The increases in frequency and duration metrics translate to 30 additional marine heatwave days per year by the end of the 35-year period (p < 0.01; based on a linear trend) from a baseline level of about 25 days in the 1980s (Fig. 2).
This is where the paper's century-scale narrative reduces to a directly measurable quantity: globally averaged total marine heatwave days per year, from daily satellite SST under a fixed 90th-percentile, five-day detection rule. Unlike the abstract's 34/17/54% figures, which rest on a monthly proxy reconstruction of the pre-satellite era, this number is computed from observed daily fields, so any independent daily SST record can test it.
Paper annotation: The increases in frequency and duration metrics translate to 30 additional marine heatwave days per year by the end of the 35-year period (p < 0.01; based on a linear trend) from a baseline level of about 25 days in the 1980s (Fig. 2).