Field Guide to: Climate's Impact on Coral & Reef Systems

  • Introduction
  • Warmer Oceans
  • Ocean Uptake
  • Impacts of Greenhouse Gases
  • Results

Click to enlarge. A healthy coral reef and aquatic community. (Photo Credit: Florida Keys National Marine Sanctuary)

Click to enlarge. An unhealthy coral reef showing signs of coral bleaching and algae domination. (Photo Credit: Professor Terry Hughes, James Cook University, Australia)

Coral Reefs and Climate Change

Home to a diverse variety of plant and animal species, coral reefs are the rainforests of the sea. More than simply tourist attractions or economic drivers that add $30 billion to local economies, these formations also protect nearby shorelines from storm surges and coastal erosion, and create habitats for other important marine ecosystems such as mangroves and seagrass beds.

But, because coral reefs are sensitive to environmental perturbations, reef degradation is increasingly common worldwide. Slight changes in incoming sunlight, water temperature, salinity, and acidity affect coral’s ability to thrive. Human impacts, both indirect (greenhouse gas emissions) and direct (sewage or sediment runoff flowing into the ocean, or blast fishing using dynamite) stress these ecosystems and may threaten the long-term survival of many coral species.

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Click to enlarge. 27% of the world's reefs are considered dead. Half of this is due to climate change. (figure: Joannie Kleypas, NCAR)

Click to enlarge. A bleached and dying coral reef.

Click to enlarge. Coral bleaching is a “canary in the coal mine”. (figure: Joannie Kleypas, NCAR)

Greenhouse Gases and Atmospheric Affects on the Ocean

Global climate change and the buildup of greenhouse gases in the atmosphere affect oceans. Not only are average sea surface temperatures increasing as a result, but more CO2 is dissolving in the oceans, causing ocean acidification.

Sea surface temperatures in tropical and sub-tropical oceans have increased since the 1950s. Warmer oceans are causing a dramatic increase in coral “bleaching.” In response to the stress of warmer sea surface temperatures, coral will jettison the single-celled microalgae that help it produce adequate nutrients, and give it its striking colors. The coral tissues become transparent, so that the underlying white skeleton becomes visible, leaving the coral with a bleached appearance. Even when corals do not bleach, warmer temperatures are thought to be a major factor in the increased incidence of coral diseases.

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Click to enlarge. As greenhouse gases rise, they push down the levels of calcium carbonate in the oceans, resulting in a slower rate of coral skeletal growth.

Click to enlarge. Slowing down the 'carbonate factory' adversely effects ecosystems that are dependent on the reefs. (figure: Joannie Kleypas, NCAR).

Ocean's Role in CO2 Uptake

About half of the CO2 released to the atmosphere by human activities has been taken up by the ocean. While this reduces greenhouse warming, the addition of CO2 to seawater creates more carbonic acid, which lowers the ocean’s pH over time.

Indeed, long-term measurements of the ocean’s chemistry show decreased ocean pH levels. Although these changes are slight, continued ocean acidification will dramatically reduce the ability of corals and other organisms to produce their calcium carbonate skeletons and shells. Returning oceans to pre-industrial pH levels would require more than a human lifetime of reduced emissions. Unless reversed, today’s increases in atmospheric CO2 will affect coral reefs for centuries.

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Click to enlarge. The natural distribution of coral is affected by ocean temperature, light, clear and shallow water, salinity, ocean chemistry and available nutrients. (figure: Joannie Kleypas, NCAR)

Click to enlarge. The effects of temperature on coral reef health. (figure: Joannie Kleypas, NCAR)

Impacts on Coral

Ocean temperature and CO2 concentrations vary by location, as do the resilience of different coral species and the degree to which local, human activities impact coral reefs. NCAR’s Joanie Kleypas focuses her attention on better understanding this problem. A marine ecologist and geologist and avid scuba diver, Joanie studies climate and human impacts in coral-reef regions, identifying how these factors interact and exacerbate each other, and how different coral species in different locales react to similar stressors.

Using climate models and geological evidence—cores from coral reefs—Joanie recreates environmental conditions that control coral reef growth to better understand how today’s trends in sea water chemistry and temperature will affect reefs of the future. She is also searching for ways to mitigate and manage local and regional non-climate stressors by identifying those reefs least vulnerable to climate change and most likely to benefit from conservation measures.

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Click to enlarge. Projected changes in climate will drive temperature and seawater chemistry to levels outside the envelope of modern reef experience. Researchers are trying to determine what the possible long-range changes to our oceans might be, whether ecosystems can adapt to the predicted changes, and why certain reef systems are adapting to current changes better than others. This will help us to understand how our oceans, and the whole earth system, may change. (figure: Joannie Kleypas, NCAR)


Conservationists, policy makers and scientists agree: finding solutions to local-scale environmental problems (sedimentation, chemical spills, or decreased water salinity due to, for example, flooding) make an obvious starting point for improving the health of coral colonies. As a result, decision makers rely on work like Joanie’s, and that of her colleagues, to create strategies for effective management of these ecosystems. Understanding the impacts and interactions of ocean, atmosphere, land, human influences, and the behavior of the coral itself all factor into the equation. Using this research, national and international organizations are steering action toward environmental adaptation and coral reef conservation.

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