The Effects of Climate Change on Coral Reef Health

Originally published at

By Caitlin Martin

Coral reefs are one of the most diverse ecosystems on this planet. They are home to numerous species of marine life and offer a plethora of benefits both to natural ecosystems and to the human population. Coral reefs bring in enormous funds to coastal countries through tourism, fishing, and discoveries of new biochemicals and drugs (Hoegh-Guldberg 1999). Additionally, they provide natural coastal protection and building materials (Hoegh-Guldberg 1999). However, coral reefs are experiencing massive die-outs all around the world. At first, many thought the biggest threats to coral reef health were direct anthropogenic effects such as water pollution and sedimentation, but now it is clear that the problem is much larger in scale (Wilkinson 2011). 50-70% of coral reefs are directly affected by anthropogenic global climate change (Hoegh-Guldberg 1999). Rising global temperatures, increasing oceanic CO2, and other consequences of climate change are all affecting coral reef health in a negative way. This blog explores some of the most pressing issues regarding climate change and coral reef health, with a special focus on the coral reefs in Guam and Palau.

NOAA thermal stress map (Sep-Dec 2010)

NOAA thermal stress map (Sep-Dec 2010)

According to the Intergovernmental Panel on Climate Change’s (IPCC) 2007 report: “There is high confidence, based on substantial new evidence, that observed changes in marine and freshwater biological systems are associated with rising water temperatures, as well as related changes in ice cover, salinity, oxygen levels and circulation” (Bernstein et al. 2007). These changes are a result of climate change and they are affecting marine organisms in a negative way. However, the report follows this statement with: “While there is increasing evidence of climate change impacts on coral reefs, separating the impacts of climate-related stresses from other stresses (e.g., over- fishing and pollution) is difficult” (Bernstein et al. 2007). This last sentence was added because even though changes to coral reef health appear to largely be a result of climate change, there are additional variables that may affect their health, and need to be studied further. This does not eliminate climate change as the main cause of the problems with coral reef health, it merely states that there are other factors, as well. The phenomenon of anthropogenic climate change is a rapid and relatively recent event, so more research needs to be conducted overall to fully understand the changes it is causing.

Even though climate change needs to be studied further, significant evidence exists that it has a notable impact on coral reefs. The IPCC compiled a list of these effects in its 2007 report: coastal conditions of small islands will deteriorate because of coral bleaching and beach erosion, intense tropical cyclones will likely increase, damaging coral reefs, and certain regions of the planet are “likely” to be affected by climate change; coral reefs are one of these regions (Bernstein et al. 2007). Coral reefs are likely to be affected by these changes because of their sensitive nature. They are sensitive to thermal stress and their ability to adapt to changing conditions is low (Bernstein et al. 2007). Research shows that an increase of 1 to 3°C in sea surface temperature will cause widespread mortality and more frequent bleaching (Bernstein et al. 2007). These kinds of observations about corals and their relationship with temperature have been seen all around the world as water temperatures increase. Many of the reefs in Guam have declined in health during the past 40 years, with live coral coverage about 50% in the 1960s and less than 25% in the 1990s (Puglise et al. 2007). This pattern of bleaching and coral reef die-offs is projected to become more common as global temperatures continue to increase. With global temperatures rising, coral reefs are feeling the change.

Bleached coral at a site in Palau monitored by the Palau International Coral Reef Center. The number and extent of bleaching events have been increasing rapidly (Jane Thomas, Integration and Application Network, University of Maryland Center for Environmental Science (

Bleached coral at a site in Palau monitored by the Palau International Coral Reef Center. The number and extent of bleaching events have been increasing rapidly (Jane Thomas, Integration and Application Network, University of Maryland Center for Environmental Science (

Another major concern regarding climate change and coral reefs is the increasing levels of CO2 in oceans that cause ocean acidification. This process impacts the ability of corals to make calcium carbonate (Pandolfi et al. 2011). Numerous studies have been conducted to investigate the impact of increasing CO2 levels on corals, and they all arrive at the conclusion that high CO2 levels make it difficult for marine organisms to create their calcium carbonate shells. For example, a study in the Red Sea reef showed that, globally, when CO2 concentrations are at 560 ppm, corals will dissolve instead of accumulate calcium carbonate, resulting in massive die-offs (Pandolfi et al. 2011). In another study, the experiment exposed calcifying algae to four different temperatures and four different CO2 levels; the greater amounts of CO2 caused significant decline in photosynthetic efficiency, ability to accumulate calcium carbonate, and growth in all species (Sinutok 2011). The results also showed that after five weeks, in the 34 °C trial under all CO2 levels, all species died (Sinutok 2011). Therefore, increased oceanic CO2 levels have detrimental effects on coral reefs. Furthermore, linking increasing CO2 levels with climate change and global warming shows the cause of the processes that are affecting the health of coral reef ecosystems. As long as greenhouse gases continue to be emitted in large amounts, CO2 concentrations will increase in oceans and corals will continue to live in a chemically unhealthy ecosystem. If CO2 emissions continue at the rate they are emitted now, there will most certainly be reductions in the extent and diversity of coral reefs in the future (Wilkinson 1999).

