The latitudinal temperature gradient between the Equator and the poles influences atmospheric stability, the strength of the jet stream and extratropical cyclones1,2,3. Recent global warming is weakening the annual surface gradient in the Northern Hemisphere by preferentially warming the high latitudes4; however, the implications of these changes for mid-latitude climate remain uncertain5,6. Here we show that a weaker latitudinal temperature gradient—that is, warming of the Arctic with respect to the Equator—during the early to middle part of the Holocene coincided with substantial decreases in mid-latitude net precipitation (precipitation minus evapotranspiration, at 30° N to 50° N). We quantify the evolution of the gradient and of mid-latitude moisture both in a new compilation of Holocene palaeoclimate records spanning from 10° S to 90° N and in an ensemble of mid-Holocene climate model simulations. The observed pattern is consistent with the hypothesis that a weaker temperature gradient led to weaker mid-latitude westerly flow, weaker cyclones and decreased net terrestrial mid-latitude precipitation. Currently, the northern high latitudes are warming at rates nearly double the global average4, decreasing the Equator-to-pole temperature gradient to values comparable with those in the early to middle Holocene. If the patterns observed during the Holocene hold for current anthropogenically forced warming, the weaker latitudinal temperature gradient will lead to considerable reductions in mid-latitude water resources.
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All of the proxy and instrumental climate records that were analysed in this study are from published sources. Supplementary Tables 1 and 2 include the citations to the original publications for each of the Holocene-long temperature and hydroclimate proxy records, respectively. The proxy data and basic metadata for the time series compiled for this study from these sources are available at the World Data Service for Paleoclimatology hosted by NOAA (https://www.ncdc.noaa.gov/paleo/study/25890). The landing page includes links to digital versions of the primary results (time series) generated by this study, including the (1) Holocene temperature composites by latitude (Fig. 3a–e), (2) Northern Hemisphere LTG (Fig. 3f) and (3) mid-latitude net precipitation reconstruction (Fig. 3h). The proxy temperature records for the past 2,000 years were compiled by the PAGES2k Consortium16 and are available at: https://www.ncdc.noaa.gov/paleo-search/study/21171. The CRU instrumental data are available at http://www.cru.uea.ac.uk/. PMIP3 model output is available at https://esgf-node.llnl.gov/projects/esgf-llnl/.
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Funding for this research was provided by the Science Foundation Arizona Bisgrove Scholar award (BP 0544-13), the National Science Foundation (AGS-1602105 and EAR-1347221), and the State of Arizona Technology and Research Initiative Fund administered by the Arizona Board of Regents. We acknowledge support by the USGS Climate and Land Use Program (any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the US government). H.G. is Research Director at the Fonds de la Recherche Scientifique–FNRS (Belgium). We thank D. Coumou and USGS reviewers M. Robinson and T. Cronin for comments on the manuscript. This work benefited from the new compilation of proxy temperature records for the past 2,000 years that was led by the Past Global Changes (PAGES) project, and from discussions with colleagues at PAGES-sponsored workshops. We thank all original data contributors who made their data available. We also thank the leaders of, and the many contributors to, the data compilations that were integrated in this study.
Nature thanks Matthew Kirby, Fredrik Charpentier Ljungqvist and the other anonymous reviewer(s) for their contribution to the peer review of this work.