Status of Caribbean coral reefs after bleaching and hurricanes in 2005

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11. Pr e d i c t i o n s f o r t h e Fu t u r e o f t h e Ca r i b b e a nSim o n Do n n e rIn t r o d u c t i o nThe 2005 coral bleaching event focused attention on the threat that continued ocean warmingposes to Caribbean coral reefs. The most recent assessment by the Intergovernmental Panel onClimate Change (IPCC) concluded that human-induced climate change will cause a 1.8–4.0ºCrise in global surface temperature by the end of the 21st century, with slightly less warmingover most of the Caribbean. The challenge for climate scientists and coral reef ecologists hasbeen to translate these more coarse projected changes in climate into impacts on coral reefsthroughout the diverse Caribbean region. This chapter presents the most recent findings andassesses the overall threat of coral bleaching in the Caribbean over the coming century.Challenges in Pr e d i c t i n g t h e Fu t u r eGlobal climate models provide the means to predict how Caribbean coral reefs will respond tofuture climate change. The models use basic physical and chemical principles to simulate themovement of heat, moisture and energy, through a three-dimensional grid representing theatmosphere, oceans and the land surface in response to natural (solar output, volcanic activity)and human (greenhouse gases, aerosols) heating forces. The simulation of future climatesdepends in large part on assumptions about demographic, economic and technologicalchange over the 21st century. For example, a ‘business-as-usual’ emissions scenario envisionsa continuation of current activities, like fossil fuel burning and consequent increases ingreenhouse gases in the atmosphere, throughout this century.It is important when examining climate models to understand the difference betweengreenhouse gas emissions and atmospheric greenhouse gas concentrations. The concentrationof greenhouse gases in the atmosphere depends on the rate of emissions and the rate at which theworld’s ecosystems and the oceans take up or remove greenhouse gases from the atmosphere.Since the rate of carbon dioxide (CO 2) emissions currently exceeds the rate of uptake, stoppingthe rise in emissions each year is not enough to stop the build-up of atmospheric CO 2. Stabilizingatmospheric CO 2concentrations and eventually global temperature will require reducing CO 2emissions well below today’s rate.129
Status of Caribbean Coral Reefs after Bleaching and Hurricanes in 2005Scientists use sea surface temperatures (SSTs) from these models to estimate the frequencyand severity of coral bleaching events under past, present and future climate scenarios. TheNOAA Coral Reef Watch program uses the accumulation of ‘Degree Heating Weeks’ (equal toone week of temperatures that are 1°C warmer than the maximum monthly temperature thecoral reef experiences in the average year) to predict the likelihood of coral bleaching in realtime.Observations indicate coral bleaching begins to occur when the degree heating week orDHW value exceeds 4°C-week and becomes severe when the DHW value exceeds 8°C-week. Thepreferred method for predicting coral bleaching from the more coarse data provided by globalclimate models is the accumulation of ‘degree heating months’. Similar to a degree heatingweek, a degree heating month or DHM is equal to one month of temperatures that are 1°Cwarmer than the maximum monthly temperature (e.g. the average temperature for Septemberin much of the Caribbean). Historical data analysis has shown that DHM values of more than1°C-month and more than 2°C-month are strong proxies for the lower and upper bleachingthresholds used by the NOAA Coral Reef Watch Program.The resolution of these climate model forecasts (the size of the grid cells in the computer’spicture of the world) is limited by the speed of computers and the complexity of the model.Although the resolution of climate models has improved in recent years, the size of model gridcells is still much larger than features of a coral reef. For example, most of the global climatemodels used in the most recent IPCC report have a horizontal resolution of only 1 degree oflatitude by 1 degree of longitude (~100 km x 100 km). Therefore, the models cannot provide adirect representation of the complex bathymetry and hydrodynamics of many coral reefs.Scientists studying climate change and coral bleaching have resolved the problem either bydownscaling (using secondary models or statistical relationships to translate the climate modeldata to a specific location) or by studying large-scale events, like the 2005 coral bleachingevent, that can be described directly by the global climate model itself. This work has provideda general picture of the expected effect of future climate change on the frequency and severityof coral bleaching in the Caribbean.Co r a l Bl e a c h i n g in t h e Fu t u r eThe various studies on climate change and coral bleaching conclude that the majority of theCaribbean is expected to experience conditions that currently lead to coral bleaching (DHM >1 or 2°C-month) every 2 years, or more, within the next 20 to 50 years.Whether coral bleaching occurs at these thresholds will depend on the ability of differentcorals and their symbionts to adapt or acclimate to warmer water temperatures. The warmingscenarios indicate that on the majority (>75%) of Caribbean reefs, corals and their symbiontswill need to increase their thermal tolerance by 0.2-0.3°C per decade over the next 30-50 yearsto avoid coral bleaching occurring more than once every 5 years. On about 25% of the reefs,mostly in the north-eastern Caribbean and Lesser Antilles, corals and their symbionts mayneed to raise their thermal tolerance by twice that amount over the same time-frame.This forecast for the next 50 years is not very sensitive to the particular future emissionsscenario, for two reasons. First, greenhouse gases like CO 2remain in the atmosphere fordecades and even centuries. This causes a lag between the emissions of greenhouse gases andtheir mean effect on climate. Second, even if the world was determined to dramatically reduce130
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3. Hu r r i ca n e s a n d t h e i
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Hurricanes and their Effect on Cora
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Hurricanes and their Effect on Cora
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4. Th e 2005 Bl e a c h i n g Ev e
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The 2005 Bleaching Event: Coral-Lis
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5. Stat u s o f t h e Mesoamerican
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Coral Reefs of the U.S. Caribbean19
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7. Th e Effects o f Co r a l Bl e a
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Status of Caribbean coral reefs after bleaching and hurricanes in 2005

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