  We introduce a new methodology to study marine heat waves, extreme events in
the sea surface temperature (SST) of the global ocean. Motivated by previously
large and impactful marine heat waves and by theoretical expectation that the
dominant heating processes coherently affect large regions of the ocean, we
introduce a methodology from computer vision to construct marine heat wave
systems (MWHSs) -- the collation of SST extrema in dimensions of area and time.
We identify 649,475 MHWSs in the 37 year period (1983-2019) of daily SST
records and find that the duration t_dur (days), maximum area A_max (km$^2$),
and total ``volume'' N_vox (days km$^2$) for the majority of MHWSs are
well-described by power-law distributions: t_dur^(-3), A_max^(-2) and
N_vox^(-2). These characteristics confirm SST extrema exhibit strong spatial
coherence that define the formation and evolution of marine heat waves.
Furthermore, the most severe MHWSs deviate from these power-laws and are the
dominant manifestation of marine heat waves: extrema in ocean heating are
driven by the ~200 systems with largest area and duration. We further
demonstrate that the previously purported rise in the incidence of marine heat
wave events over the past decade is only significant in these severe systems. A
change point analysis reveals a rapid increase in days under a severe MHW in
most regions of the global ocean over the period of 2000-2005. Understanding
the origin and impacts of marine heat waves in the current and future ocean,
therefore, should focus on the production and evolution of the largest-scale
and longest-duration heating phenomena.
