decade of the satellite record, 27 large MHWs
occurred with an average duration of 32 days,
an average peak temperature anomaly of 4.8°C,
and an average cumulative intensity of 11,000°C
days km^2. By contrast, 172 large MHWs oc-
curred in the past decade, with an average
duration of 48 days, an average peak temper-
ature anomaly of 5.5°C, and an average cumu-
lative intensity of 14,000°C days km^2.
For most of these large MHWs, little is
known about their individual environmental
consequences. In this study, we focus on all
recent, large MHWs that have had a major
documented impact in the region in which
they occurred ( 10 ) and that have extended
across at least 1 million km^2 —seven MHWs in
total (Fig. 1, B, D, and F; and Table 1). These
seven heatwaves (Table 1) include the North-east Pacific 2013 to 2015 MHW, which is the
largest and longest heatwave that occurred
between 1981 to 2017 and had major ecolog-
ical consequences ( 11 , 12 ); the Northwest At-
lantic 2012 heatwave that had substantial
ramifications for fisheries ( 17 ); large heat-
waves around Australia, which caused long-
lasting, major losses in corals, kelp, and sea
grass meadows ( 1 , 30 , 31 ); the Southwest1622 25 SEPTEMBER 2020•VOL 369 ISSUE 6511 sciencemag.org SCIENCE
123456701002003001985 1990 1995 2000 2005 2010 201531245670 10 20 304050607080 90 >100
Duration [days]Duration [days]1234567 8 >91985 1990 1995 2000 2005 2010 20150481216Peak temperature anomaly [°C]Peak temperature anomaly [°C]A
C
BD6Cumulative intensity [°C days]E F
1985 1990 1995 2000 2005 2010 2015101010Cumulative intensity [°C days km²] 103456Fig. 1. Characteristics of MHWs during 1981 to 2017.(A,C, andE) Duration (A), peak temperature anomaly (C), and cumulative intensity (E) of the 300 largest
heatwaves that occurred globally between September 1981 and December 2017. Each bar represents a spatiotemporal heatwave, which is defined as a
spatiotemporally contiguous area where each grid cell exhibits daily temperature anomalies above the 99.5th percentile (materials and methods). The bars
corresponding to the numbered heatwaves on the maps are marked in red. (B,D, andF) The maps show the duration (B), peak temperature anomaly (D), and
the cumulative intensity (F) of the seven prominent recent heatwaves that are analyzed in this study.
RESEARCH | REPORTS