Comment on “Canada's Fourth Generation of Homogenized Surface Air Temperature and Its Trends for 1948–2023” by Wan et al. (2025)

The Fourth-Generation homogenized surface air temperature dataset for Canada (V4) produced by Wan et al. (Citation2025) exhibits differences from the updated Third Generation (V3.1) dataset and other international datasets that appear to be the result of methodological deficiencies. This short comment compares the V4 dataset with V3.1 and six international datasets produced using different methods and source data collections, including the ERA5 reanalysis, illustrating that warming in the V4 dataset is substantially greater than in other datasets. The primary causes of the higher warming appear to be related to a combination of the homogenization methodology choices that depart from established practices and the presence of segments of estimated data, mostly prior to 1970, that are systematically different from, but were not made homogeneous to, the observed data series.