A marine heatwave is defined as periods of prolonged high sea surface temperatures . More specifically, this was defined in a recent study as sea surface temperatures (SST) in the 90th percentile based on a 30-year average for five consecutive days for a given area of the ocean. MHWs can have a devastating impact on ocean ecosystems, resulting in coral bleaching, loss of kelp forests, marine deaths due to heat stress, reduced chlorophyll levels, and shifts in species habitat ranges. The rise in greenhouse gases is considered to be a major attributor to the subsequent rise in sea surface temperatures .
Researchers compiled temperature data from satellite observations, and both monthly and daily on site temperature readings to look at frequency and duration of marine heatwaves (MHWs) in order to understand trends and patterns. Researchers used in situ temperature recordings dating back to 1900 and satellite imagery dating back to 1982 to look at global trends. They found that between 1925 to 2016, “global average marine heatwave frequency and duration increased by 34% and 17%.”
 Hobday, A. J., Alexander, L. V., Perkins, S. E., Smale, D. A., Straub, S. C., Oliver, E. C., … & Holbrook, N. J. (2016). A hierarchical approach to defining marine heatwaves. Progress in Oceanography, 141, 227-238.
 Bindoff, N. L., Stott, P. A., AchutaRao, K. M., Allen, M. R., Gillett, N., Gutzler, D., … & Mokhov, I. I. (2013). Detection and attribution of climate change: from global to regional.