Abstract Extreme warm events in the ocean, known as marine heatwaves (MHWs), can have severe and even irreversible impacts on marine ecosystems, underscoring the imperative need to quantify their anthropogenic changes based on observations. In situ temperature profiles over the past three decades reveal an outsized global increase (over 10% per decade) in intensity of MHWs, whereas both ocean reanalysis and climate simulations suggest changes should be an order of magnitude smaller. Here we show that the paradox arises primarily from an artificial trend of MHWs caused by sparse‐to‐abundant temporal evolution of amount of temperature profiles. Sparsity of temperature profiles in the early period systematically underestimates intensity of MHWs, while subsequent densification of temperature profiles alleviates such underestimation, introducing an artificial positive trend of MHW intensity. Our findings indicate that temporal heterogeneity of in situ ocean observing capacity may severely contaminate the genuine response of extreme events to global warming.