FIGURE 2 | The role of anthropogenic forcing in increasing the probability of MHW days. Each panel shows the globally aggregated probability ratio of a MHW day occurring in a given year from the historical (black), RCP4.5 (brown), and RCP8.5 (red) experiments relative to the probability of a MHW day occurring in the historicalNat experiment (calculated from the full 1850–2005 time series). The panels show the probability ratio but for only MHW days that are (A) at least category moderate, (B) at least category strong, (C) at least category severe, and (D) category extreme. The shaded regions indicate the minimum and maximum values based on individual ensemble members. The dashed line indicates a value of one (i.e., the probability in the historical or projected experiment is the same as in the historicalNat experiment).