Interpreting Differential Temperature Trends at the Surface and in the Lower Troposphere

Estimated global-scale temperature trends at Earth's surface (as recorded by thermometers) and in the lower troposphere (as monitored by satellites) diverge by up to 0.14°C per decade over the period 1979 to 1998. Accounting for differences in the spatial coverage of satellite and surface measurements reduces this differential, but still leaves a statistically significant residual of roughly 0.1°C per decade. Natural internal climate variability alone, as simulated in three state-of-the-art coupled atmosphere-ocean models, cannot completely explain this residual trend difference. A model forced by a combination of anthropogenic factors and volcanic aerosols yields surface-troposphere temperature trend differences closest to those observed.

Glacial-interglacial temperature change in the tropical West Pacific: a comparison of stalagmite-based paleo-thermometers

In the tropics, geochemical records from stalagmites have so far mainly been used to qualitatively reconstruct changes in precipitation, but several new methods to reconstruct past temperatures from stalagmite material have emerged recently: i) liquid–vapor homogenization of fluid inclusion water ii) noble gas concentrations in fluid inclusion water, iii) the partitioning of oxygen isotopes between fluid inclusion water and calcite, and iv) the abundance of the 13C18O16O (‘clumped’) isotopologue in calcite. We present, for the first time, a direct comparison of these four paleo-thermometers by applying them to a fossil stalagmite covering nearly two glacial–interglacial cycles (Marine Isotope Stages (MIS) 12–9) and to two modern stalagmites, all from northern Borneo. The temperature estimates from the different methods agree in most cases within errors for both the old and recent samples; reconstructed formation temperatures of the recent samples match within 2-sigma errors with measured cave temperatures. However, slight but systematic deviations are observed between noble gas and liquid–vapor homogenization temperatures. Whereas the temperature sensitivity of fluid inclusion δ18O and clumped isotopes is currently debated, we find that the calibration of Tremaine et al. (2011) for fluid inclusion δ18O and a synthetic calcite-based clumped isotope calibration (Ziegler et al., in prep.) yield temperature estimates consistent with the other methods. All methods (with the potential exception of clumped isotopes) show excellent agreement on the amplitude of glacial–interglacial temperature change, indicating temperature shifts of 4–5 °C. This amplitude is similar to the amplitude of Mg/Ca-based regional sea surface temperature records, when correcting for sea level driven changes in cave elevation. Our reconstruction of tropical temperature evolution over the time period from 440 to 320 thousand years ago (ka) adds support to the view that climate sensitivity to varying greenhouse forcing is substantial also in the deep tropics.