Coral reefs occupy only 0.1% of the ocean’s surface area, but support 25% of all marine life. Reef-building corals are slow-growing and exist in a mutualistic (or symbiotic) relationship with microscopic photosynthetic algae living within their tissues. The algae provide corals with nutritional compounds such as glucose, glycerol and amino acids, which are products of photosynthesis, and the corals provide shelter and reactants necessary for photosynthesis. The relationship between these organisms provides intricate habitat structure for some of the most complex ecosystems on Earth.
When a coral is stressed, most commonly by ambient water that is too warm, it expels the algae living in its tissues causing it to bleach, or turn white (Figure 1). In recent decades, local and widespread coral reef bleaching events attributed to warming water have become more frequent, greater in intensity and longer in duration (Figure 2).
I recently attended a showing of Chasing Coral, a new documentary on Netflix that follows a team of divers hoping to capture time-lapse imagery of coral bleaching events in tropical seas around the world. Despite setbacks with novel undersea technologies, the divers’ dedication, persistence and passion revealed dramatic imagery of a massive bleaching event in 2016 that killed vast swaths of coral reefs in northern Great Barrier Reef (Figure 3). In total, an estimated 29% of the shallow water corals in the Great Barrier Reef died as a direct result of the 2016 bleaching event.
The Great Barrier Reef – 2016
The Great Barrier Reef experienced a record breaking heatwave in 2016. Hughes et al. (2018) documented the broad, ecosystem-scale impacts that the prolonged heat exposure had on coral. Forecasts of thermal stress events by the US National Oceanic and Atmospheric Administration (NOAA) follow guidance that coral bleaching begins with exposure to 4°C degree heating weeks (DHW, a moving sum that expresses how much heat stress has accumulated in an area over the prior three months) and that mortality may result from 8°C-weeks. However, Hughes et al. (2018) showed that substantial coral mortality began at 3-4°C-weeks with coral losses exceeding 50% at 4-5°C-weeks during the 2016 heat wave. Further, their analysis indicated that coral assemblages breakdown at approximately 6°C-weeks. The northern third and, to a lesser extent, the central third of the Great Barrier Reef experienced prolonged exposure to these acute thermal conditions in 2016, leading to severe losses of coral communities.
Hughes et al., (2018) determined that coral mortality was highly correlated with both the amount of bleaching and the level of heat exposure. Heat-sensitive coral species began dying almost immediately from exposure to the stress of 3-4°C-weeks; many millions of corals in the northern region died in a period of only 2-3 weeks. The amount of initial mortality increased with increasing heat exposure. Responses to the heat varied significantly among species of coral. As a result, mortality following the bleaching event transformed the structural and functional diversity of corals in reefs that experienced significant bleaching.
The authors highlight that the prospects for full recovery following severe bleaching events are bleak for a few reasons. First, the corals that survived the events can continue to die due to their sustained injuries. Second, corals do not recovery quickly. Even fast-growing and quickly-reproducing species can take a decade to recruit and form a colony. Third, establishing longer-lived and slower-growing coral species is complicated by the increasing frequency of intense heating events.
Where are we headed?
The oceans are stressed by more than just heat. Since the Industrial Revolution, 25-30% of the carbon dioxide emitted by humans has been absorbed by the oceans. The ongoing uptake of carbon dioxide is causing a progressive decrease in ocean pH known as ocean acidification. The ongoing decrease in pH decreases the saturation state of aragonite (a form of calcium carbonate), the primary mineral of reef-building corals. As ocean acidification continues, growth rates for corals slow, which is of particular concern given that the occurrences of severe coral bleaching events are projected to be more common in the future.
