Tropical Storms, Climate Change and Reef Resilience

The increasing number, duration and intensity of tropical storms in response to a warming climate is another possible threat to coral reef ecosystems. This issue has only been discussed briefly in this blog, and since tropical storms have the potential to cause widespread damage to coral reefs it is a threat that should be examined. While coral reefs act as protection for coastal settlements, they can often be severely damaged during tropical storms. Global Circulation Models predict rising global average sea surface temperatures, increasing the likelihood of tropical storms (IPCC, 2007). Recovery from tropical storm damage can take many years, depending on the resilience of the reef. While the prediction of future changes to storm intensity inevitably involves uncertainty, data from the last thirty yeas suggests that the number of intense storms is increasing (Figure 1).




Figure 1: Total number of intense hurricanes, showing a significant increase in category four and five storms over the period 1970-2004 (Webster et al, 2005).




The assessment of coral damage following hurricanes and cyclones has been carried out for many years. Rogers et al (1991) monitored coral reef off the south coast of St John, US Virgin Islands, at a depth of 11-13m. Nine months after permanent transects were set up Hurricane Hugo struck, allowing the analysis of storm destruction. The category 5 hurricane caused patchy damage to the reef, with large waves moving and damaging significant quantities of coral and depositing sediment on the sea floor. Following the storm there was a decrease in the percentage of living coral by 40%, 12 months later there had been no significant increase in the amount of live coral. There was also a decline in species diversity following the storm. The ability of a coral reef to recover following storm damage is highly influenced by the extent of human activities, with overfished ecosystems showing poor recovery compared with less disturbed reefs. Similar results were found in a more recent study by Guillemot et al (2010), following Cyclone Erica in New Caledonia, South Pacific, 2003. The cover of live coral decreased by 45% in the two year period following this storm. After four years the coral reef displayed good signs of recovery, with a return to pre-cyclone fish community. It is suggested that the moderate anthropogenic pressure in this region was conducive to good resilience and recovery.




In order to predict how future changes in tropical storm intensity are likely to affect coral reef ecosystems it is useful to assess long-term variability as well as their role in ecosystem disturbances. Through topographical surveys and geological dating of coral ridges and terraces (which are produced by storms) it is possible to estimate historic storm surge and wave heights. Nott and Hayne (2001) determine the intensity of tropical cyclones along the Great Barrier Reef over the past 5,000 years using this method. Their results indicate a long history of intense tropical cyclones in this region, with the majority leading to widespread coral damage, including a weakening of the substrate which enables severe damage to occur during less intense storms. This study suggests that storm events are central to coral community structure and function, including the life span of individual corals. Therefore, if the frequency and intensity of these events is to change it may have significant negative repercussions for coral reef ecosystems. The authors of this study consequently believe that coral reef ‘vulnerability, exposure and risk is much higher than previously estimated’ (Nott and Hayne, 2001 p.511).




Once again it appears that coral reefs are likely to be negatively affected by climate change. Although there is strong evidence that coral reefs are resilient to storm damage, and are able to recover in the long-term, the ability of these ecosystems to do so is significantly reduced in regions where human pressures are having damaging impacts. The need to reinforce reef resilience through effective policies and management appears a necessity if these fragile, yet globally important, habitats are to be maintained.