But corals are simple creatures with thin tissues spread over large surfaces; this makes them particularly sensitive to their environment. Coastal development and increased pollution, run-off from land, coral disease, habitat destruction and overfishing mean that nearly all coral reefs are in decline. With the additional impacts of sea-level rise, temperature increases, and ocean acidification, the world's coral reefs face mounting threats; their future is a matter of much research and debate (Pandolfi et al. 2011a, Hoegh-Guldberg et al. 2011, Pandolfi et al. 2011b).

Despite their sensitive nature, the geological record shows that corals have persisted for many millions of years. The first scleractinians, or modern-day hard corals, appeared during the Triassic period about 230 million years ago. Since then, coral reefs have experienced a lot of climatic changes and undergone periods of growth and decline. Significant reductions in reef development correspond with periods of combined ocean acidification and global warming – conditions that are predicted for the future. Yet the persistence of coral reefs suggests that corals may be highly resilient and adaptive species. This seeming contradiction begs the following questions:

  • how will corals and coral reefs fare in a future of climate-related change – increased temperatures, rising sea-level and higher acidity – and local impacts such as pollution, habitat destruction and over-fishing?
  • will the combined effect of multiple stressors result in a complete extinction of corals or will they adapt to the changes and survive?
  • what can we do to increase the chances of these ecosystems persisting?

Reef research

Several researchers have summarised how reef-building corals react to climate change and other man-made stresses. Hughes et al. (2003) focused on the need to understand how much corals can adapt or acclimate to environmental change. More recent reviews concentrate on the large fraction of coral species that are under threat (Carpenter et al. 2008, Hoegh-Guldberg et al. 2007) and the overall decline in reef health because of the multiple stressors imposed by human communities (Pandolfi et al. 2011).

One controversial issue is the extent to which corals can cope with changing local conditions. If corals can acclimate or adapt to climate change, a better outcome than the dire prediction of more than 60% reduction in coral calcification by the year 2065 (Cao and Caldeira, 2008) may be possible. Another key question is how multiple stressors combine to affect coral health, particularly the relationship between corals and their symbionts – in many respects a coral's welfare depends on its symbiosis with the algal cells living alongside it, yet key details of the link between symbiosis and the environment are as yet unknown.

Cutting-edge coral

Ongoing research in coral environmental biology that will help address these questions includes: determining the role of symbionts in coral environmental response; the relationships between corals and fast growing benthic [sea-floor] algae; the biogeochemistry of reef carbonate systems; the mechanism of calcification; the response of coral growth, calcification, physiology, symbiosis, immune system and disease to low pH and other environmental changes; and the genomic mechanisms through which corals interact with their environment, including adaptation and acclimation and the genetic variability among corals.

New information on biogeochemistry, the physiological and genomic responses of corals and symbionts to changing conditions, the genetic variations of population and species, models of adaptation, and the links between coral and ecological health is set to revolutionize understanding of how corals are likely to fare in the future. Guidance from the fossil record, calcification physiology, connectivity and resilience studies, field observations, and new oceanographic models of biogeochemistry will also provide novel contributions. Together these approaches could inform our view of the likely future of coral reefs. For example, the first full coral genome sequence was announced in 2011. Such advances have shown that corals house complex and robust mechanisms for reacting to environmental variation, and that their algal symbionts can additionally help determine coral health. For example, mechanisms involved in increasing thermal tolerance include algal clade shuffling and switching, improved photoprotective defences by symbiotic algae, and up-regulated stress protein and antioxidant enzyme responses in both symbiotic algae and coral host (Brown and Cossins, 2011).

From my own research on corals that grow near acidic springs, it's clear that the ability to survive under predicted future conditions will differ considerably across coral species and reef ecosystems. This is both good news and bad. Some corals and other organisms important to an ecosystem will be able to acclimate and survive, and possibly adapt to the new conditions, but others will not. Almost inevitably, this will result in shifts in species composition and diversity, reef structural decay, and considerable changes to reef ecosystems, with potential loss of reef resources and removal of benefits to human communities.

Accordingly, as well as focusing on the research listed above, scientists, planners, managers and people that depend on or care about coral reef ecosystems should together identify the more tolerant species, and increase protection in areas that may form ecological refuges for corals more likely to adapt and survive in the face of future changes. This "management for resilience" approach needs widespread action to solve the local problems of pollution and damage. But ultimately the only long-term solution to the problems of climate change is a dedicated and consistent effort to reduce carbon-dioxide emissions.

Resources

  • Hoegh-Guldberg, O., Ortiz, J.C. and Dove, S. 2011. The future of coral reefs. Science 334: 1494–1495.
  • Pandolfi, J.M., Connolly, S.R., Marshall, D.J. and Cohen, A.L. 2011a. Projecting coral reef futures under global warming and ocean acidification. Science 333: 418–422.
  • Pandolfi, J.M., Connolly, S.R., Marshall, D.J. and Cohen, A.L. 2011b. Response. Science 334: 1495–1496.
  • Hughes, T. P. et al. Climate change, human impacts, and the resilience of coral reefs. Science 301, 929–933 (2003).
  • Cao, L., Caldeira, K. Atmospheric CO2 stabilization and ocean acidification. Geophys. Res. Lett. 35, L19609 (2008).
  • Carpenter, K. E. et al. One-third of reef building corals face elevated extinction risk from climate change and local impacts. Science 321, 560–563 (2008).
  • Hoegh-Guldberg, O. et al. Coral reefs under rapid climate change and ocean acidification. Science 318, 1737–1742 (2007)
  • Brown BE, Cossins AR (2011) The potential for temperature acclimatisation of reef corals in the face of climate change. In: Dubinsky Z, Stambler N, editors. Coral reefs: an ecosytem in transition. Dordrecht Heidelberg London New York: Springer. pp. 421–433.