Van Hooidonk et al. (2013) presented global projections of ocean acidification for coral reef areas. Ensembles of the IPCC AR5 for all four Representative Concentration Pathways (RCPs, Box 1) were used to generate projections of the year severe bleaching events begin to occur annually and of change in the saturation state of aragonite. Timing of the onset of severe annual bleaching showed that higher latitude reefs are projected to experience these conditions a decade or more later (2050s) than reefs in lower latitudes (~2040) based on RCP8.5 (business-as-usual climate future). However, the continuing decrease in ocean pH and aragonite saturation could cancel out any benefits associated with the delay in onset of severe annual bleaching at higher latitudes. The onset of severe annual bleaching is extended by around two decades under the RCP6.0 climate future implying there are important benefits of moving toward a lower greenhouse gas emissions future.
The researchers interpreted their results based on the NOAA thermal stress guidance described above. Hughes et al. (2018) observed severe bleaching effects and mortality at lower levels of heat stress than the NOAA guidance suggesting that the projections in van Hooidonk et al. (2013) could be too optimistic. Further, anthropogenic emissions are currently outpacing the RCP8.5 climate future.
—Box 1. The Intergovernmental Panel on Climate Change (IPCC) adopted four Representative Concentration (RCP) pathways for its fifth Assessment Report (AR5). The RCPs describe four realistic climate futures (RCP2.6, RCP4.5, RCP6.0 and RCP8.5) dependent on levels of anthropogenic greenhouse gas emissions. The RCPs are not measures of greenhouse gas emissions, but of possible ranges of radiative forcing in the year 2100 relative to pre-industrial times (+2.6, +4.5, +6.0 and +8.5 watts/meter squared, respectively). RCP2.6 represents the future with the most aggressive greenhouse gas mitigation. This RCP assumes that annual greenhouse gas emissions peak between 2010 and 2020 followed by a substantial decrease. RCP8.5 is the least aggressive, business-as-usual, future with respect to greenhouse gas mitigation. Here, greenhouse gas emissions continue to rise throughout the 21st century.—
What can we do?
Following the viewing of Chasing Coral, the audience participated in a discussion with a panel that included marine science faculty from the University of South Carolina, a contributor to the film and a representative from the Conservation Voters of South Carolina. The prospect of addressing the many aspects and challenges of global climate change is admittedly overwhelming; however, the panel highlighted a number of ways we can make strides toward the more aggressive greenhouse gas mitigation futures.
Change happens locally. From a personal stance, adopting a diet to include a higher percentage of plant-based food options reduces methane (an even more potent greenhouse gas than carbon dioxide) production from animal agriculture, a significant source of annual greenhouse gas emissions. Driving less, supporting local conservation efforts, conserving energy and water use at home, planting trees on your property, purchasing items that use minimum plastic packaging and avoiding single-use plastics represent a few other approaches that will reduce our personal environmental footprint. These steps seem obvious, but achieving them can require a change in philosophy and perspective.
Community engagement is also important. Identifying and advocating for local political candidates that support sustainable futures for the next generations is critical, especially in election years. Scientists often feel that it’s difficult to communicate with people from non-science backgrounds. It’s important to identify commonalities and find ways to easily share difficult science-heavy concepts. Conservation and advocacy groups such as 4Ocean, World Wildlife Fund and Environmental Defense Fund can help visualize and simplify messages.
It might feel as though an individual can’t make an impact, but the sum of the efforts of many can move markets, influence policy and make change.
Hughes, T.P., J.T. Kerry, A.H. Baird, S.R. Connolly, A. Dietzel, C.M. Eakin, S.F. Heron, A.S. Hoey, M.O. Hoogenboom, G. Liu, M.J. McWilliam, R.J. Pears, M.S. Pratchett, W.J. Skirving, J.S. Stella and G. Torda. 2018. Global warming transforms coral reef assemblages. Nature 556, 492-496. doi:10.1038/s41586-018-0041-2
van Hooidonk, R., J.A. Maynard, D. Manzello and S. Planes. 2013. Opposite latitudinal gradients in projected ocean acidification and bleaching impacts on coral reefs. Global Change Biology. https://doi.org/10.1111/gcb.12394.