Substantial warming occurred across North America, Europe and the Arctic over the early twentieth century1, including an increase in global drought2, that was partially forced by rising greenhouse gases (GHGs)3. The period included the 1930s Dust Bowl drought4,5,6,7 across North America’s Great Plains that caused widespread crop failures4,8, large dust storms9 and considerable out-migration10. This coincided with the central United States experiencing its hottest summers of the twentieth century11,12 in 1934 and 1936, with over 40 heatwave days and maximum temperatures surpassing 44 °C at some locations13,14. Here we use a large-ensemble regional modelling framework to show that GHG increases caused slightly enhanced heatwave activity over the eastern United States during 1934 and 1936. Instead of asking how a present-day heatwave would behave in a world without climate warming, we ask how these 1930s heatwaves would behave with present-day GHGs. Heatwave activity in similarly rare events would be much larger under today’s atmospheric GHG forcing and the return period of a 1-in-100-year heatwave summer (as observed in 1936) would be reduced to about 1-in-40 years. A key driver of the increasing heatwave activity and intensity is reduced evaporative cooling and increased sensible heating during dry springs and summers.
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Source Data for Figs. 3 and 4 are available with the manuscript, and Source Data for Figs. 1, 3 and 4 as well as Extended Data Fig. 3 are available at https://github.com/tcowan80/Cowan_et_al_2020_DustBowl_GHG. The BEST gridded product can be downloaded from http://berkeleyearth.org/. The GHCN-D archive can be accessed from ftp://ftp.ncdc.noaa.gov/pub/data/ghcn/daily/by_year/. The WAH2 experiments were coordinated through the Environmental Change Institute at the University of Oxford and can be made available on request.
The code to generate the main figures and Extended Data figures is available at: https://github.com/tcowan80/Cowan_et_al_2020_DustBowl_GHG. The code to calculate weather analogues, including installation, is publicly available from https://github.com/sradanov/castf90. Information on its use is available at https://flyingpigeon.readthedocs.io/en/latest/processes_des.html. All supplementary figure code is available on request. Spatial plots are produced using NCAR Command Language (v.6.4.0; https://doi.org/10.5065/D6WD3XH5). Return period two-dimensional plots are generated using Grace v.5.1.25 (http://plasma-gate.weizmann.ac.il/Grace/).
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This study forms part of the Transition into the Anthropocene project, funded by European Research Council Advanced grant no. EC-320691 and was further supported by the EUCLEIA project funded by the European Union’s Seventh Framework Programme (FP7/200713) under grant agreement no. 607085 and the EUPHEME ERA4CS grant no. 690462 and the Sigrist Foundation. G.H. was also supported by the Wolfson Foundation and the Royal Society as a Royal Society Wolfson Research Merit Award holder (WM130060). T.C. was also supported by the Northern Australian Climate Program, with funding provided by Meat and Livestock Australia, the Queensland Government and University of Southern Queensland. S.U. was also supported by the Horizon 2020 project EUCP.
The authors declare no competing interests.
Peer review information Nature Climate Change thanks Markus Donat, Deepti Singh and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
A comparison between observations from (left) Global Historical Climatology Network-Daily (GHCN-D) stations, and (right) Berkley Earth Surface Temperature (BEST) for summer heatwave conditions averaged over 1934. These include a, b heatwave frequency (HWF), c, d, heatwave duration (HWD), and e, f, heatwave amplitude (HWA). The heatwave metrics are calculated against a 1920–2012 reference period. The outlined GHCN-D stations are those where 1934 was the year with the most heatwave days, and the longest and hottest events.
a–c, Average over top 200 ranked experiments that simulate the most summer heatwave days over the central US in 1931 for a, WAH21930s, b, WAH2PD; compared to c, WAH22015. d–f, Average over the bottom ranked experiments for d, WAH21930s, e, WAH2PD; compared to f, WAH22015.
Return period of summer HWF for (a–c) 1934 and (d–f) 1936, for a, d, WAH21930s, b, e, WAH2PD, and c, f, WAH22015.
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Cowan, T., Undorf, S., Hegerl, G.C. et al. Present-day greenhouse gases could cause more frequent and longer Dust Bowl heatwaves. Nat. Clim. Chang. 10, 505–510 (2020). https://doi.org/10.1038/s41558-020-0771-7
Journal of Geophysical Research: Atmospheres (2020)
Nature Communications (2020)