Global warming refers to the increase in the temperature of the Earth's atmosphere due to increases in CO2 and other pollutants, collectively referred to as greenhouse gases. This increase in the concentration of greenhouse gases is leading to the climate changes experienced around the world. The warming of the Earth's atmosphere has caused the temperature of water in the oceans and seas to rise. The coral reef ecosystem is highly sensitive to even the smallest changes in temperature. When sea temperatures increase, corals exceed their thermal threshold and enter a state of stress. When they are stressed, the symbiotic zooxanthellae that live inside them leave. Zooxanthellae and coral share a symbiotic relationship, meaning they both benefit from each other's presence. When the zooxanthellae leave, they take some of the coral's energy source with them. Furthermore, when this expulsion of the zooxanthellae occurs, the coral loses its color, hence the term coral bleaching. Coral bleaching does not always cause the death of corals, but it does make them more susceptible to disease and UV rays. Global warming is also causing ocean acidification, which weakens the structure of corals making them easy targets for bacteria and disease. Oceans absorb CO2 from the atmosphere which in turn lowers the pH of the water and reduces the rate of calcification of reef-building corals (NOAA). Coral bleaching is a global problem as Earth's sea temperatures continue to rise. Coral reefs located along the equator, such as the Belize Barrier Reef, the Florida Barrier Reef, the Hawaiian Barrier Reefs and the Great Barrier Reef in Australia are all affected. Organizations such as Coral Watch and Save the Reef have directed their actions towards monitoring and preserving death over the next century. Coral reefs are a highly sensitive ecosystem. Even the smallest changes in the environment, such as pH fluxes and temperature increases, cause catastrophic events including coral bleaching. The Belize Barrier Reef and the Great Barrier Reef are two coral reefs that have experienced the most severe coral bleaching. This is harmful not only to the fish that depend on the coral, but also to the economies of surrounding countries that thrive on various water-related tourist attractions. With the increasing concentration of greenhouse gases, this trend only appears to be increasing, unless something is done to reverse it. Corals may be able to adapt to these stresses over time, but the real solution comes from reducing human production of carbon and other greenhouse gases. If this is accomplished, the environment may be able to recover and thrive.