observed the wastage of ice masses on the Kerguelen Islands (Indian Ocean, 49S, 69E) using historical information and recent satellite data. Overall, the total ice-covered area on the islands declined from 703 to 552 km2 between 1963 and 2001, a reduction of 21%. The area of Cook ice cap (the main ice body) decreased asymmetrically from 501 to 403 km2. West flowing glaciers lost 11% of their area, while east flowing glaciers lost 28%. After 1991, the retreat rate accelerated from 1.9 km2/a (1963–1991) to 3.8 km2/a (1991–2003). Between 1963 and 2000, the ice volume loss was 25–30 km3, equivalent to an area-average ice-thinning rate of 1.4–1.7 m/a. The glacial retreat took place in the climatic context of a relatively low level of precipitation (compared to the 1950s) and a ~1C warming that occurred between 1964 and 1982. The acceleration of the ice losses since at least the 1990s indicates that the state of the ice bodies on the Kerguelen Islands is still far from balanced. Together with other studies in Patagonia, South Georgia, and Heard Island, our analysis is consistent with a pattern of strong and accelerated wastage of ice masses influenced by the Southern Ocean.

Abstract: In an ice sheet, a preferred crystal orientation fabric affects deformation rates because ice crystals are strongly anisotropic: shear along the basal plane is significantly easier than shear perpendicular to the basal plane. The effect of fabric can be as important as temperature in defining deformation rates. Fabric is typically measured using analysis of thin sections under the microscope with co-polarized light. Due to the time-consuming and destructive nature of these measurements, however, it is difficult to capture the spatial variation in fabric necessary for evincing ice sheet flow patterns. Because an ice crystal is similarly elastically anisotropic, the speed of elastic waves through ice can be used as a proxy for quantify anisotropy. We use borehole sonic logging measurements and thin section data from Dome C, East Antarctica to define the relations between apparent fabric and borehole measured elastic speeds (compressional VP and vertically polarized shear VSV). These relations, valid for single maximum fabrics, allow in-situ, depth-continuous fabric estimates of unimodal fabric strength from borehole sonic logging. We describe the single maximum fabric using a1: the largest eigenvalue of the second-order orientation tensor. For ice at -16C and a1 in the 0.7-1 range the relations are VP = 248 a13.7 + 3755 m s-1 and VSV = -210a17.3 + 1968 m s-1.