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Abstract: We revisit the portion of (Nature 391 (1998) 141) devoted to the abrupt temperature increase reconstruction at the Younger Dryas/Preboreal transition. The original estimate of +5 to +10°C abrupt warming is revised to +10±4°C. The gas isotope data from the original work were employed, combined with recently measured precise air thermal diffusion constants (Geochim. Cosmochim. Acta 67 (2003a) 345; J. Phys. Chem. 23A (2003b) 4636). The new constants allow a robust interpretation of the gas isotope signal in terms of temperature change. This was not possible at the time of the original work, when no air constants were available. Three quasi-independent approaches employed in this work all give the same result of a +10°C warming in several decades or less. The new result provides a firm target for climate models that attempt to predict future climates. [Copyright &y& Elsevier]

Rapid temperature change causes fractionation of isotopic gaseous species in air in firn (snow) by thermal diffusion, producing a signal that is preserved in trapped air bubbles as the snow forms ice. Using a model of heat penetration and gas diffusion in the firn, as well as the values of appropriate thermal diffusion constants, it is possible to reconstruct the magnitude of a particular paleoclimate change. Isotopic nitrogen in air serves as a convenient tracer for such paleoreconstruction, because the ratio 29N2/28N2 has stayed extremely constant in the atmosphere for ≥106 years. However, prior to this work no data were available for thermal diffusion of 29N2/28N2 in air, but only in pure N2. We devised a laboratory experiment allowing fractionation of gases by thermal diffusion in a small, tightly controlled temperature difference. A mass spectrometer was employed in measuring the resulting fractionations yielding measurement precision greater than was attainable by earlier thermal diffusion investigators.Our laboratory experiments indicate that the value of the thermal diffusion sensitivity (Ω) for 29N2/28N2 in air is +(14.7 ± 0.5) × 10−3 per mil/°C when the average temperature is –30.0°C. The corresponding value for 29N2/28N2 in pure N2 that we find is +(15.3 ± 0.4) × 10−3 per mil/°C at –30.6°C, in agreement with the previously available literature data within their large range of uncertainty. We find that an empirical equation, Ω = (8.656/TK − 1232/T K2) ± 3% per mil/°C, describes the slight variation of the sensitivity values for 29N2/28N2 in air with temperature in the range of –60 to 0°C. A separate set of experiments also described in this paper rules out adsorption as a candidate for producing additional temperature change-driven fractionation of 29N2/28N2 in the firn air. The combined newly obtained data constitute a calibration of the fossil–air paleothermometer with respect to isotopic nitrogen and will serve to improve the estimates of the magnitudes of past abrupt climate changes recorded in ice cores. [Copyright &y& Elsevier]

Abstract: How fast and how much climate can change has significant implications for concerns about future climate changes and their potential impacts on society. An abrupt climate change 8200 years ago (8.2ka event) provides a test case to understand possible future climatic variability. Here, methane concentration (taken as an indicator for terrestrial hydrology) and nitrogen isotopes (Greenland temperature) in trapped air in a Greenland ice core (GISP2) are employed to scrutinize the evolution of the 8.2ka event. The synchronous change in methane and nitrogen implies that the 8.2ka event was a synchronous event (within ±4 years) at a hemispheric scale, as indicated by recent climate model results [Legrande, A. N., Schmidt, G. A., Shindell, D. T., Field, C. V., Miller, R. L., Koch, D. M., Faluvegi, G., Hoffmann, G., 2006. Consistent simulations of multiple proxy responses to an abrupt climate change event. Proceedings of the National Academy of Sciences 103, 837–842]. The event began with a large-scale general cooling and drying around ∼8175±30 years BP (Before Present, where Present is 1950 AD). Greenland temperature cooled by 3.3±1.1°C (decadal average) in less than ∼20 years, and atmospheric methane concentration decreased by ∼80±25ppb over ∼40 years, corresponding to a 15±5% emission reduction. Hemispheric scale cooling and drying, inferred from many paleoclimate proxies, likely contributed to this emission reduction. In central Greenland, the coldest period lasted for ∼60 years, interrupted by a milder interval of a few decades, and temperature subsequently warmed in several steps over ∼70 years. The total duration of the 8.2ka event was roughly 150 years. [Copyright &y& Elsevier]