Increasing thermal stress for tropical coral reefs: 1871-2017

Abstract

Tropical corals live close to their upper thermal limit making them vulnerable to unusually warm summer sea temperatures. The resulting thermal stress can lead to breakdown of the coral-algal symbiosis, essential for the functioning of reefs, and cause coral bleaching. Mass coral bleaching is a modern phenomenon associated with increases in reef temperatures due to recent global warming. Widespread bleaching has typically occurred during El Niño events. We examine the historical level of stress for 100 coral reef locations with robust bleaching histories. The level of thermal stress (based on a degree heating month index, DHMI) at these locations during the 2015-2016 El Niño was unprecedented over the period 1871-2017 and exceeded that of the strong 1997-1998 El Niño. The DHMI was also 5 times the level of thermal stress associated with the 'pre-industrial', 1877-1878, El Niño. Coral reefs have, therefore, already shown their vulnerability to the modest (~0.92 °C) global warming that has occurred to date. Estimates of future levels of thermal stress suggest that even the optimistic 1.5 °C Paris Agreement target is insufficient to prevent more frequent mass bleaching events for the world's reefs. Effectively, reefs of the future will not be the same as those of the past.

Figure 1

(a) Annual average global land and sea temperature (HadCRUT4, red and blue bars) and average annual tropical coral reef sea surface temperature (HadISST1, black line) as anomalies from 1961–1990 average, 1880–2017, and (b) annual tropical sea surface temperature warming as percentage of global land and sea temperature warming, 1880–2017. Black symbols indicate 1-degree latitude by longitude boxes containing 1,670 tropical coral reefs.

Figure 2

Average annual degree heating month index (DHMI) for 100 coral reef locations, 1871–2017 (grey bars). Black bars mark years with Niño 3.4 index ≥1.0 °C.

Figure 3

Average annual degree heating month index (DHMI), 1871–2017, (grey bars) for (a) Indian Ocean and Middle East, (b) Southeast Asia, (c) Pacific Ocean and (d) Caribbean and Atlantic. Black bars mark years with Niño 3.4 index ≥1.0 °C.

Figure 4

Frequency distribution of annual maximum monthly SST for 100 reef locations for (a) 5 sub-periods since the late 19th century and (b) 4 strong El Niño events.

Figure 5

Average degree heating month index DHMI (blue) and global land and sea temperature (red) for 10-year periods, 1951–1960 through 2001–2010 and most recent 8-year period, 2011–2017.