Verifying Southern Hemisphere trends in atmospheric carbon dioxide stable isotopes

Stable isotopes of carbon dioxide distinguish between oceanic and terrestrial uptake of accumulating atmospheric carbon dioxide. At Cape Grim (41°S, 141°E), two Commonwealth Scientific and Industrial Research Organisation programs provide δ 13 C and δ 18 O of CO 2 employing different collection, sample pretreatment, and calibration strategies that exemplify calibration problems. A comprehensive reexamination of methods results in δ 13 C agreement of 0.005 ± 0.004‰ through 14 overlapping years of data and suggests we have described and accounted for the main sources of bias. For δ 18 O, flask storage effects and early standard assignment problems contribute to larger variation and the comparable differences are 0.024 ± 0.184‰. Cape Grim records are reissued with uncertainties reflecting the histories of each method, as well as selected “best” records for δ 13 C from 1981, and for δ 18 O from 1978. The uncertainties reflect the internal consistency against an in‐house reference, of most interest in determining the limits of temporal behavior, and can be combined with a larger fixed uncertainty if a Cape Grim record is used to quantify spatial differences from sites operated by different measurement laboratories. Marked variability on seasonal and interannual timescales is observed in both isotope records. The geographical location and sampling strategy at Cape Grim ensures large‐scale (hemispheric) representativeness on interannual timescales, and we show differences from comparable records maintained by the University of Colorado (using National Oceanic and Atmospheric Administration flasks) and Scripps Institution of Oceanography. The long‐term Southern Hemisphere relationships between δ 13 C and CO 2 are explored for the seasonal and interannual variability in the Cape Grim record. The interannual relationship is characterized by distinct periods of δ 13 C leveling immediately following increased global CO 2 growth rate.