Abstract We examine how the structural compactness of North Atlantic tropical cyclone (TC) seeds responds to warming in ERA5 reanalysis and CAM5 simulations. In ERA5, comparing 1985–1999 with 2000–2014, developing seeds in the later, warmer period become slightly more compact and exhibit stronger mid‐level ascent and low‐level convergence, whereas non‐developing seeds change much less. In the historical CAM5 experiment, by contrast, seed compactness and secondary circulation change little across the same two periods. Under RCP4.5 and RCP8.5 forcing, CAM5 produces fewer developing seeds, more non‐developing seeds, and lower survival rates with stronger warming. However, developing seeds become less compact even as their secondary circulation strengthens, while non‐developing seeds show stronger secondary circulation but little compactness change. Compactness remains a useful discriminator of seed development, but its climate response differs substantially between ERA5 and CAM5.