Coral reefs are facing new challenges that could impact their health. Researchers recently launched the International "Year of the Reef" in an effort to highlight the issues regarding these reefs. Climate warming has become an increasing threat to reefs because corals have an upper limit of temperatures they can tolerate. Carbon dioxide in the ocean water is increasing the acidity of the water and making it harder for corals to form their shells. Researchers also see increasing amounts of blue-green algae, which may be killing coral.