Severe tropical storms frequently occur between 10 to 30 degrees latitude north and south, known as hurricanes in North America, tropical cyclones (TC) in the southern hemisphere, and typhoons in the northwest Pacific. They cause significant disturbances in marine ecosystems including coral reefs (Harmelin-Vivien 1994, Done 1992; Gardner et al. 2005). The aquatic ecosystem is the most affected, and they cause flattening of the reef structures.
However, unlike rocks, corals are alive. Corals are, in fact, animals. The branch or mound that we often call “a coral” is actually made up of thousands of tiny animals called polyps. A coral polyp is an invertebrate that can be no bigger than a pinhead to up to a foot in diameter. The skeletons of marine invertebrates that comprise calcium carbonate are involved in reef building. According to Coral Reef Alliance (CORAL), the important coral reefs seen today are expected to be 5,000 to 10,000 years old.
The major threats for the corals are two kinds: natural like temperature, sediment deposition, pH, salinity, etc.; and anthropogenic like mining, bottom fishing, pollution, etc. Tropical cyclones cause huge damage to both inshore and offshore coral reefs. The parameters to measure the disturbances are the erosion of the top layer of reefs, the clearance of massive corals, breakage of corals, disfiguration, disturbance of algal blooms, shifting of sedimentation.
A depression with low pressure acquires strength from warm tropical oceans and forms a storm called a tropical cyclone. Tropical cyclones can cause extensive damage to individual corals and to the structure of the reef, and can affect large areas. Cyclonic winds can also cause substantial changes in the shape of islands and coastlines, affect ocean currents and increase inshore ocean turbidity through suspension of sediments. Large storm waves may result in significant coral reef damage.
However, a hurricane will also cool surface waters and can often mitigate coral bleaching. Hurricanes help to regulate the earth’s temperature, extracting heat from the ocean and redistributing it into the atmosphere, thereby moving tropical heat polewards. The effects of tropical cyclones also include coral bleaching and diseases.
Coral bleaching is a coral disease caused by the symbiotic algae, the zooxanthellae. They are responsible for a healthy stony coral, and the coral’s death occur when the polyps of algae cells are expelled, which turns the coral into white stark-like structures. This explains the importance of reef-building corals that require clear, nutrient-less water, yet they are the most productive and diverse marine ecosystems.
The damage is extensive to inshore reefs than offshore. These cyclones ruin the ecosystem, infrastructure and coastal communities. Here, the coral reef productivity and species diversity are more affected despite being resilient to these cyclones. If we go through the timeline of tropical cyclones, for instance, Yasi in Queensland, Australia that occured in February, 2011, with a damaging wind speed of 285km/s and hit the coral gardens extensively. The fragile corals were more affected by the sediment and pesticides, which were swept away before the cyclone.
Similarly, cyclone Roanu destroyed Bangladesh and India’s east coast causing blanching corals in high intensity. In the past 15 years, cyclones like Sidr, Aila, Phailin, Hudhud, Bulbul and Fani drew devastating costs. The Ganga-Brahmaputra delta and Sunderbans have been the focus of most of the above cyclones. The recent powerful cyclone Amphan in the Bay of Bengal surpassed the Odisha cyclone of 2009.
Generally, warm waters develop cyclones. The marine heatwave intensified rapidly into a super cyclone from category 1 to category 5 with 250km/h winds. In recent years, we see that cyclones are forming quite quickly: Amphan intensified from a category 1 to a category 5 cyclone within 18 hours.
Our research shows that high ocean temperatures most likely result in such rapid intensification of cyclones in the North Indian Ocean. The buoys recorded the temperatures in the range of 30°C–33°C that are the high values until now in the basins of Bay of Bengal and Arabian Sea. The forecast models of the intensification of the rapid cyclone, which is temperature-driven climate change effect, is still a challenge that needs to be addressed. Last year’s cyclone Fani, which struck Odisha in early May, had shown somewhat similar intensification, but Amphan had no peer in how rapidly it intensified.
The efficiency of the communication between government departments in forecasting climate change-based natural calamities is necessary. However, the India Meteorological Department gave a skillfully accurate forecast of the track and intensity, but the damage to the coral reefs is still unavoidable.
While increasing heat, stress is considered a pervasive risk to coral reefs. Quantitative estimates of threats from cyclone intensification are lacking due to limited data on cyclone impact to inform projections. According to the Intergovernmental Panel on Climate Change’s Special Report on the Ocean and Cryosphere in a Changing Climate, marine heatwaves are a result of climate change that also led to massive bleaching of coral reefs in the Gulf of Mannar through April and May.
The global ocean has absorbed 90% of the excess heat generated by greenhouse gas emissions since 1970. The greenhouse gases that cause more heat are absorbed by the ocean causing the rapid, devastating intensification of tropical cyclones, and the destructive bleaching of coral reefs is the result. Hence, climate change is one leading threat to coral survival.
In order to save coral reefs, we as humans could conserve water, reduce pollution, use organic fertilizers, be responsible in disposing trash, reef-friendly livelihood. Is it too tough for mankind to be responsible and resilient at the same time?