The exclusive use of carbonate reference materials is a robust method for the standardization of clumped isotope measurements • Measurements using different acid temperatures, designs of preparation lines, and mass spectrometers are statistically indistinguishable • We propose new consensus values for a set of 7 carbonate reference materials and updated guidelines to report clumped isotope measurements

Changes in atmospheric circulation over the past five decades have enhanced the wind-driven inflow of warm ocean water onto the Antarctic continental shelf, where it melts ice shelves from below 1-3 . Atmospheric circulation changes have also caused rapid warming 4 over the West Antarctic Ice Sheet, and contributed to declining sea-ice cover in the adjacent Amundsen-Bellingshausen seas 5 . It is unknown whether these changes are part of a longer-term trend. Here, we use waterisotope (δ 18 O) data from an array of ice-core records to place recent West Antarctic climate changes in the context of the past two millennia. We find that the δ 18 O of West Antarctic precipitation has increased significantly in the past 50 years, in parallel with the trend in temperature, and was probably more elevated during the 1990s than at any other time during the past 200 years. However, δ 18 O anomalies comparable to those of recent decades occur about 1% of the time over the past 2,000 years. General circulation model simulations suggest that recent trends in δ 18 O and climate in West Antarctica cannot be distinguished from decadal variability that originates in the tropics. We conclude that the uncertain trajectory of tropical climate variability represents a significant source of uncertainty in projections of West Antarctic climate and ice-sheet change.The West Antarctic Ice Sheet (WAIS), which is grounded largely below sea level, is potentially unstable. Mass loss from the WAIS is contributing to present sea-level rise, owing to the widespread thinning of ice shelves and the acceleration of the large outlet glaciers that drain the ice sheet into the ocean 1 . Contemporaneous with the loss of mass from the WAIS, air temperatures over the WAIS have increased significantly in the past 50 years 4,6,7 .Climate and ice-sheet changes in West Antarctica are closely linked with one another by changes in regional atmospheric circulation 8 . Observations beneath the floating ice shelf of Pine Island Glacier, a major drainage system for the flow of the WAIS into the Amundsen Sea, show that the primary cause of ice-shelf thinning is the presence of warm Circumpolar Deep Water on

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Carbonate clumped isotope thermometry provides a new method for investigating longstanding questions regarding seasonal biases in soil carbonate formation and the relationship between soil carbonate formation temperatures recorded by clumped isotopes (T(Δ 47)) and surface temperatures. We address these questions by comparing C, O, and clumped isotope data from Holocene soil carbonates to meteorological and in-situ soil monitoring data along a 170 km transect with >4 km of relief in Chile (30 °S). This arid transect experiences a winter wet season, and a >20 °C range in mean annual air temperature. We test the hypothesis that, regardless of soil moisture conditions, soil carbonates from arid regions record warm season biases and form in isotopic equilibrium with soil water and soil CO 2. Below 3200 m, precipitation falls as rain and soil carbonate T(Δ 47) values at depths >40 cm resemble summer soil temperatures. Above 3200 m, precipitation falls as snow and T(Δ 47) values resemble mean annual soil temperatures. Soil