Augmented In Situ Ocean Observing Capacity Could Cause an Artificial Intensification of Extreme Warm Water Events Globally

One of the devastating effects of global warming is potentially more frequent and stronger extreme events. In particular, 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 during the same period suggest that the changes should be an order of magnitude smaller. Here we show that the paradox arises primarily from an artificial trend of MHWs caused by increasing amount of temperature profiles with time. Sparsity of temperature profiles in the early period systematically underestimates the intensity of MHWs, while subsequent densification of temperature profiles alleviates such underestimation, introducing an artificial positive trend of MHW intensity. This artificial trend is more dominant in historically observation-sparse regions like the Southern Ocean. Our findings indicate that the augmented in situ ocean observing capacity with time may severely contaminate the genuine response of extreme events to the global warming so that careful handling of these observations is essential to reach valid conclusions.