El Niño causing Coral Bleaching

Variations in Pacific Ocean sea surface height compared to a long term average for the 2015 and 1997/98 El Niño events. Courtesy NASA/JPL-Caltech

Variations in Pacific Ocean sea surface height compared to a long term average for the 2015 and 1997/98 El Niño events. Courtesy NASA/JPL-Caltech

Coral reefs are currently undergoing their third worldwide bleaching event linked to the El Niño effect in the last 20 years, and scientists fear this one could be the worst.

El Niño is the warm phase of the El Niño Southern Oscillation and is associated with a band of warm water that develops in the central and east-central equatorial Pacific. This means waters in the Pacific Ocean are nutrient-poor, and are accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. This causes disruption to weather patterns worldwide including: droughts in Indonesia and Australia, and altering the path of the atmospheric jet stream over America.

Coral reefs cover less than 1% of the earth’s surface, but are some of the most valuable, diverse and vulnerable ecosystems on the planet. Reef building corals thrive in water temperatures between 73° and 84° Fahrenheit, but struggle outside of this range. Climate change is providing a challenge, and when you add on a warming effect like El Niño, the danger for coral reef ecosystems is clear. Worldwide coral bleaching events have occurred in 1997/98 and 2009/10, both of which were El Nino years; and the fear is that this year’s event, which will extend into 2016, could be significant.

Warmer water stresses the coral causing them to expel the photosynthetic algae, called zooxanthellae, living in their tissues. This turns them completely white, hence the term bleaching. Although this does not kill the coral immediately, it does put them at greater risk of dying. For example, half the coral reefs in the Caribbean were lost following a local bleaching event in 2005 – a weak El Niño year.

Satellite data has provided a valuable source of data to monitor the changes in coral reefs. For example, the French Centre National d’etudes Spatiales (CNES) and the USA’s National Aeronautics and Space Administration (NASA), began a mission in 1987 to monitor global ocean changes including measuring sea height by radar altimetry. It began with the TOPEX/Poseidon mission launched in 1992, which provided major data on the way the El Niño effect operated. This was followed Jason-1, launched in 2001, and Jason-2 in 2008. The value of this type of data to monitoring effects like El Niño can be seen at the top of the blog that shows side by side the variations in the Pacific Ocean sea surface height compared to a long term average for the 2015 El Niño and the strong event of 1997/98, with data collected by TOPEX/Poseidon for 1997 and the OSTM/Jason-2 for 2015. Further images and animations from NASA/JPL-Caltech can be found here.

Coral reef ecosystems are a source of food, protection against coastal erosion and provide spawning and nursing grounds for fish. They also provide jobs through fishing and tourism, and are estimated to contribute $29.8 billion to the global economy every year. However, scientists estimate that between 40 and 50% of corals worldwide have been destroyed or lost in the last 50 years. They expect this decline to continue, which could have significant consequences to the human and marine populations that are dependent on them.

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