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Widespread six degrees Celsius cooling on land during the Last Glacial Maximum

Abstract

The magnitude of global cooling during the Last Glacial Maximum (LGM, the coldest multimillennial interval of the last glacial period) is an important constraint for evaluating estimates of Earth’s climate sensitivity1,2. Reliable LGM temperatures come from high-latitude ice cores3,4, but substantial disagreement exists between proxy records in the low latitudes1,5,6,7,8, where quantitative low-elevation records on land are scarce. Filling this data gap, noble gases in ancient groundwater record past land surface temperatures through a direct physical relationship that is rooted in their temperature-dependent solubility in water9,10. Dissolved noble gases are suitable tracers of LGM temperature because of their complete insensitivity to biological and chemical processes and the ubiquity of LGM-aged groundwater around the globe11,12. However, although several individual noble gas studies have found substantial tropical LGM cooling13,14,15,16, they have used different methodologies and provide limited spatial coverage. Here we use noble gases in groundwater to show that the low-altitude, low-to-mid-latitude land surface (45 degrees south to 35 degrees north) cooled by 5.8 ± 0.6 degrees Celsius (mean ± 95% confidence interval) during the LGM. Our analysis includes four decades of groundwater noble gas data from six continents, along with new records from the tropics, all of which were interpreted using the same physical framework. Our land-based result broadly supports a recent reconstruction based on marine proxy data assimilation1 that suggested greater climate sensitivity than previous estimates5,6,7.

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Fig. 1: Overview of the noble gas palaeothermometer and its key components.
Fig. 2: Noble gases in young groundwater accurately record modern temperatures.
Fig. 3: Noble gases suggest around 6 °C of low-elevation, low-latitude LGM cooling on land.

Data availability

All original groundwater data (noble gas concentrations, ages, water temperatures (if available)), recharge elevations, study locations, fitted parameters and statistical uncertainties are freely available for download through PANGAEA (https://doi.org/10.1594/PANGAEA.929176). NGT time series plots of each study are available as supplementary files. Source data are provided with this paper.

Code availability

All MATLAB scripts for NGT fitting (including documentation) are freely available from zenodo (https://doi.org/10.5281/zenodo.4589442).

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Acknowledgements

We thank the entire community of groundwater noble gas geochemists, particularly the pioneers E. Mazor and J. N. Andrews, for decades of careful work in analysing groundwater from around the world and furthering our physical understanding of inert gases in groundwater; A. Moulla and T. Condesso de Melo for sharing data, and D. Bekaert for helpful discussions. The manuscript was improved by helpful suggestions from A. Manning, J. Clark and David McGee. This work was supported in part by NSF-EAR-1702704, NSF-EAR-1702571, and NSF-OCE-1923915.

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Contributions

A.M.S. wrote the manuscript and carried out modelling and data analysis. J.N. and A.M.S. created the database. J.P.S. developed the LGM atmospheric pressure model. A.M.S., J.N., W.A., M.S., J.T.K. and R.K. contributed groundwater datasets, all authors contributed to weekly discussions about data interpretation and modelling. All authors edited and revised the manuscript.

Corresponding author

Correspondence to Alan M. Seltzer.

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The authors declare no competing interests.

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Peer review information Nature thanks Jordan Clark, Andrew Manning, David McGee and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Evaluation of leading noble gas models.

Comparison of Late Holocene NGTs to ERA5-Land 1981–2019 MASTs across three NGT models and χ2 goodness-of-fit histogram comparison of all groundwater samples (n = 753) included in this work (normalized by degrees of freedom, n) (inset). The closed-system equilibration (CE) model agrees closest with ERA5-Land temperatures (r.m.s.d. = 1.4 °C), followed by the partial re-equilibration (PR) (r.m.s.d. = 1.5 °C) and oxygen depletion (OD) (r.m.s.d. = 5.7 °C) models. The closed-system equilibration model also exhibits the best goodness-of-fit (lowest median χ2/n). Data are mean ± 1 s.e.m.

Extended Data Fig. 2 ERA5-Land temperatures are unbiased above 5 °C.

Comparison of 1981–2019 mean annual ERA5-Land ground (upper soil) temperatures to a global database of modern mean annual measured ground temperatures34, using the approach described in the Methods to project ERA5-Land temperatures to the observation elevations. Whereas below approximately 5 °C, ERA5-Land temperatures appear to be systematically biased to be warmer than the observed temperature, above 5 °C they consistently overlap the 1:1 line with an r.m.s.d. of 1.6 °C. Observed temperatures tend to be slightly warmer than ERA5-Land temperatures on average, perhaps because of the typical locations of micrometeorological stations in barren fields, with little cooling from the shade provided by vegetation.

Extended Data Fig. 3 Leading sources of systematic error in LGM noble gas palaeothermometry.

ad, Modelled sensitivity of apparent NGTs to leading sources of systematic error. In each case, NGT bias (T′) is reported with respect to a starting recharge temperature of 10 °C at 1 km elevation (except for mixing tests (d) in which T′ is given relative to the temperature of an equal-parts mixture of 10 °C equilibrated water with a given equilibrium mixing end-member temperature). The NGT bias associated with a source of error is shown, including LGM–Late-Holocene changes in recharge elevation (a), water table depth (ΔWTD, b) and pressure (ΔP, c), and the direct NGT bias induced by mixing (d) relative to the admixture temperature. Green squares indicate the ±1σ confidence region for the range of likely glacial–interglacial variability (Supplementary Table 1; see Methods for a detailed description of each sensitivity test and Supplementary Table 2 for a compilation of the results).

Extended Data Fig. 4 Atmospheric pressure changes during the LGM.

Box model result for changes in LGM atmospheric pressure with elevation at a fixed point. a, The absolute pressure (P) is shown during the LGM and in the modern atmosphere. b, LGM anomalies in pressure (ΔP) relative to the modern atmosphere are shown. In brief, the model assumes a fixed lapse rate (6.5 °C km−1) and uses the barometric equation to estimate the vertical distribution of atmospheric pressure during the LGM, accounting for loss of atmospheric air by dissolution into a colder ocean and occlusion in high-latitude ice sheets, as well as displacement of air by the growth of large ice sheets (see Methods for further details).

Extended Data Fig. 5 Comparison of AP2 LGM cooling estimates to literature values.

Comparison by latitude of noble-gas-derived ΔTLGM (this study, approach AP2) to zonal-mean land-surface (solid lines) and sea-surface (dashed lines) estimates of ΔTLGM from key previous studies1,5,7. Data are mean ± 1 s.e.m. Our AP2 low latitude (45° S–35° N) mean estimate of LGM cooling (4.8 ± 0.6 °C; thick green dashed line, with 95% confidence error envelope) is around 1 °C smaller in magnitude (warmer) than AP1. Although the AP2 estimate seems to more closely overlap the previously published land cooling data1, we note that this data-assimilation study was entirely constrained by marine proxies and therefore the implications for cooling over land should be treated with caution. For the physical and statistical reasons described in the main text, we suggest that AP1 is more robust, and we emphasize that the relatively good agreement between AP1 and AP2, compared with the range of disagreement among literature values, adds confidence to the reliability of the NGT reconstruction.

Extended Data Table 1 Leading sources of systematic error
Extended Data Table 2 Overview of groundwater noble gas datasets

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Seltzer, A.M., Ng, J., Aeschbach, W. et al. Widespread six degrees Celsius cooling on land during the Last Glacial Maximum. Nature 593, 228–232 (2021). https://doi.org/10.1038/s41586-021-03467-6

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