Climate Change

Close-up image of coral polyps that have bleached and are white

These coral polpys have lost their symbiotic zooxanthellae due to heat stress, causing them to appear white. Photo credit: Emma Hickerson/ Flower Garden Banks National Marine Sanctuary

Climate change impacts have been identified as one of the greatest global threats to coral reef ecosystems. As temperature rise, mass bleaching, and infectious disease outbreaks are likely to become more frequent. Additionally, carbon dioxide (CO₂) absorbed into the ocean from the atmosphere has already begun to reduce calcification rates in reef-building and reef-associated organisms by altering sea water chemistry through decreases in pH (ocean acidification). In the long term, failure to address carbon emissions and the resultant impacts of rising temperatures and ocean acidification could make many other coral ecosystem management efforts futile.

Climate change and ocean acidification have been identified by many groups as the most important threat to coral reefs on a global basis. In 2007, the Intergovernmental Panel on Climate Change (IPCC) noted that the evidence is now "unequivocal" that the earth's atmosphere and oceans are warming. They concluded that these changes are primarily due to anthropogenic greenhouse gases (i.e.those derived from human activities), especially the accelerating increase in emissions of CO₂.

partially bleached Acropora corals in Tumon Bay Marine Preserve in Guam

In 2007, these Acropora corals in the Tumon Bay Marine Preserve in Guam bleached, likely due to higher-than-normal water temperatures caused by global climate change. Photo credit: Dave Burdick

While reducing CO₂ and other greenhouse gas emissions is vital to stabilize the climate in the long term, excess CO₂ already in the atmosphere has changed and will continue to change global climate throughout the next century. Global ocean temperature has risen by 0.74°C (1.3°F) since the late 19^th century causing more frequent and severe bleaching of corals around the world. At the current increasing rate of greenhouse gas emissions, a temperature rise of up to 4.0°C (7.2°F) this century is a distinct possibility. These changes have already had harmful impacts on coral reef ecosystems and will continue to affect coral reef ecosystems globally over the coming century. At the same time, the ocean absorbs approximately one-third of the additional CO₂ generated every year by human activities, making the ocean more acidic. The resulting change to ocean chemistry has important consequences for corals and other marine life, especially other important reef builders. Warming seas and ocean acidification are already affecting reefs by causing mass coral bleaching events and slowing the growth of coral skeletons. Bleaching and infectious disease outbreaks are likely to be more frequent and severe as temperatures rise, increasing coral mortality. Climate changes will have other impacts on marine systems such as sea level rise; altered frequency, intensity, and distribution of tropical storms; altered ocean circulation; and others. All of these impacts will combine, often synergistically, to eliminate important ecosystem function and reduce global biodiversity.

For more information:

NOAA Coral Reef Conservation Program Goals & Objectives 2010-2015
An Introduction to Ocean Acidification / NOAA's Coral Reef Watch
Coral Bleaching Outlook / NOAA's Coral Reef Watch
Climate Change Information Sheet / UN Framework Convention on Climate Change
Global Climate Change and Coral Reefs / Status of Coral Reefs of the World: 2008 (pdf, 1.0 mb)
ReefBase Threats - Climate / Global Coral Reef Monitoring Network

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