Marine heatwaves in global sea surface temperature records since 1850

The adverse impacts of marine heatwaves (MHWs) on marine ecosystems and human activities are well-documented, yet observational studies tend to largely rely on recent records. Long-term records of MHWs can put the recent increase in frequency and intensity of MHWs in the context of past variability. We used long-term monthly sea surface temperature (SST) data and night marine air temperatures to characterise past MHW activity. A persistent increase in the global extent of MHWs is demonstrated, beginning around 1970. The average annual MHW extent post-2010 is estimated to be increased at least four fold compared to that pre-1970. A strong correlation between spatial variance of recorded average monthly SSTs and the average inverse number of monthly observations implies both frequency and amplitude of MHWs is overestimated when the number of monthly observations is low. Nevertheless, many identified early MHWs appear genuine, such as a multi-month event in the North Atlantic in 1851–1852. MHWs are also affected by poorer sampling during the world wars. The most extensive MHW years globally coincide with El Niño years, and MHW extent in the North Atlantic is correlated with the Atlantic Multidecadal Oscillation.