Internal tide-driven cooling may buffer some shallow-water coral reefs from future global warming

Abstract:

Observations show ocean temperatures are rising due to climate change, resulting in a 5-fold increase in the incidence of regional-scale coral bleaching events since the 1980s. Analyses based on global climate models forecast bleaching will become annual events for most of the world’s shallow-water coral reefs within 30-50 years. Internal waves at tidal frequencies can regularly flush shallow-water reefs with cooler waters, buffering the thermal stress from rising sea-surface temperatures. Here we present the first global maps of these effects these processes have on bleaching projections for three IPCC-AR5 emissions scenarios. Incorporating semidiurnal temperature fluctuations into the projected water temperatures at 10-, 20-, and 30-m depths for shallow coral reefs creates a delay in the timing of annual severe bleaching ≥10 year (≥20 year) for 38% (9%), 15% (1%), and 1% (0%) of sites for the low, moderate, and high emission scenarios, respectively; regional averages can reach twice as high. The cooling effects are greatest later in 21st century for the low and moderate emission scenarios, and around the middle 21st century for the highest emission scenario. Our results demonstrate that internal wave-driven cooling effects could delay thermally induced bleaching for many shallow-water corals, providing refugia where these effects are pronounced. Identification of such areas could be a factor for the selection of area for coral reef conservation, restoration, and increased protection.

FIGURE CAPTION. Map showing coral reef locations and the projected delay (∆) in the Year of Annual Severe Bleaching (YASB) for different Representative Concentration Pathway (RCP) scenarios. (A) RCP4.5. (B) RCP6.0. (C) RCP8.5. ∆YASB is the difference between YASB for sea-surface temperature (SST) projections and YASB with site-specific semidiurnal temperature fluctuations for coral reef sites at 20-m depths. Sites that are outside the range of the global, mode-1 internal tide observations and sites that never reach YASB by 2090 are omitted. For many sites, ∆YASB attributed to semidiurnal temperature fluctuations is small (<10 yr); however, for some reefs, the delay can be on the order of decades. The thermal benefit provided by the subsurface temperature fluctuations decreases for higher emission scenarios such as RCP8.5.