Exposure
We considered two exposure models, one based on a single variable of excess temperature exposure (cumulative degree heating weeks, the CTA model) and one based on a multivariate environmental index of climate exposure (CE) that integrates temperature, winds and water flow into a weighted metric of exposure. The CTA model is based on the concept of cumulative degree heating weeks or months, or the amount of excess temperatures above a summer baseline, and is the most commonly used metric to assess the probability of coral bleaching and the future state of coral reefs (Eakin et al. 2010; van Hooidonk et al. 2013; Donner & Carilli 2019). For each site, we extracted daily SST time series from the 5-km NOAA Coral Reef Watch version 3.1 products between 1985 and 2015, available from the NOAA website (https://coralreefwatch.noaa.gov/satellite/bleaching5km/index.php). Daily temperature measurements were used to calculate monthly hotspots, defined as positive SST anomalies referenced to the maximum of the monthly mean SST climatology (i.e. MMM climatology of Strong et al. 2004). We then calculated the Degree-Heating Months (DHM) as the sum of hotspots with monthly means ≥0oC. To derive the cumulative DHM product for each site, we summed cells with DHM > 0oC for each year over the 1985 – 2015 time series. We did not include the 2016 SST satellite measurements in this metric to maintain independence of the sensitivity metric established from underwater bleaching surveys during 2016. Cumulative thermal anomalies (CTA) between 1985-2015 ranged from 7.8 to 48.0 DHM across the 226 reef sites included in this study.
Climate Exposure (CE) is a multivariate metric that combines radiation variables to optimize predictions for coral bleaching (Maina et al. 2008; 2011). The model was developed using variables derived from historical SST (mean, variability, maximum, minimum), ocean current velocity in zonal and meridional direction, wind velocity (number of doldrum days and wind speed magnitude), and average satellite derived UV and photosynthetic active radiation measurements. These measurements were synthesized using fuzzy logic and weighted based on bleaching observation to produce values ranging between 0 (least exposure) and 1 (highest exposure). Overall, this multivariate metric of coral exposure has a strong relationship to previous compilations of coral bleaching data (Maina et al. 2008). Comparing the two exposure models showed that the distributions of CTA and CE values for the 226 sites found the CTA means were more frequently clustered around lower values (<0.50 normalized values) of the distribution and had a right skew. CE values were distributed more frequently towards the high ends (>0.50 normalized values) and had a left skew (Supplementary Fig. 1).