Specifically relating to the coral reefs of Guam and Palau, many of the problems associated with climate change are evident. According to Paul Collins, a biologist in Palau, there have been numerous small bleaching events in the coral reefs in Palau that extend into waters as deep as 70 meters (P. Collins, pers. comm.). He says this bleaching did not exist six years ago when he first arrived at the island, showing the rapid progression of the issue (P. Collins, pers. comm.). Collins specifically monitors the Ngederrak Reef in Palau, which has experienced dramatic change in the last ten years.  His remote-sensing research shows there is “habitat changing on the barrier reef side of Ngederrak”, which may be due to climate change, more specifically rising sea levels, or stronger wave surges (P. Collins 2012). However, one of the most significant comments Collins offered is that since these changes are relatively recent, he can make educated guesses as to their causes, but it is difficult at this time to provide quantitative data (P. Collins, pers. comm.). Much more research needs to be conducted to provide specific evidence that climate change is the cause in this specific regional problem. This is the same comment that was brought up in the IPCC report; while there is evidence pointing to climate change as the cause of problems with coral reef health, there are still other factors that need to be studied. Additionally, claims can be made for change to coral reef health globally, but each reef deals with regional problems that are specific to the local conditions it is exposed to. Brent Tibbatts, a fish and wildlife biologist in Guam, also agrees that he has not seen any changes in Guam that can be directly attributed to climate change yet (B. Tibbatts, pers. comm.). Overall, the impression is that climate change is most likely causing the degradation of coral reefs, but it cannot be proven for sure with the data available.

Tibbatts has also noticed that weather patterns are continuing to change in Guam, increasing the amount of rainfall. The influx of freshwater increases sedimentation and changes the salinity of the ocean water (B. Tibbatts, pers. comm.). Corals need water that has a specific salinity and low turbidity, and these changes in weather create an environment that does not support healthy coral. As weather patterns continue to be more extreme and frequent in nature, these types of living conditions will become more common for corals. Since corals are not able to adapt easily, the stressful environment may be too much for them to handle.

Recently, 55 different species of Pacific corals were recommended for inclusion as either threatened and/or endangered species (J. Biggs, pers. comm.) with ocean warming and ocean acidification being identified as one of the primary mechanisms for their demise (NOAA 2013). This shows that the effects of climate change are a large concern for corals, both on the local and on the global scale. These effects are so significant that corals are moving toward being threatened. Unless something is done to change the causes of the ocean warming and acidification, more corals will be threatened. Even though there are other variables that need to be tested as to their connection with ocean warming and ocean acidification, it appears ever more clear that climate change is one of the earliest contributing factors.  Whatever the causes of the changes are, they are negatively affecting corals and at alarming rates.

As stated earlier, coral reefs are an essential ecosystem to this planet and to lose them would be detrimental on many levels. Many signs point to climate change as the key cause of problems associated with coral reefs, but additional data need to be collected.  More research needs to be conducted to discover the exact factors that cause coral bleaching and problems making calcium carbonate shells. Once these facts are established, most likely, efforts to help save coral reefs will increase. Since corals offer such a wide range of benefits to humans, widespread efforts should be made to save them and to do so, action needs to be taken immediately. One of the most significant changes would be decreasing CO2 emissions.  This targets the root of the entire problem and would significantly help both the warming problem and the calcium carbonate problem. Climate change is a widespread issue and will not be solved with one simple solution, but with the rate at which coral reefs are dying, some sort of change needs to be made.

photo: David Ginsburg

Caitlin Martin. Photo: David Ginsburg

Author Bio: Caitlin Martin is a freshman at the University of Southern California, where she is majoring in environmental studies. She plans to pursue a career in environmental law or environmental policy, specifically focusing on climate change (Photo: David Ginsburg)


Berstein L (2007) An Assessment of the Intergovernmental Panel on Climate Change. Climate Change 2007: Synthesis Report 1: 1-52

Biggs J (2013) Personal Communication

Collins P (2012) Ngederrak Site Summary and Stakeholder Matrix. 1-15,d.cGE

Collins P (2013) Personal Communication

Hoegh-Guldberg O (1999) Climate change, coral bleaching and the future of the world’s coral reefs. Marine and Freshwater Research 50(8): 839-866

Pandolfi J, Connolly S, Marshall D, Cohen A (2011) Projecting Coral Reef Futures Under Global Warming and Ocean Acidification. Science 333 (6041): 418-422

Puglise, K.A., Kelty R (eds.).  2007.  NOAA Coral Reef Ecosystem Research Plan for Fiscal Years 2007 to 2011. Silver Spring, MD: NOAA Coral Reef Conservation Program. NOAA Technical Memorandum CRCP 1.128 p. 91

Tibbatts B (2013) Personal Communication

Thomas J (2009) Integration and Application Network, University of Maryland Center for Environmental Science (

Sinutok S, Hill R, Doblin M, Wuhrer R, Ralph P (2011) Warmer more acidic conditions cause decreased productivity and calcification in subtropical coral reef sediment-dwelling calcifiers. Association for the Sciences of Limnology and Oceanography 56(4): 1200-1212

Wilkinson C (1999) Global and local threats to coral reef functioning and existence: review and predictions. Marine and Freshwater Research 50(8): 867-878

Yulsman T (2010) The Wages of Heat. CE Journal:

NOAA (2013) Corals Proposed for Listing under the ESA. NOAA Fisheries: Office of Protected Resources 1

Editor’s note: Scientific Research Diving at USC Dornsife is offered as part of an experiential summer program offered to undergraduate students of the USC Dana and David Dornsife College of Letters, Arts and Sciences through the Environmental Studies Program.   This course takes place on location at the USC Wrigley Marine Science Center on Catalina Island and throughout Micronesia. Students investigate important environmental issues such as ecologically sustainable development, fisheries management, protected-area planning and assessment, and human health issues. During the course of the program, the student team will dive and collect data to support conservation and management strategies to protect the fragile coral reefs of Guam and Palau in Micronesia.

Instructors for the course include Jim Haw, Director of the Environmental Studies Program in USC Dornsife, Assistant Professor of Environmental Studies David Ginsburg, Lecturer Kristen Weiss, SCUBA instructor and volunteer in the USC Scientific Diving Program Tom Carr and USC Dive Safety Officer Gerry Smith of the USC Wrigley Institute for Environmental Studies.

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