William F. Budd

Partitioning of the global fossil CO2 sink using a 19‐year trend in atmospheric O2

O2/N2 is measured in the Cape Grim Air Archive (CGAA), a suite of tanks filled with background air at Cape Grim, Tasmania (40.7°S, 144.8°E) between April 1978 and January 1997. Derived trends are compared with published O2/N2 records and assessed against limits on interannual variability of net terrestrial exchanges imposed by trends of δ13C in CO2. Two old samples from 1978 and 1987 and eight from 1996/97 survive critical selection criteria and give a mean 19-year trend in δ(O2/N2) of −16.7±0.5 per meg yr−1, implying net storage of +2.3 ± 0.7 GtC (1015 g carbon) yr−1 of fossil fuel CO2 in the oceans and +0.2 ± 0.9 GtC yr−1 in the terrestrial biosphere. The uptake terms are consistent for both O2/N2 and δ13C tracers if the mean 13C isotopic disequilibrium flux, combining terrestrial and oceanic contributions, is 93±15 GtC ‰ yr−1.

Collapse and reorganisation of the Southern Ocean overturning under global warming in a coupled model

This study investigates the long-term behaviour of the thermohaline circulation (THC) in the CSIRO climate model, under a scenario of transient increase of atmospheric (equivalent) CO2 concentration followed by a perpetual stabilisation at triple the initial level (3 × CO2). The North Atlantic Deep Water Formation (NADWF) declines substantially and the Antarctic Bottom Water Formation (AABWF) essentially ceases by the time of CO2 tripling. During the subsequent millennium of stable 3 × CO2, NADWF recovers slightly but the AABWF shows no sign of returning and the residual deep overturning dies away. Accelerating the convergence to equilibrium of the deep ocean under the 3 × CO2 condition, the global THC eventually reaches a near-stable state with the entire ocean warming by about 7°C, NADWF fully recovered and AABWF partly re-established. This result shows a possible new quasi-equilibrium of the ocean under long-term global warming induced by the anthropogenic CO2 increase.

A further assessment of surface temperature changes at stations in the Antarctic and Southern Ocean, 1949–2002

Abstract Statistical analyses are carried out, of the annual mean surface air temperature at occupied stations and automatic weather stations in the Antarctic and Southern and Pacific Oceans. The data are studied in four groupings: coastal Antarctica (excluding the Antarctic Peninsula), inland Antarctica, the Antarctic Peninsula and the Southern Ocean/Pacific Ocean islands. We find that within each of these four groupings the average trend indicates warming. For coastal Antarctica the trend is ∼0.8°C(100 a)–1. Inland, the results are less clear, but the mean trend is to a warming of ∼1.0°C(100 a)–1. For the Peninsula stations it is ∼4.4°C(100 a)–1, and for the ocean stations the average trend is ∼0.8°C(100 a)–1. The results indicate a reduction in the warming trend since our last analysis 6 years ago. While the Pinatubo (Philippines) volcanic eruption may have had some influence on this reduction in the warming rate, examination of the interannual variations in the temperature record shows variability has continued high since the recovery from any such effect. There has been a further period of cooler temperatures in coastal and inland Antarctica in that time, yet a warmer period in the Peninsula and ocean islands.

Meteoric and marine ice crystal orientation fabrics from the Amery Ice Shelf, East Antarctica

The northwestern sector of the Amery Ice Shelf, East Antarctica, has a layered structure, due to the presence of both meteoric ice and a marine ice layer resulting from sub-shelf freezing processes. Crystal orientation fabric and grain-size data are presented for ice cores obtained from two boreholes � 70 km apart on approximately the same flowline. Multiple-maxima crystal orientation fabrics and large mean grain sizes in the meteoric ice are indicative of stress relaxation and subsequent grain growth in ice that has flowed into the Amery Ice Shelf. Strongly anisotropic single-maximum crystal orientation fabrics and rectangular textures near the base of the � 200 m thick marine ice layer suggest accretion occurs by the accumulation of frazil ice platelets. Crystal orientation fabrics in older marine ice exhibit vertical large circle girdle patterns, influenced by the complex stress configurations that exist towards the margins of the ice shelf. Post-accumulation grain growth and fabric development in the marine ice layer are restricted by a high concentration of brine and insoluble particulate inclusions. Differences in the meteoric and marine ice crystallography are indicative of the contrasting rheological properties of these layers, which must be considered in relation to large-scale ice-shelf dynamics.

Derivation of ice thickness and bedrock topography in data-gap regions over Antarctica

Abstract An approach to deriving the thickness of the Antarctic ice sheet and inferring the bedrock elevation to fill in gaps in the compilations of ice-thickness observations is presented. It combines assumptions about the general state of balance of the ice sheet and of the dynamics of ice flow, with information about ice accumulation and the topography of the ice-sheet surface elevation to infer the ice thickness and bedrock. A simplified version of the scheme already shows the potential of this approach, as is demonstrated by an application to the Lambert Glacier basin, East Antarctica.

Laboratory studies of the flow rates of debris-laden ice

Abstract Ice-sheet basal ice is warmer than that above because of the heat from the Earth’s interior. The stresses acting on the basal ice are greatest. In addition, the basal ice often contains debris consisting of silt and small stones picked up from the rock over which the ice flows. Because the base is the warmest part of an ice sheet and the stress there is greatest, flow rates in the basal ice are large and often contribute most of the ice movement. It is therefore important, for accurate modelling of the ice sheets, to know whether the debris within the basal ice enhances or retards the flow of the ice. In this paper, we describe laboratory deformation tests in uniaxial compression and in simple shear, on sand-laden ice. We find no significant dependence of flow rate on sand content (up to 15% volume) in the stress range 0.13–0.5 MPa and temperature range –0.02 to –18.0°C. Further work needs to include laboratory tests on debris-laden ice extracted from the polar ice sheets. This work is underway.

Ice-flow properties at Dome Summit South, Law Dome, East Antarctica

Abstract Ice-flow properties within a polar ice sheet are examined using the comprehensive data gathered from ice-core drilling by Australian National Antarctic Research Expeditions (ANARE) at Dome Summit South (DSS), on Law Dome, East Antarctica. Using the shear strain rates derived from borehole inclination measurements we demonstrate the need to modify the ice-flow relations to treat enhanced shear deformation deep within the ice sheet. We show that the relation between enhanced flow and the measured crystallographic properties is generally in accord with expectations, at least in the upper parts of the ice sheet, but it becomes clear that nearer to the bedrock the situation is more complicated. We also compare the observed shear strain-rate profile with results from a model that describes flow enhancement as a function of the applied stresses.