Over this century, coral reefs will run the gauntlet of climate change, as marine heatwaves (MHWs) become more intense and frequent, and ocean acidification (OA) progresses. However, we still lack a quantitative assessment of how, and to what degree, OA will moderate the responses of corals to MHWs as they intensify throughout this century. Here, we first projected future MHW intensities for tropical regions under three future greenhouse gas emissions scenario (representative concentration pathways, RCP2.6, RCP4.5 and RCP8.5) for the near‐term (2021–2040), mid‐century (2041–2060) and late‐century (2081–2100). We then combined these MHW intensity projections with a global data set of 1,788 experiments to assess coral attribute performance and survival under the three emissions scenarios for the near‐term, mid‐century and late‐century in the presence and absence of OA. Although warming and OA had predominately additive impacts on the coral responses, the contribution of OA in affecting most coral attributes was minor relative to the dominant role of intensifying MHWs. However, the addition of OA led to greater decreases in photosynthesis and survival under intermediate and unrestricted emissions scenario for the mid‐ and late‐century than if intensifying MHWs were considered as the only driver. These results show that role of OA in modulating coral responses to intensifying MHWs depended on the focal coral attribute and extremity of the scenario examined. Specifically, intensifying MHWs and OA will cause increasing instances of coral bleaching and substantial declines in coral productivity, calcification and survival within the next two decades under the low and intermediate emissions scenario. These projections suggest that corals must rapidly adapt or acclimatize to projected ocean conditions to persist, which is far more likely under a low emissions scenario and with increasing efforts to manage reefs to enhance resilience.

Climate change impact syntheses, such as those by the Intergovernmental Panel on Climate Change, consistently assert that limiting global warming to 1.5 °C is unlikely to safeguard most of the world’s coral reefs. This prognosis is primarily based on a small subset of available models that apply similar ‘excess heat’ threshold methodologies. Our systematic review of 79 articles projecting coral reef responses to climate change revealed five main methods. ‘Excess heat’ models constituted one third (32%) of all studies but attracted a disproportionate share (68%) of citations in the field. Most methods relied on deterministic cause-and-effect rules rather than probabilistic relationships, impeding the field’s ability to estimate uncertainty. To synthesize the available projections, we aimed to identify models with comparable outputs. However, divergent choices in model outputs and scenarios limited the analysis to a fraction of available studies. We found substantial discrepancies in the projected impacts, indicating that the subset of articles serving as a basis for climate change syntheses may project more severe consequences than other studies and methodologies. Drawing on insights from other fields, we propose methods to incorporate uncertainty into deterministic modeling approaches and propose a multi-model ensemble approach to generating probabilistic projections for coral reef futures.

Responses of cnidarian-Symbiodiniaceae associations to warming are determined, in part, by high-frequency temperature variability. Yet, the role of such variability in determining specific maximum temperature thresholds of cnidarian holobionts (the ecological units comprised of cnidarian hosts and associated microorganisms, including Symbiodiniaceae) remains untested. Here we contrasted the thermal resilience (that is the ability to resist stress) of a model symbiotic cnidarian from the Red Sea (jellyfish of the genus Cassiopea) under stable and diel oscillating temperature conditions that provide night-time reprieves from daily maximum temperatures. Holobionts were subjected to two thermal trajectories; one that increased but plateaued at 2°C below identified bleaching thresholds and another that increased incrementally until holobionts bleached. We used behavior, growth, photochemical efficiency, Symbiodiniaceae (symbiont) cell density, and total chlorophyll cell content to characterize thermal resilience and examined Symbiodiniaceae community composition responses at 1 and 13 days of exposure, and post-bleaching. Lower night-time temperatures, resulting in lower daily mean temperatures, allowed holobionts to withstand daily maximum temperatures close to their bleaching thresholds for two extra days than those under stable maximum temperature conditions. Lower night-time temperatures increased the bleaching threshold of the holobionts, whereby holobionts exposed to night-time thermal reprieves tolerated a more extreme daily mean temperature of 40.6°C and reached a daily thermal maxima 4°C higher than those under stable temperature conditions. However, post-bleaching observations indicate that night-time temperature reprieves did not prevent symbiont cell or pigment loss. Symbiodiniaceae communities were unaffected by lower night-time temperatures and no directional changes indicative of symbiont shuffling/selection of thermally tolerant lineages were observed. We show that stable experimental treatments may fail to accurately identify maximum thermal thresholds of non-calcifying cnidarians and limit their relevance to in situ environments that are often characterized by high levels of temperature fluctuations.