Question iii): Role of circulation and greenhouse gas increase
At the peak of the early 20th century warming, record seasonal, monthly, and daily maximum temperatures were measured in many locations, including Eastern North America (DeGaetano and Allen, 2002) and Northern Europe in the 1930s, and Switzerland (Schaer et al., 2004) in the 1940s. TITAN used recently developed methods to attribute changes in the risk of these extreme events to external influences on climate (Pall et al., 2011; Otto et al., 2012). These high impact events have the potential to yield considerable insights on more recent extreme events.
The Dust Bowl heat waves in the Central US in the 1930s (Cowan et al., J Climate 2017) were affected by unusual climate variability, slightly enhanced by greenhouse gas increases even in the 1930s (Cowan et al., Nature Climate Change 2020), yet the biggest contributing factor is likely to have been devegetation due to agricultural practices and drought conditions (Cowan et al., Nature Comm. 2020). This should be a warning for potential threshold behaviour of heat waves related to surface feedbacks and vegetation in the future.