As carbon dioxide rises in the atmosphere, climate change and ocean acidification are modifying important physical and chemical parameters in the oceans with resulting impacts on coral reef ecosystems. On one hand, rising CO₂, a major greenhouse gas, is warming the world’s oceans and causing corals to bleach, or expel their symbiotic algae (zooxanthellae) with both alarming frequency and severity. The frequent return of stressful temperatures has already resulted in major damage to many of the world’s coral reefs and is expected to continue in the foreseeable future. Warmer oceans also have contributed to a rise in coral infectious diseases. Both bleaching and infectious disease can result in coral mortality and threaten one of the most diverse ecosystems on Earth and the important ecosystem services they provide. Additionally, rising CO₂ is reducing the pH of the oceans and reducing the availability of carbonate ions needed by corals and many other marine organisms to build structural components like skeletons and shells. 
Results/Conclusions

Using a combination of satellite data, in situ observations, and models, this talk will explore the consequences of these two stressors on corals and the importance of limiting absolute levels of atmospheric CO₂—not just emissions. While thresholds vary among species and locations, it is clear that corals around the world are already experiencing anomalous temperatures that are too high, too often, and that warming is exceeding the rate at which corals can adapt. This is despite a complex adaptive capacity that involves both the coral host and the zooxanthellae, including changes in the relative abundance of the latter in their coral hosts. The safe upper limit for atmospheric CO₂ is probably somewhere below 350ppm, a level we passed decades ago, and for temperature is a sustained global temperature increase of less than 1.5°C above pre-industrial levels. At somewhere between 450-500ppm coral reefs are probably going to be unable to grow quickly enough to exceed natural forces of erosion and dissolution, resulting in net reef loss. As we are likely to exceed 1.5°C warming soon as well, local actions to protect reefs are even more important to sustain reef organisms until we can restore more favorable environmental conditions.