Based on global temperature, the early 20th century warming is attributed to a combination of anthropogenic influences, reduced volcanism, and internal climate variability. However, it also featured extreme events (e.g., Dust Bowl decade in the 1930s) and analysing such events is one way to test methods for event attribution. The 1930s Dust Bowl devastated North America's central plains through a combination of severe droughts and deadly dust storms, as well as record-breaking summer heat waves and temperature highs that are only now being surpassed (https://www.noaa.gov/news/summer-2018-ranked-4th-hottest-on-record-for-us).

Here we determine to what extent the Dust Bowl mega-heat waves were ‘black swan’ events, triggered by oceanic and atmospheric conditions, influenced by greenhouse gases, and/or caused by land degradation. We show that the exceptional heat waves in many coupled climate models occur coincident with warm North Atlantic sea surface temperature (SST) anomalies, as observed in the 1930s. These warm SST anomalies favour dry conditions across the central United States in spring which in turn yield summer heat waves that are hotter, earlier and more frequent. A large ensemble of regional climate model experiments suggest that, even as early as 1936 (the warmest contiguous United States summer on record), greenhouse gas increases likely contributed to more frequent and intense heat waves along the United States east coast. Both of these factors are important but cannot explain the exceptional nature of the 1930s heat waves, which exceed the envelope of historical simulations in coupled climate models. Simulations with an atmospheric model suggest that de-vegetation, which occurred due to a combination of the drought with rapid land development at the time, has the potential to exacerbate heat wave frequency and magnitude. This study highlights the potential for vegetation feedbacks to create tipping points for the most extreme heat waves in a warmer world.